Laboratory transformer power supply. Good DIY laboratory power supply

Laboratory power supply is an equipment in demand among professionals, which is actively used by engineers involved in the development and repair of various electronic devices. At the moment there are a huge number laboratory power supplies . The number of different variations is so large that it will be difficult for a beginner to navigate such a variety of equipment. To choose the optimal power source for certain purposes, it is recommended to understand the features of various types of units, and only then make a purchasing decision.

Classification of laboratory power supplies

Laboratory Power Supplies can be classified according to a variety of parameters. The most popular classification method is based on the operating principle, according to which all power supplies can be divided into switching and linear. The latter are also called transformer.

Each block type has its own advantages. So, for example, impulse power block characterized by a high efficiency and significantly greater power compared to transformer units. In the same time linear power supply has such advantages as simplicity and reliability of design, as well as low cost of repairs and affordable spare parts.

Linear power supply

The traditional power supply is a linear unit. Its design consists of an autotransformer and a step-down transformer. There is also a rectifier that converts AC voltage to DC. The vast majority of models are equipped with a rectifier consisting of one or four diodes, making up the so-called diode bridge. At the same time, there are other design schemes, but they are used much less frequently. In some models, a special filter can be installed after the rectifier, which stabilizes fluctuations in the network. As a rule, this function is performed by a high-capacity capacitor. Some models provide high-frequency noise filters, current and voltage stabilizers, and much more. The simplest linear power supply can be made with your own hands, but the main and most expensive component is the step-down transformer - T1.

Linear power supply circuit

Among craftsmen who specialize in the repair and maintenance of electronics and radio equipment, the most popular linear power supply is considered to be a model with output characteristics of voltage in the adjustable range of 0-30 V and current in the range of 0-5A, for example, a DC power supply. This unit is a high-precision unit with which you can easily and finely tune the parameters of alternating current and voltage within the established nominal limits. The equipment operates in dual mode - a digital indicator simultaneously shows current voltage and output current indicators. In addition, this model has a protection mode against short circuit (short circuit), overcurrent and self-healing function.

Impulse power block

These days, the vast majority of power supplies used are switching type units. These units are essentially an inverter system. The principle of their operation is simple - the input voltage is pre-rectified, after which it is converted into pulses with an increased frequency and the necessary duty cycle parameters. Switching power supplies use small transformers, which are more than enough, since increasing the frequency increases the efficiency of the transformer, which means there is no need for large dimensions. Often the transformer core is made of ferromagnetic materials, which, among other things, significantly facilitates the design.

What ensures voltage stabilization? This function is performed by negative feedback, which maintains the output voltage at the same level. This does not take into account the load size and input voltage fluctuations. It is also possible to make a switching power supply with your own hands, but in this case the main components are a linear regulator - LM7809, or a PWM controller TL494, as well as a T1 pulse transformer.

Circuit diagram of a simple switching power supply

The most popular switching unit among professionals, which is in demand among both amateurs and professionals, is considered a switching power supply - the standard of compactness and convenience. This laboratory switching source is ideal for stable operation of a wide variety of electronic circuits and devices. The design provides the ability to adjust the parameters of alternating current in the range from 0 to 5 A and voltage from 0 to 30 V, protection against short circuit, overheating and overcurrent. This model is equipped with smooth regulators that facilitate precise selection of voltage and current. The device is equipped with a convenient digital display, which displays voltage and AC current parameters in real time.

What to choose? Advantages and disadvantages of linear and switching power supplies.

Today, switching power supplies are used everywhere, and they are actively displacing less convenient linear units from the market. However, only in work can one evaluate the strengths and weaknesses of switching and transformer power supplies.

The advantages of pulse units include:
High stabilization coefficient;
High efficiency;
Wider input voltage range;
Higher power compared to linear devices.
Lack of sensitivity to power supply quality and input voltage frequency;
Small dimensions and decent transportability;
Affordable price.

The obvious disadvantages of switching power supplies include:
Presence of impulse noise;
Complexity of circuits, which negatively affects reliability;
Repairs are not always possible to do yourself.

Transformer power supplies also have a number of advantages, including:
Simplicity and reliability of design;
High maintainability and low cost of spare parts;
No radio interference;

As you understand, transformer power supplies also have disadvantages, including:
Large weight and dimensions, which often makes transportation very inconvenient;
There is an inverse relationship between efficiency and output voltage stability;
Metal consumption of the structure.

Laboratory power supplies today are represented by a huge range of units. Both pulse and transformer units are in demand. The successful choice of equipment directly depends on what goals you are pursuing when purchasing a power supply. If you always want to have on hand a reliable unit with no radio interference, which rarely breaks down and is easy to repair, then you should pay attention to transformer power supplies. If power and efficiency are important to you, then you should study pulsed devices in more detail.

The most powerful laboratory power supplies are represented by switching models:

Making a power supply with your own hands makes sense not only for enthusiastic radio amateurs. A homemade power supply unit (PSU) will create convenience and save a considerable amount in the following cases:

• To power low-voltage power tools, to save the life of an expensive rechargeable battery;
• For electrification of premises that are particularly dangerous in terms of the degree of electric shock: basements, garages, sheds, etc. When powered by alternating current, a large amount of it in low-voltage wiring can create interference with household appliances and electronics;
• In design and creativity for precise, safe and waste-free cutting of foam plastic, foam rubber, low-melting plastics with heated nichrome;
• In lighting design, the use of special power supplies will extend the life of the LED strip and obtain stable lighting effects. Powering underwater illuminators, etc. from a household electrical network is generally unacceptable;
• For charging phones, smartphones, tablets, laptops away from stable power sources;
• For electroacupuncture;
• And many other purposes not directly related to electronics.

Acceptable simplifications

Professional power supplies are designed to power any kind of load, incl. reactive. Possible consumers include precision equipment. The pro-BP must maintain the specified voltage with the highest accuracy for an indefinitely long time, and its design, protection and automation must allow operation by unqualified personnel in difficult conditions, for example. biologists to power their instruments in a greenhouse or on an expedition.

An amateur laboratory power supply is free from these limitations and therefore can be significantly simplified while maintaining quality indicators sufficient for personal use. Further, through also simple improvements, it is possible to obtain a special-purpose power supply from it. What are we going to do now?

Abbreviations

1. KZ – short circuit.
2. XX – idle speed, i.e. sudden disconnection of the load (consumer) or a break in its circuit.
3. VS – voltage stabilization coefficient. It is equal to the ratio of the change in input voltage (in % or times) to the same output voltage at a constant current consumption. Eg. The network voltage dropped completely, from 245 to 185V. Relative to the norm of 220V, this will be 27%. If the VS of the power supply is 100, the output voltage will change by 0.27%, which, with its value of 12V, will give a drift of 0.033V. More than acceptable for amateur practice.
4. IPN is a source of unstabilized primary voltage. This can be an iron transformer with a rectifier or a pulsed network voltage inverter (VIN).
5. IIN - operate at a higher (8-100 kHz) frequency, which allows the use of lightweight compact ferrite transformers with windings of several to several dozen turns, but they are not without drawbacks, see below.
6. RE – regulating element of the voltage stabilizer (SV). Maintains the output at its specified value.
7. ION – reference voltage source. Sets its reference value, according to which, together with the OS feedback signals, the control device of the control unit influences the RE.
8. SNN – continuous voltage stabilizer; simply “analog”.
9. ISN – pulse voltage stabilizer.
10. UPS is a switching power supply.

Note: both SNN and ISN can operate both from an industrial frequency power supply with a transformer on iron, and from an electrical power supply.

About computer power supplies

UPSs are compact and economical. And in the pantry many people have a power supply from an old computer lying around, obsolete, but quite serviceable. So is it possible to adapt a switching power supply from a computer for amateur/working purposes? Unfortunately, a computer UPS is a rather highly specialized device and the possibilities of its use at home/at work are very limited:

It is perhaps advisable for the average amateur to use a UPS converted from a computer one only to power power tools; about this see below. The second case is if an amateur is engaged in PC repair and/or creation of logic circuits. But then he already knows how to adapt a power supply from a computer for this:

1. Load the main channels +5V and +12V (red and yellow wires) with nichrome spirals at 10-15% of the rated load;
2. The green soft start wire (low-voltage button on the front panel of the system unit) pc on is shorted to common, i.e. on any of the black wires;
3. On/off is performed mechanically, using a toggle switch on the rear panel of the power supply unit;
4. With mechanical (iron) I/O “on duty”, i.e. independent power supply of USB ports +5V will also be turned off.

Get to work!

Due to the shortcomings of UPSs, plus their fundamental and circuitry complexity, we will only look at a couple of them at the end, but simple and useful, and talk about the method of repairing the IPS. The main part of the material is devoted to SNN and IPN with industrial frequency transformers. They allow a person who has just picked up a soldering iron to build a power supply of very high quality. And having it on the farm, it will be easier to master “fine” techniques.

IPN

First, let's look at the IPN. We’ll leave pulse ones in more detail until the section on repairs, but they have something in common with “iron” ones: a power transformer, a rectifier and a ripple suppression filter. Together, they can be implemented in various ways depending on the purpose of the power supply.

Pos. 1 in Fig. 1 – half-wave (1P) rectifier. The voltage drop across the diode is the smallest, approx. 2B. But the pulsation of the rectified voltage is with a frequency of 50 Hz and is “ragged”, i.e. with intervals between pulses, so the pulsation filter capacitor Sf should be 4-6 times larger in capacity than in other circuits. The use of power transformer Tr for power is 50%, because Only 1 half-wave is rectified. For the same reason, a magnetic flux imbalance occurs in the Tr magnetic circuit and the network “sees” it not as an active load, but as inductance. Therefore, 1P rectifiers are used only for low power and where there is no other way, for example. in IIN on blocking generators and with a damper diode, see below.

Note: why 2V, and not 0.7V, at which the p-n junction in silicon opens? The reason is through current, which is discussed below.

Pos. 2 – 2-half-wave with midpoint (2PS). The diode losses are the same as before. case. The ripple is 100 Hz continuous, so the smallest possible Sf is needed. Usage of Tr – 100% Disadvantage – double consumption of copper on the secondary winding. At the time when rectifiers were made using kenotron lamps, this did not matter, but now it is decisive. Therefore, 2PS are used in low-voltage rectifiers, mainly at higher frequencies with Schottky diodes in UPSs, but 2PS have no fundamental limitations on power.

Pos. 3 – 2-half-wave bridge, 2RM. Losses on diodes are doubled compared to pos. 1 and 2. The rest is the same as 2PS, but the secondary copper is needed almost half as much. Almost - because several turns have to be wound to compensate for the losses on a pair of “extra” diodes. The most commonly used circuit is for voltages from 12V.

Pos. 3 – bipolar. The “bridge” is depicted conventionally, as is customary in circuit diagrams (get used to it!), and is rotated 90 degrees counterclockwise, but in fact it is a pair of 2PS connected in opposite polarities, as can be clearly seen further in Fig. 6. Copper consumption is the same as 2PS, diode losses are the same as 2PM, the rest is the same as both. It is built mainly to power analog devices that require voltage symmetry: Hi-Fi UMZCH, DAC/ADC, etc.

Pos. 4 – bipolar according to the parallel doubling scheme. Provides increased voltage symmetry without additional measures, because asymmetry of the secondary winding is excluded. Using Tr 100%, ripples 100 Hz, but torn, so Sf needs double capacity. Losses on the diodes are approximately 2.7V due to the mutual exchange of through currents, see below, and at a power of more than 15-20 W they increase sharply. They are built mainly as low-power auxiliary ones for independent power supply of operational amplifiers (op-amps) and other low-power, but demanding analog components in terms of power supply quality.

How to choose a transformer?

In a UPS, the entire circuit is most often clearly tied to the standard size (more precisely, to the volume and cross-sectional area Sc) of the transformer/transformers, because the use of fine processes in ferrite makes it possible to simplify the circuit while making it more reliable. Here, “somehow in your own way” comes down to strict adherence to the developer’s recommendations.

The iron-based transformer is selected taking into account the characteristics of the SNN, or is taken into account when calculating it. The voltage drop across the RE Ure should not be taken less than 3V, otherwise the VS will drop sharply. As Ure increases, the VS increases slightly, but the dissipated RE power grows much faster. Therefore, Ure is taken at 4-6 V. To it we add 2(4) V of losses on the diodes and the voltage drop on the secondary winding Tr U2; for a power range of 30-100 W and voltages of 12-60 V, we take it to 2.5 V. U2 arises primarily not from the ohmic resistance of the winding (it is generally negligible in powerful transformers), but due to losses due to magnetization reversal of the core and the creation of a stray field. Simply, part of the network energy, “pumped” by the primary winding into the magnetic circuit, evaporates into outer space, which is what the value of U2 takes into account.

So, we calculated, for example, for a bridge rectifier, 4 + 4 + 2.5 = 10.5 V extra. We add it to the required output voltage of the power supply unit; let it be 12V, and divide by 1.414, we get 22.5/1.414 = 15.9 or 16V, this will be the lowest permissible voltage of the secondary winding. If TP is factory-made, we take 18V from the standard range.

Now the secondary current comes into play, which, naturally, is equal to the maximum load current. Let us say we need 3A; multiply by 18V, it will be 54W. We have obtained the overall power Tr, Pg, and we will find the rated power P by dividing Pg by the efficiency Tr η, which depends on Pg:

• up to 10W, η = 0.6.
• 10-20 W, η = 0.7.
• 20-40 W, η = 0.75.
• 40-60 W, η = 0.8.
• 60-80 W, η = 0.85.
• 80-120 W, η = 0.9.
• from 120 W, η = 0.95.

In our case, there will be P = 54/0.8 = 67.5 W, but there is no such standard value, so you will have to take 80 W. In order to get 12Vx3A = 36W at the output. A steam locomotive, and that's all. It’s time to learn how to calculate and wind the “trances” yourself. Moreover, in the USSR, methods for calculating transformers on iron were developed that make it possible, without loss of reliability, to squeeze 600 W out of a core, which, when calculated according to amateur radio reference books, is capable of producing only 250 W. "Iron Trance" is not as stupid as it seems.

SNN

The rectified voltage needs to be stabilized and, most often, regulated. If the load is more powerful than 30-40 W, short-circuit protection is also necessary, otherwise a malfunction of the power supply may cause a network failure. SNN does all this together.

Simple reference

It is better for a beginner not to immediately go into high power, but to make a simple, highly stable 12V ELV for testing according to the circuit in Fig. 2. It can then be used as a source of reference voltage (its exact value is set by R5), for checking devices, or as a high-quality ELV ION. The maximum load current of this circuit is only 40mA, but the VSC on the antediluvian GT403 and the equally ancient K140UD1 is more than 1000, and when replacing VT1 with a medium-power silicon one and DA1 on any of the modern op-amps it will exceed 2000 and even 2500. The load current will also increase to 150 -200 mA, which is already useful.

0-30

The next stage is a power supply with voltage regulation. The previous one was done according to the so-called. compensation comparison circuit, but it is difficult to convert one to a high current. We will make a new SNN based on an emitter follower (EF), in which the RE and CU are combined in just one transistor. The KSN will be somewhere around 80-150, but this will be enough for an amateur. But the SNN on the ED allows, without any special tricks, to obtain an output current of up to 10A or more, as much as the Tr will give and the RE will withstand.

The circuit of a simple 0-30V power supply is shown in pos. 1 Fig. 3. IPN for it is a ready-made transformer such as TPP or TS for 40-60 W with a secondary winding for 2x24V. Rectifier type 2PS with diodes rated at 3-5A or more (KD202, KD213, D242, etc.). VT1 is installed on a radiator with an area of ​​50 square meters or more. cm; An old PC processor will work very well. Under such conditions, this ELV is not afraid of a short circuit, only VT1 and Tr will heat up, so a 0.5A fuse in the primary winding circuit of Tr is enough for protection.

Pos. Figure 2 shows how convenient a power supply on an electric power supply is for an amateur: there is a 5A power supply circuit with adjustment from 12 to 36 V. This power supply can supply 10A to the load if there is a 400W 36V power supply. Its first feature is the integrated SNN K142EN8 (preferably with index B) acts in an unusual role as a control unit: to its own 12V output is added, partially or completely, all 24V, the voltage from the ION to R1, R2, VD5, VD6. Capacitors C2 and C3 prevent excitation on HF DA1 operating in an unusual mode.

The next point is the short circuit protection device (PD) on R3, VT2, R4. If the voltage drop across R4 exceeds approximately 0.7V, VT2 will open, close the base circuit of VT1 to the common wire, it will close and disconnect the load from the voltage. R3 is needed so that the extra current does not damage DA1 when the ultrasound is triggered. There is no need to increase its denomination, because when the ultrasound is triggered, you need to securely lock VT1.

And the last thing is the seemingly excessive capacitance of the output filter capacitor C4. In this case it is safe, because The maximum collector current of VT1 of 25A ensures its charge when turned on. But this ELV can supply a current of up to 30A to the load within 50-70 ms, so this simple power supply is suitable for powering low-voltage power tools: its starting current does not exceed this value. You just need to make (at least from plexiglass) a contact block-shoe with a cable, put on the heel of the handle, and let the “Akumych” rest and save resources before leaving.

About cooling

Let's say in this circuit the output is 12V with a maximum of 5A. This is just the average power of a jigsaw, but, unlike a drill or screwdriver, it takes it all the time. At C1 it stays at about 45V, i.e. on RE VT1 it remains somewhere around 33V at a current of 5A. Power dissipation is more than 150 W, even more than 160, if you consider that VD1-VD4 also needs to be cooled. It is clear from this that any powerful adjustable power supply must be equipped with a very effective cooling system.

A finned/needle radiator using natural convection does not solve the problem: calculations show that a dissipating surface of 2000 sq. m. is needed. see and the thickness of the radiator body (the plate from which the fins or needles extend) is from 16 mm. To own this much aluminum in a shaped product was and remains a dream in a crystal castle for an amateur. A CPU cooler with airflow is also not suitable; it is designed for less power.

One of the options for the home craftsman is an aluminum plate with a thickness of 6 mm and dimensions of 150x250 mm with holes of increasing diameter drilled along the radii from the installation site of the cooled element in a checkerboard pattern. It will also serve as the rear wall of the power supply housing, as in Fig. 4.

An indispensable condition for the effectiveness of such a cooler is a weak, but continuous flow of air through the perforations from the outside to the inside. To do this, install a low-power exhaust fan in the housing (preferably at the top). A computer with a diameter of 76 mm or more is suitable, for example. add. HDD cooler or video card. It is connected to pins 2 and 8 of DA1, there is always 12V.

Note: In fact, a radical way to overcome this problem is a secondary winding Tr with taps for 18, 27 and 36V. The primary voltage is switched depending on which tool is being used.

And yet the UPS

The described power supply for the workshop is good and very reliable, but it’s hard to carry it with you on trips. This is where a computer power supply will fit in: the power tool is insensitive to most of its shortcomings. Some modification most often comes down to installing an output (closest to the load) electrolytic capacitor of large capacity for the purpose described above. There are a lot of recipes for converting computer power supplies for power tools (mainly screwdrivers, which are not very powerful, but very useful) in RuNet; one of the methods is shown in the video below, for a 12V tool.

Video: 12V power supply from a computer

With 18V tools it’s even easier: for the same power they consume less current. A much more affordable ignition device (ballast) from a 40 W or more energy saving lamp may be useful here; it can be completely placed in the case of a bad battery, and only the cable with the power plug will remain outside. How to make a power supply for an 18V screwdriver from ballast from a burnt housekeeper, see the following video.

Video: 18V power supply for a screwdriver

High class

But let’s return to SNN on ES; their capabilities are far from being exhausted. In Fig. 5 – bipolar powerful power supply with 0-30 V regulation, suitable for Hi-Fi audio equipment and other fastidious consumers. The output voltage is set using one knob (R8), and the symmetry of the channels is maintained automatically at any voltage value and any load current. A pedant-formalist may turn gray before his eyes when he sees this circuit, but the author has had such a power supply working properly for about 30 years.

The main stumbling block during its creation was δr = δu/δi, where δu and δi are small instantaneous increments of voltage and current, respectively. To develop and set up high-quality equipment, it is necessary that δr does not exceed 0.05-0.07 Ohm. Simply, δr determines the ability of the power supply to instantly respond to surges in current consumption.

For the SNN on the EP, δr is equal to that of the ION, i.e. zener diode divided by the current transfer coefficient β RE. But for powerful transistors, β drops significantly at a large collector current, and δr of a zener diode ranges from a few to tens of ohms. Here, in order to compensate for the voltage drop across the RE and reduce the temperature drift of the output voltage, we had to assemble a whole chain of them in half with diodes: VD8-VD10. Therefore, the reference voltage from the ION is removed through an additional ED on VT1, its β is multiplied by β RE.

The next feature of this design is short circuit protection. The simplest one, described above, does not fit into a bipolar circuit in any way, so the protection problem is solved according to the principle “there is no trick against scrap”: there is no protective module as such, but there is redundancy in the parameters of powerful elements - KT825 and KT827 at 25A and KD2997A at 30A. T2 is not capable of providing such a current, and while it warms up, FU1 and/or FU2 will have time to burn out.

Note: It is not necessary to indicate blown fuses on miniature incandescent lamps. It’s just that at that time LEDs were still quite scarce, and there were several handfuls of SMOKs in the stash.

It remains to protect the RE from the extra discharge currents of the pulsation filter C3, C4 during a short circuit. To do this, they are connected through low-resistance limiting resistors. In this case, pulsations may appear in the circuit with a period equal to the time constant R(3,4)C(3,4). They are prevented by C5, C6 of smaller capacity. Their extra currents are no longer dangerous for RE: the charge drains faster than the crystals of the powerful KT825/827 heat up.

Output symmetry is ensured by op-amp DA1. The RE of the negative channel VT2 is opened by current through R6. As soon as the minus of the output exceeds the plus in absolute value, it will slightly open VT3, which will close VT2 and the absolute values ​​of the output voltages will be equal. Operational control over the symmetry of the output is carried out using a dial gauge with a zero in the middle of the scale P1 (its appearance is shown in the inset), and adjustment, if necessary, is carried out by R11.

The last highlight is the output filter C9-C12, L1, L2. This design is necessary to absorb possible HF interference from the load, so as not to rack your brain: the prototype is buggy or the power supply is “wobbly”. With electrolytic capacitors alone, shunted with ceramics, there is no complete certainty here; the large self-inductance of the “electrolytes” interferes. And chokes L1, L2 divide the “return” of the load across the spectrum, and to each their own.

This power supply unit, unlike the previous ones, requires some adjustment:

1. Connect a load of 1-2 A at 30V;
2. R8 is set to maximum, in the highest position according to the diagram;
3. Using a reference voltmeter (any digital multimeter will do now) and R11, the channel voltages are set to be equal in absolute value. Maybe, if the op-amp does not have the ability to balance, you will have to select R10 or R12;
4. Use the R14 trimmer to set P1 exactly to zero.

About power supply repair

PSUs fail more often than other electronic devices: they take the first blow of network surges, and they also get a lot from the load. Even if you do not intend to make your own power supply, a UPS can be found, in addition to a computer, in a microwave oven, washing machine, and other household appliances. The ability to diagnose a power supply and knowledge of the basics of electrical safety will make it possible, if not to fix the fault yourself, then to competently bargain on the price with repairmen. Therefore, let's look at how a power supply is diagnosed and repaired, especially with an IIN, because over 80% of failures are their share.

Saturation and draft

First of all, about some effects, without understanding which it is impossible to work with a UPS. The first of them is the saturation of ferromagnets. They are not capable of absorbing energies of more than a certain value, depending on the properties of the material. Hobbyists rarely encounter saturation on iron; it can be magnetized to several Tesla (Tesla, a unit of measurement of magnetic induction). When calculating iron transformers, the induction is taken to be 0.7-1.7 Tesla. Ferrites can withstand only 0.15-0.35 T, their hysteresis loop is “more rectangular”, and operate at higher frequencies, so their probability of “jumping into saturation” is orders of magnitude higher.

If the magnetic circuit is saturated, the induction in it no longer grows and the EMF of the secondary windings disappears, even if the primary has already melted (remember school physics?). Now turn off the primary current. The magnetic field in soft magnetic materials (hard magnetic materials are permanent magnets) cannot exist stationary, like an electric charge or water in a tank. It will begin to dissipate, the induction will drop, and an EMF of the opposite polarity relative to the original polarity will be induced in all windings. This effect is quite widely used in IIN.

Unlike saturation, through current in semiconductor devices (simply draft) is an absolutely harmful phenomenon. It arises due to the formation/resorption of space charges in the p and n regions; for bipolar transistors - mainly in the base. Field-effect transistors and Schottky diodes are practically free from drafts.

For example, when voltage is applied/removed to a diode, it conducts current in both directions until the charges are collected/dissolved. That is why the voltage loss on the diodes in rectifiers is more than 0.7V: at the moment of switching, part of the charge of the filter capacitor has time to flow through the winding. In a parallel doubling rectifier, the draft flows through both diodes at once.

A draft of transistors causes a voltage surge on the collector, which can damage the device or, if a load is connected, damage it through extra current. But even without that, a transistor draft increases dynamic energy losses, like a diode draft, and reduces the efficiency of the device. Powerful field-effect transistors are almost not susceptible to it, because do not accumulate charge in the base due to its absence, and therefore switch very quickly and smoothly. “Almost”, because their source-gate circuits are protected from reverse voltage by Schottky diodes, which are slightly, but through.

TIN types

UPS trace their origins to the blocking generator, pos. 1 in Fig. 6. When turned on, Uin VT1 is slightly opened by current through Rb, current flows through winding Wk. It cannot instantly grow to the limit (remember school physics again); an emf is induced in the base Wb and load winding Wn. From Wb, through Sb, it forces the unlocking of VT1. No current flows through Wn yet and VD1 does not start up.

When the magnetic circuit is saturated, the currents in Wb and Wn stop. Then, due to the dissipation (resorption) of energy, the induction drops, an EMF of the opposite polarity is induced in the windings, and the reverse voltage Wb instantly locks (blocks) VT1, saving it from overheating and thermal breakdown. Therefore, such a scheme is called a blocking generator, or simply blocking. Rk and Sk cut off HF interference, of which blocking produces more than enough. Now some useful power can be removed from Wn, but only through the 1P rectifier. This phase continues until the Sat is completely recharged or until the stored magnetic energy is exhausted.

This power, however, is small, up to 10W. If you try to take more, VT1 will burn out from a strong draft before it locks. Since Tp is saturated, the blocking efficiency is no good: more than half of the energy stored in the magnetic circuit flies away to warm other worlds. True, due to the same saturation, blocking to some extent stabilizes the duration and amplitude of its pulses, and its circuit is very simple. Therefore, blocking-based TINs are often used in cheap phone chargers.

Note: the value of Sb largely, but not completely, as they write in amateur reference books, determines the pulse repetition period. The value of its capacitance must be linked to the properties and dimensions of the magnetic circuit and the speed of the transistor.

Blocking at one time gave rise to line scan TVs with cathode ray tubes (CRT), and it gave birth to an INN with a damper diode, pos. 2. Here the control unit, based on signals from Wb and the DSP feedback circuit, forcibly opens/locks VT1 before Tr is saturated. When VT1 is locked, the reverse current Wk is closed through the same damper diode VD1. This is the working phase: already greater than in blocking, part of the energy is removed into the load. It’s big because when it’s completely saturated, all the extra energy flies away, but here there’s not enough of that extra. In this way it is possible to remove power up to several tens of watts. However, since the control device cannot operate until Tr has approached saturation, the transistor still shows through strongly, the dynamic losses are large and the efficiency of the circuit leaves much more to be desired.

The IIN with a damper is still alive in televisions and CRT displays, since in them the IIN and the horizontal scan output are combined: the power transistor and TP are common. This greatly reduces production costs. But, frankly speaking, an IIN with a damper is fundamentally stunted: the transistor and transformer are forced to work all the time on the verge of failure. The engineers who managed to bring this circuit to acceptable reliability deserve the deepest respect, but it is strongly not recommended to stick a soldering iron in there except for professionals who have undergone professional training and have the appropriate experience.

The push-pull INN with a separate feedback transformer is most widely used, because has the best quality indicators and reliability. However, in terms of RF interference, it also sins terribly in comparison with “analog” power supplies (with transformers on hardware and SNN). Currently, this scheme exists in many modifications; powerful bipolar transistors in it are almost completely replaced by field-effect ones controlled by special devices. IC, but the principle of operation remains unchanged. It is illustrated by the original diagram, pos. 3.

The limiting device (LD) limits the charging current of the capacitors of the input filter Sfvkh1(2). Their large size is an indispensable condition for the operation of the device, because During one operating cycle, a small fraction of the stored energy is taken from them. Roughly speaking, they play the role of a water tank or air receiver. When charging “short”, the extra charge current can exceed 100A for a time of up to 100 ms. Rc1 and Rc2 with a resistance of the order of MOhm are needed to balance the filter voltage, because the slightest imbalance of his shoulders is unacceptable.

When Sfvkh1(2) are charged, the ultrasonic trigger device generates a trigger pulse that opens one of the arms (which one does not matter) of the inverter VT1 VT2. A current flows through the winding Wk of a large power transformer Tr2 and the magnetic energy from its core through the winding Wn is almost completely spent on rectification and on the load.

A small part of the energy Tr2, determined by the value of Rogr, is removed from the winding Woc1 and supplied to the winding Woc2 of a small basic feedback transformer Tr1. It quickly saturates, the open arm closes and, due to dissipation in Tr2, the previously closed one opens, as described for blocking, and the cycle repeats.

In essence, a push-pull IIN is 2 blockers “pushing” each other. Since the powerful Tr2 is not saturated, the draft VT1 VT2 is small, completely “sinks” into the magnetic circuit Tr2 and ultimately goes into the load. Therefore, a two-stroke IPP can be built with a power of up to several kW.

It's worse if he ends up in XX mode. Then, during the half cycle, Tr2 will have time to saturate itself and a strong draft will burn both VT1 and VT2 at once. However, now there are power ferrites on sale for induction up to 0.6 Tesla, but they are expensive and degrade from accidental magnetization reversal. Ferrites with a capacity of more than 1 Tesla are being developed, but in order for IINs to achieve “iron” reliability, at least 2.5 Tesla is needed.

Diagnostic technique

When troubleshooting an “analog” power supply, if it is “stupidly silent,” first check the fuses, then the protection, RE and ION, if it has transistors. They ring normally - we move on element by element, as described below.

In the IIN, if it “starts up” and immediately “stalls out”, they first check the control unit. The current in it is limited by a powerful low-resistance resistor, then shunted by an optothyristor. If the “resistor” is apparently burnt, replace it and the optocoupler. Other elements of the control device fail extremely rarely.

If the IIN is “silent, like a fish on ice,” the diagnosis also begins with the OU (maybe the “rezik” has completely burned out). Then - ultrasound. Cheap models use transistors in avalanche breakdown mode, which is far from being very reliable.

The next stage in any power supply is electrolytes. Fracture of the housing and leakage of electrolyte are not nearly as common as they write on the RuNet, but loss of capacity occurs much more often than failure of active elements. Electrolytic capacitors are checked with a multimeter capable of measuring capacitance. Below the nominal value by 20% or more - we lower the “dead” into the sludge and install a new, good one.

Then there are the active elements. You probably know how to dial diodes and transistors. But there are 2 tricks here. The first is that if a Schottky diode or zener diode is called by a tester with a 12V battery, then the device may show a breakdown, although the diode is quite good. It is better to call these components using a pointer device with a 1.5-3 V battery.

The second is powerful field workers. Above (did you notice?) it is said that their I-Z are protected by diodes. Therefore, powerful field-effect transistors seem to sound like serviceable bipolar transistors, even if they are unusable if the channel is “burnt out” (degraded) not completely.

Here, the only way available at home is to replace them with known good ones, both at once. If there is a burnt one left in the circuit, it will immediately pull a new working one with it. Electronics engineers joke that powerful field workers cannot live without each other. Another prof. joke – “replacement gay couple.” This means that the transistors of the IIN arms must be strictly of the same type.

Finally, film and ceramic capacitors. They are characterized by internal breaks (found by the same tester that checks the “air conditioners”) and leakage or breakdown under voltage. To “catch” them, you need to assemble a simple circuit according to Fig. 7. Step-by-step testing of electrical capacitors for breakdown and leakage is carried out as follows:

• We set on the tester, without connecting it anywhere, the smallest limit for measuring direct voltage (most often 0.2V or 200mV), detect and record the device’s own error;
• We turn on the measurement limit of 20V;
• We connect the suspicious capacitor to points 3-4, the tester to 5-6, and to 1-2 we apply a constant voltage of 24-48 V;
• Switch the multimeter voltage limits down to the lowest;
• If on any tester it shows anything other than 0000.00 (at the very least - something other than its own error), the capacitor being tested is not suitable.

This is where the methodological part of the diagnosis ends and the creative part begins, where all the instructions are based on your own knowledge, experience and considerations.

A couple of impulses

UPSs are a special article due to their complexity and circuit diversity. Here, to begin with, we will look at a couple of samples using pulse width modulation (PWM), which allows us to obtain the best quality UPS. There are a lot of PWM circuits in RuNet, but PWM is not as scary as it is made out to be...

For lighting design

You can simply light the LED strip from any power supply described above, except for the one in Fig. 1, setting the required voltage. SNN with pos. 1 Fig. 3, it’s easy to make 3 of these, for channels R, G and B. But the durability and stability of the LEDs’ glow does not depend on the voltage applied to them, but on the current flowing through them. Therefore, a good power supply for LED strip should include a load current stabilizer; in technical terms - a stable current source (IST).

One of the schemes for stabilizing the light strip current, which can be repeated by amateurs, is shown in Fig. 8. It is assembled on an integrated timer 555 (domestic analogue - K1006VI1). Provides a stable tape current from a power supply voltage of 9-15 V. The amount of stable current is determined by the formula I = 1/(2R6); in this case - 0.7A. The powerful transistor VT3 is necessarily a field-effect transistor; from a draft, due to the base charge, a bipolar PWM simply will not form. Inductor L1 is wound on a ferrite ring 2000NM K20x4x6 with a 5xPE 0.2 mm harness. Number of turns – 50. Diodes VD1, VD2 – any silicon RF (KD104, KD106); VT1 and VT2 – KT3107 or analogues. With KT361, etc. The input voltage and brightness control ranges will decrease.

The circuit works like this: first, the time-setting capacitance C1 is charged through the R1VD1 circuit and discharged through VD2R3VT2, open, i.e. in saturation mode, through R1R5. The timer generates a sequence of pulses with the maximum frequency; more precisely - with a minimum duty cycle. The VT3 inertia-free switch generates powerful impulses, and its VD3C4C3L1 harness smooths them out to direct current.

Note: The duty cycle of a series of pulses is the ratio of their repetition period to the pulse duration. If, for example, the pulse duration is 10 μs, and the interval between them is 100 μs, then the duty cycle will be 11.

The current in the load increases, and the voltage drop across R6 opens VT1, i.e. transfers it from the cut-off (locking) mode to the active (reinforcing) mode. This creates a leakage circuit for the base of VT2 R2VT1+Upit and VT2 also goes into active mode. The discharge current C1 decreases, the discharge time increases, the duty cycle of the series increases and the average current value drops to the norm specified by R6. This is the essence of PWM. At minimum current, i.e. at maximum duty cycle, C1 is discharged through the VD2-R4-internal timer switch circuit.

In the original design, the ability to quickly adjust the current and, accordingly, the brightness of the glow is not provided; There are no 0.68 ohm potentiometers. The easiest way to adjust the brightness is by connecting, after adjustment, a 3.3-10 kOhm potentiometer R* into the gap between R3 and the VT2 emitter, highlighted in brown. By moving its engine down the circuit, we will increase the discharge time of C4, the duty cycle and reduce the current. Another method is to bypass the base junction of VT2 by turning on a potentiometer of approximately 1 MOhm at points a and b (highlighted in red), less preferable, because the adjustment will be deeper, but rougher and sharper.

Unfortunately, to set up this useful not only for IST light tapes, you need an oscilloscope:

1. The minimum +Upit is supplied to the circuit.
2. By selecting R1 (impulse) and R3 (pause) we achieve a duty cycle of 2, i.e. The pulse duration must be equal to the pause duration. You cannot give a duty cycle less than 2!
3. Serve maximum +Upit.
4. By selecting R4, the rated value of a stable current is achieved.

For charging

In Fig. 9 – diagram of the simplest ISN with PWM, suitable for charging a phone, smartphone, tablet (a laptop, unfortunately, will not work) from a homemade solar battery, wind generator, motorcycle or car battery, magneto flashlight “bug” and other low-power unstable random sources power supply See the diagram for the input voltage range, there is no error there. This ISN is indeed capable of producing an output voltage greater than the input. As in the previous one, here there is the effect of changing the polarity of the output relative to the input; this is generally a proprietary feature of PWM circuits. Let's hope that after reading the previous one carefully, you will understand the work of this tiny little thing yourself.

Incidentally, about charging and charging

Charging batteries is a very complex and delicate physical and chemical process, the violation of which reduces their service life several times or tens of times, i.e. number of charge-discharge cycles. The charger must, based on very small changes in battery voltage, calculate how much energy has been received and regulate the charging current accordingly according to a certain law. Therefore, the charger is by no means a power supply, and only batteries in devices with a built-in charge controller can be charged from ordinary power supplies: phones, smartphones, tablets, and certain models of digital cameras. And charging, which is a charger, is a subject for a separate discussion.

Question-remont.ru said:

There will be some sparking from the rectifier, but it's probably not a big deal. The point is the so-called. differential output impedance of the power supply. For alkaline batteries it is about mOhm (milliohms), for acid batteries it is even less. A trance with a bridge without smoothing has tenths and hundredths of an ohm, i.e. approx. 100 – 10 times more. And the starting current of a brushed DC motor can be 6-7 or even 20 times greater than the operating current. Yours is most likely closer to the latter - fast-accelerating motors are more compact and more economical, and the huge overload capacity of the batteries allows you to give the engine as much current as it can handle. for acceleration. A trans with a rectifier will not provide as much instantaneous current, and the engine accelerates more slowly than it was designed for, and with a large slip of the armature. From this, from the large slip, a spark arises, and then remains in operation due to self-induction in the windings.

What can I recommend here? First: take a closer look - how does it spark? You need to watch it in operation, under load, i.e. during sawing.

If sparks dance in certain places under the brushes, it’s okay. My powerful Konakovo drill sparkles so much from birth, and for goodness sake. In 24 years, I changed the brushes once, washed them with alcohol and polished the commutator - that’s all. If you connected an 18V instrument to a 24V output, then a little sparking is normal. Unwind the winding or extinguish the excess voltage with something like a welding rheostat (a resistor of approximately 0.2 Ohm for a dissipation power of 200 W or more), so that the motor operates at the rated voltage and, most likely, the spark will go away. If you connected it to 12 V, hoping that after rectification it would be 18, then in vain - the rectified voltage drops significantly under load. And the commutator electric motor, by the way, doesn’t care whether it is powered by direct current or alternating current.

Specifically: take 3-5 m of steel wire with a diameter of 2.5-3 mm. Roll into a spiral with a diameter of 100-200 mm so that the turns do not touch each other. Place on a fireproof dielectric pad. Clean the ends of the wire until shiny and fold them into “ears”. It is best to immediately lubricate with graphite lubricant to prevent oxidation. This rheostat is connected to the break in one of the wires leading to the instrument. It goes without saying that the contacts should be screws, tightened tightly, with washers. Connect the entire circuit to the 24V output without rectification. The spark is gone, but the power on the shaft has also dropped - the rheostat needs to be reduced, one of the contacts needs to be switched 1-2 turns closer to the other. It still sparks, but less - the rheostat is too small, you need to add more turns. It is better to immediately make the rheostat obviously large so as not to screw on additional sections. It’s worse if the fire is along the entire line of contact between the brushes and the commutator or spark tails trail behind them. Then the rectifier needs an anti-aliasing filter somewhere, according to your data, from 100,000 µF. Not a cheap pleasure. The “filter” in this case will be an energy storage device for accelerating the motor. But it may not help if the overall power of the transformer is not enough. Efficiency of brushed DC motors is approx. 0.55-0.65, i.e. trans is needed from 800-900 W. That is, if the filter is installed, but still sparks with fire under the entire brush (under both, of course), then the transformer is not up to the task. Yes, if you install a filter, then the diodes of the bridge must be rated for triple the operating current, otherwise they may fly out from the surge of charging current when connected to the network. And then the tool can be launched 5-10 seconds after being connected to the network, so that the “banks” have time to “pump up”.

And the worst thing is if the tails of sparks from the brushes reach or almost reach the opposite brush. This is called all-round fire. It very quickly burns out the collector to the point of complete disrepair. There can be several reasons for a circular fire. In your case, the most probable is that the motor was turned on at 12 V with rectification. Then, at a current of 30 A, the electrical power in the circuit is 360 W. The anchor slides more than 30 degrees per revolution, and this is necessarily a continuous all-round fire. It is also possible that the motor armature is wound with a simple (not double) wave. Such electric motors are better at overcoming instantaneous overloads, but they have a starting current - mother, don’t worry. I can’t say more precisely in absentia, and there’s no point in it – there’s hardly anything we can fix here with our own hands. Then it will probably be cheaper and easier to find and purchase new batteries. But first, try turning on the engine at a slightly higher voltage through the rheostat (see above). Almost always, in this way it is possible to shoot down a continuous all-round fire at the cost of a small (up to 10-15%) reduction in power on the shaft.

Evgeniy said:

Need more cuts. So that all the text is made up of abbreviations. Fuck that no one understands, but you don’t have to write the same word that is repeated THREE times in the text.

By clicking the “Add comment” button, I agree with the site.

Every novice radio amateur needs a laboratory power supply. To do it correctly, you need to choose a suitable scheme, and with this there are usually many problems.

Types and features of power supplies

There are two types of power supplies:

• Pulse;
• Linear.

A pulse-type block can generate interference, which will affect the settings of receivers and other transmitters. A linear power supply may not be able to supply the required power.

How to properly make a laboratory power supply from which you can charge the battery and power sensitive circuit boards? If you take a simple linear power supply of 1.3-30 V, and a current power of no more than 5 A, you will get a good voltage and current stabilizer.

Let's use the classic diagram to assemble a power supply with our own hands. It is designed on LM317 stabilizers, which regulate voltage in the range of 1.3-37V. Their work is combined with KT818 transistors. These are powerful radio components that can pass large currents. The protective function of the circuit is provided by LM301 stabilizers.

This scheme was developed quite a long time ago and was periodically modernized. Several diode bridges appeared on it, and the measuring head received a non-standard switching method. The MJ4502 transistor was replaced by a less powerful analogue - KT818. Filter capacitors also appeared.

DIY block installation

During the next assembly, the block diagram received a new interpretation. The capacitance of the output capacitors was increased, and several diodes were added for protection.

The KT818 type transistor was an unsuitable element in this circuit. It overheated greatly and often caused breakdowns. They found a replacement for it with a more profitable option TIP36C; in the circuit it has a parallel connection.

Step-by-step setup

A self-made laboratory power supply needs to be switched on step by step. The initial startup takes place with LM301 and transistors disconnected. Next, the function regulating the voltage through regulator P3 is checked.

If the voltage is well regulated, then transistors are included in the circuit. Their work will then be good when several resistances R7, R8 begin to balance the emitter circuit. Resistors are needed so that their resistance is as low as possible. In this case, there must be enough current, otherwise in T1 and T2 its values ​​will differ.

This adjustment step allows the load to be connected to the output end of the power supply. You should try to avoid a short circuit, otherwise the transistors will immediately burn out, followed by the LM317 stabilizer.

The next step will be the installation of LM301. First, you need to make sure that there is -6V on the op-amp in pin 4. If +6V is present on it, then there may be an incorrect connection of the BR2 diode bridge.

Also, the connection of capacitor C2 may be incorrect. After inspecting and correcting installation defects, you can supply power to the 7th leg of the LM301. This can be done from the output of the power supply.

At the last stages, P1 is adjusted so that it can operate at the maximum operating current of the power supply. A lab power supply with voltage regulation is not that difficult to adjust. In this case, it is better to double-check the installation of parts than to get a short circuit with subsequent replacement of elements.

Basic radioelements

To assemble a powerful laboratory power supply with your own hands, you need to purchase the appropriate components:

• A transformer is required for power supply;
• Several transistors;
• Stabilizers;
• Operational amplifier;
• Several types of diodes;
• Electrolytic capacitors – no more than 50V;
• Resistors of different types;
• Resistor P1;
• Fuse.

The rating of each radio component must be checked with the diagram.

Block in final form

For transistors, it is necessary to select a suitable heatsink that can dissipate heat. Moreover, a fan is mounted inside to cool the diode bridge. Another one is installed on an external radiator, which will blow air over the transistors.

For the internal filling, it is advisable to choose a high-quality case, since the thing turned out to be serious. All elements should be well fixed. In the photo of the laboratory power supply, you can see that pointer voltmeters have been replaced by digital devices.

Photo of laboratory power supply

In general, any power supply unit (PSU) is a device that, when connected to an electrical network, generates the voltage and current necessary for further use.

Most often, such devices convert alternating current from the public electrical network (~220V, frequency 50 Hz) into direct current.

All power supplies can be divided into:

• transformer (linear);
• pulsed.

In turn, transformer blocks can be:

• stabilized;
• unstabilized.

An unstabilized source is the simplest device, which includes:

• step-down transformer with a primary winding designed for mains voltage;
• a full-wave rectifier, with the help of which the alternating current voltage is converted into direct (pulsating);
• high-capacity capacitor that smoothes out ripples.

In such power supplies, the nominal values ​​of the output parameters (voltage, current) are provided only with normal values ​​of the input electrical parameters and the current consumed by the load. They are used to work with devices equipped with their own stabilizers.

Stabilized power supplies have a constant output voltage level. Moreover, even with a significant deviation from the nominal mains voltage, it remains constant.

In switching power supplies, the alternating voltage is rectified and then converted into high-frequency rectangular pulses with a given duty cycle. Stabilization in them is ensured by the use of negative feedback, which can be organized either using galvanic isolation from the supply circuit (transformer) or by applying pulses to a low-frequency filter.

Depending on the fluctuations in the feedback signal, the duty cycle of the output pulses is adjusted and thus the stability of the output voltage is maintained.

For each electronic or radio device, developers select the most optimal type of power supply. So, for example, to work with devices operating with maximum load current:

• up to 5A linear power supplies are used;
• over 5A use switching power supplies.

When comparing power supplies with similar output characteristics, it is necessary to note the advantages of switching devices, among which the most significant are:

1. High efficiency factor (efficiency), reaching in some cases 98%.
2. Light weight, which is associated with a reduction in the size of transformers when using high-frequency currents.
3. Wide range of supply voltage and frequency.
4. The presence of a large number of built-in security elements, etc.

A significant disadvantage of switching power supplies is that they are all sources of high-frequency interference, requiring special protection measures to suppress them.

Both types of units are presented in a wide range on the domestic radio-electronic equipment (REA) market. At the same time, universal power supplies are very popular, which are used to equip workplaces of employees of enterprises specializing in the production or repair of electronic equipment. Every radio amateur has them.

UNIVERSAL POWER SUPPLY

A universal power supply is a reliable power supply with stable output parameters and a double power reserve. In general, its front panel should contain:

1. Pointer and digital measuring instruments (voltmeter, ammeter). At the same time: the switch will make it possible to evaluate the dynamic changes in the controlled parameters; digital will allow you to control the output characteristics of the power supply with high accuracy.

2. Controls with which the output parameters are adjusted in the “coarse” and “fine” modes, an operating mode indicator, a toggle switch or a key switch for the power supply.

It is theoretically possible, but practically impractical to develop and manufacture a universal power supply that is suitable, as they say, “for all occasions.” Such a device will be enormous in size and weight, and its cost will exceed all acceptable limits.

Therefore, modern universal secondary voltage sources are classified by power, by the rated value of the output voltage and by the number of supply voltage outputs. Based on these gradations, the necessary device is selected.

According to the rated output voltage, universal power supplies are:

• low voltage up to 100 V;
• medium voltage up to 1000 V;
• high voltage over 1000 V.

Based on output power they are divided into:

• micropower, the output power of which does not exceed 1 W;
• low power from 1 to 10 W;
• average power 10...100 W;
• increased (from 100 to 1000 W) and high (over 1000 W) power.

In this case, universal power supplies can be single or multi-channel, that is, providing one or more supply voltages.

Adjustable power supply.

One of the simplest universal power supplies is regulated. For example, for novice radio amateurs, such a device could be a power supply with a load current of several amperes and allowing the output voltage to be adjusted in the range from 1 to 36 V.

You can connect not only a radio device or an electric motor to it, but also a car battery for charging.

The electrical circuit of such a power supply is based on a powerful power transformer, and at the output there is a powerful transistor mounted on a heat sink. The transistor is controlled by a special microcircuit. Existing low-frequency ripples and high-frequency noise are smoothed out by high-capacity electrolytic capacitors.

LABORATORY POWER SUPPLY

The laboratory power supply is nothing more than a high-quality universal power supply with standardized and thermally stable characteristics. These devices are available at any enterprise that develops, manufactures or repairs and/or repairs electronic equipment.

They are used during testing and/or calibration of various instruments. In addition, they are necessary in cases where it is necessary to supply supply voltage and current to a radio device with high precision.

As a rule, laboratory power supplies are equipped with all kinds of protection devices (overload, short circuit protection, etc.) and controls for adjusting output parameters (voltage and current).

Laboratory units are also equipped with special inputs for supplying modulating signals, which allows the user to generate an output voltage and current of arbitrary shape.

Commercially produced laboratory power supplies can be either linear or switching.

Linear.

Linear laboratory power supplies are built on the basis of large low-frequency transformers, which reduce the mains voltage ~220 V with a frequency of 50 Hz to a certain value. The AC frequency remains unchanged. Then the sinusoidal voltage is rectified, smoothed by capacitive filters and brought to a given value by a linear semiconductor stabilizer.

Devices operating on this principle provide the required output voltage with high accuracy. It is stable and pulsation-free. However, they have a number of disadvantages:

• large overall dimensions and weight, which can be more than 20 kg. Because of this, the load power of such power supplies rarely exceeds 200 W;
• low efficiency (no more than 60%), which is due to the operating principle of a linear stabilizer, where all excess voltage is converted into heat;
• the presence of high-frequency interference penetrating from the network ~220 V, 50 Hz, to eliminate which a network filter is required;
• relatively short MTBF caused by aging of electrolytic capacitors.

Pulse.

The operation of pulsed laboratory power supplies is based on the principle of charging smoothing capacitors with pulsed current. It is formed at the moment of connecting/disconnecting the inductive element. Switching occurs under the influence of specially optimized transistors, and the output voltage is regulated by changing the depth of pulse width modulation (PWM).

The main advantages of pulsed laboratory sources are provided by:

• smoothly changing the PWM depth, which in turn allows you to pump into the smoothing capacitors an amount of energy that is commensurate with the power consumption of the power supply load. In this case, the efficiency of the power supply can reach 90 percent or more;
• high-frequency component, which makes it possible to use smoothing capacitors of significantly small capacity.

Due to this, the overall dimensions of the case are small. In addition, due to higher efficiency, heat generation is significantly reduced and the temperature regime of the power source is improved.

A significant disadvantage of pulsed laboratory units, which somewhat limits their use, are:

• high-frequency pulsations at the output, which are quite difficult to filter;
• radio frequency interference and its harmonics caused by periodic current pulses.

When working with radio frequency circuits, switching power supplies must be located at the maximum distance from them or use transformer circuit solutions.

The main technical parameter of laboratory sources of electrical energy is power. Here is the following division:

• standard, power up to 700 W. Their maximum weight does not exceed 15 kg;
• high power.

Standard versions can be either transformer or pulse. They are designed to work with voltages in the range from 15 to 150 V. In this case, the maximum current is limited to about 25 A. As a rule, they have from one to three channels, two of which are adjustable.

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The materials presented on the site are for informational purposes only and cannot be used as guidelines or regulatory documents.

The master whose device was described in the first part, having set out to make a power supply with regulation, did not complicate things for himself and simply used boards that were lying idle. The second option involves the use of an even more common material - an adjustment has been added to the usual block, perhaps this is a very promising solution in terms of simplicity, given that the necessary characteristics will not be lost and even the most experienced radio amateur can implement the idea with his own hands. As a bonus, there are two more options for very simple schemes with all the detailed explanations for beginners. So, there are 4 ways for you to choose from.

We'll tell you how to make an adjustable power supply from an unnecessary computer board. The master took the computer board and cut out the block that powers the RAM.
This is what he looks like.

Let's decide which parts need to be taken and which ones not, in order to cut off what is needed so that the board has all the components of the power supply. Typically, a pulse unit for supplying current to a computer consists of a microcircuit, a PWM controller, key transistors, an output inductor and an output capacitor, and an input capacitor. For some reason, the board also has an input choke. He left him too. Key transistors - maybe two, three. There is a seat for 3 transistors, but it is not used in the circuit.

The PWM controller chip itself may look like this. Here she is under a magnifying glass.

It may look like a square with small pins on all sides. This is a typical PWM controller on a laptop board.

This is what a switching power supply looks like on a video card.

The power supply for the processor looks exactly the same. We see a PWM controller and several processor power channels. 3 transistors in this case. Choke and capacitor. This is one channel.
Three transistors, a choke, a capacitor - the second channel. Channel 3. And two more channels for other purposes.
You know what a PWM controller looks like, look at its markings under a magnifying glass, look for a datasheet on the Internet, download the pdf file and look at the diagram so as not to confuse anything.
In the diagram we see a PWM controller, but the pins are marked and numbered along the edges.

Transistors are designated. This is the throttle. This is an output capacitor and an input capacitor. The input voltage ranges from 1.5 to 19 volts, but the supply voltage to the PWM controller should be from 5 volts to 12 volts. That is, it may turn out that a separate power source is required to power the PWM controller. All the wiring, resistors and capacitors, don’t be alarmed. You don't need to know this. Everything is on the board; you do not assemble a PWM controller, but use a ready-made one. You only need to know 2 resistors - they set the output voltage.

Resistor divider. Its whole point is to reduce the signal from the output to about 1 volt and apply feedback to the input of the PWM controller. In short, by changing the value of the resistors, we can regulate the output voltage. In the case shown, instead of a feedback resistor, the master installed a 10 kilo-ohm tuning resistor. This was sufficient to regulate the output voltage from 1 volt to approximately 12 volts. Unfortunately, this is not possible on all PWM controllers. For example, on PWM controllers of processors and video cards, in order to be able to adjust the voltage, the possibility of overclocking, the output voltage is supplied by software via a multi-channel bus. The only way to change the output voltage of such a PWM controller is by using jumpers.

So, knowing what a PWM controller looks like and the elements that are needed, we can already cut out the power supply. But this must be done carefully, since there are tracks around the PWM controller that may be needed. For example, you can see that the track goes from the base of the transistor to the PWM controller. It was difficult to save it; I had to carefully cut out the board.

Using the tester in dial mode and focusing on the diagram, I soldered the wires. Also using the tester, I found pin 6 of the PWM controller and the feedback resistors rang from it. The resistor was located in the rfb, it was removed and instead of it, a 10 kilo-ohm tuning resistor was soldered from the output to regulate the output voltage; I also found out by calling that the power supply of the PWM controller is directly connected to the input power line. This means that you cannot supply more than 12 volts to the input, so as not to burn out the PWM controller.

Let's see what the power supply looks like in operation

I soldered the input voltage plug, voltage indicator and output wires. We connect an external 12 volt power supply. The indicator lights up. It was already set to 9.2 volts. Let's try to adjust the power supply with a screwdriver.

It's time to check out what the power supply is capable of. I took a wooden block and a homemade wirewound resistor made from nichrome wire. Its resistance is low and, together with the tester probes, is 1.7 Ohms. We turn the multimeter into ammeter mode and connect it in series with the resistor. See what happens - the resistor heats up to red, the output voltage remains virtually unchanged, and the current is about 4 amperes.

The master had already made similar power supplies before. One is cut out with your own hands from a laptop board.

This is the so-called standby voltage. Two sources of 3.3 volts and 5 volts. I made a case for it on a 3D printer. You can also look at the article where I made a similar adjustable power supply, also cut from a laptop board (https://electro-repair.livejournal.com/3645.html). This is also a PWM power controller for RAM.

How to make a regulating power supply from a regular printer

We will talk about the power supply for a Canon inkjet printer. Many people have them idle. This is essentially a separate device, held in the printer by a latch.
Its characteristics: 24 volts, 0.7 amperes.

I needed a power supply for a homemade drill. It's just right in terms of power. But there is one caveat - if you connect it like this, the output will only get 7 volts. Triple output, connector and we get only 7 volts. How to get 24 volts?
How to get 24 volts without disassembling the unit?
Well, the simplest one is to close the plus with the middle output and we get 24 volts.
Let's try to do it. We connect the power supply to the 220 network. We take the device and try to measure it. Let's connect and see 7 volts at the output.
Its central connector is not used. If we take it and connect it to two at the same time, the voltage is 24 volts. This is the easiest way to ensure that this power supply produces 24 volts without disassembling it.

A homemade regulator is needed so that the voltage can be adjusted within certain limits. From 10 volts to maximum. It's easy to do. What is needed for this? First, open the power supply itself. It is usually glued. How to open it without damaging the case. There is no need to pick or pry anything. We take a piece of wood that is heavier or have a rubber mallet. Place it on a hard surface and tap along the seam. The glue comes off. Then they tapped thoroughly on all sides. Miraculously, the glue comes off and everything opens up. Inside we see the power supply.

We'll get the payment. Such power supplies can be easily converted to the desired voltage and can also be made adjustable. On the reverse side, if we turn it over, there is an adjustable zener diode tl431. On the other hand, we will see the middle contact goes to the base of transistor q51.

If we apply voltage, then this transistor opens and 2.5 volts appears at the resistive divider, which is needed for the zener diode to operate. And 24 volts appears at the output. This is the simplest option. Another way to start it is to throw away transistor q51 and put a jumper instead of resistor r 57 and that’s it. When we turn it on, the output is always 24 volts continuously.

How to make the adjustment?

You can change the voltage, make it 12 volts. But in particular, the master does not need this. You need to make it adjustable. How to do it? We throw away this transistor and replace the 57 by 38 kilo-ohm resistor with an adjustable one. There is an old Soviet one with 3.3 kilo-ohms. You can put from 4.7 to 10, which is what it is. Only the minimum voltage to which it can lower it depends on this resistor. 3.3 is very low and not necessary. The engines are planned to be supplied at 24 volts. And just from 10 volts to 24 is normal. If you need a different voltage, you can use a high-resistance tuning resistor.
Let's get started, let's solder. Take a soldering iron and hair dryer. I removed the transistor and resistor.

We soldered the variable resistor and will try to turn it on. We applied 220 volts, we see 7 volts on our device and begin to rotate the variable resistor. The voltage has risen to 24 volts and we rotate it smoothly and smoothly, it drops - 17-15-14, that is, it decreases to 7 volts. In particular, it is installed on 3.3 rooms. And our rework turned out to be quite successful. That is, for purposes from 7 to 24 volts, voltage regulation is quite acceptable.

This option worked out. I installed a variable resistor. The handle turns out to be an adjustable power supply - quite convenient.

Video of the channel “Technician”.

Such power supplies are easy to find in China. I came across an interesting store that sells used power supplies from various printers, laptops and netbooks. They disassemble and sell the boards themselves, fully functional for different voltages and currents. The biggest plus is that they disassemble branded equipment and all power supplies are of high quality, with good parts, all have filters.
The photos are of different power supplies, they cost pennies, practically a freebie.

Simple block with adjustment

A simple version of a homemade device for powering devices with regulation. The scheme is popular, it is widespread on the Internet and has shown its effectiveness. But there are also limitations, which are shown in the video along with all the instructions for making a regulated power supply.

Homemade regulated unit on one transistor

What is the simplest regulated power supply you can make yourself? This can be done on the lm317 chip. It almost represents a power supply itself. It can be used to make both a voltage- and flow-regulated power supply. This video tutorial shows a device with voltage regulation. The master found a simple scheme. Input voltage maximum 40 volts. Output from 1.2 to 37 volts. Maximum output current 1.5 amperes.

Without a heat sink, without a radiator, the maximum power can be only 1 watt. And with a radiator 10 watts. List of radio components.


Let's start assembling

Let's connect an electronic load to the output of the device. Let's see how well it holds current. We set it to minimum. 7.7 volts, 30 milliamps.

Everything is regulated. Let's set it to 3 volts and add current. We’ll only set larger restrictions on the power supply. We move the toggle switch to the upper position. Now it's 0.5 ampere. The microcircuit began to warm up. There is nothing to do without a heat sink. I found some kind of plate, not for long, but enough. Let's try again. There is a drawdown. But the block works. Voltage adjustment is in progress. We can insert a test into this scheme.

Radioblogful video. Soldering video blog.

Adjustable voltage source from 5 to 12 volts

Continuing with our guide to converting an ATX power supply into a desktop power supply, one very nice addition to this is the LM317T positive voltage regulator.

The LM317T is an adjustable 3-pin positive voltage regulator capable of supplying a variety of DC outputs other than a +5 or +12V DC source, or as an AC output voltage from a few volts to some maximum value, all with currents around 1. 5 amp.

With a small amount of additional circuitry added to the output of the power supply, we can achieve a benchtop power supply capable of operating over a range of fixed or variable voltages, both positive and negative in nature. This is actually a lot easier than you think, since the transformer, rectification and smoothing have already been done by the PSU in advance, and all we need to do is connect our additional circuit to the output of the yellow +12 Volt wire. But first, let's look at fixed output voltage.

Fixed 9V power supply

A wide variety of three-pole voltage regulators are available in the standard TO-220 package, with the most popular fixed voltage regulator being the 78xx series positive regulators, which range from the very common 7805 +5V fixed voltage regulator to the 7824, +24V fixed voltage regulator. There is also a series of 79xx series fixed negative voltage regulators that create an additional negative voltage of -5 to -24 volts, but in this tutorial we will only use the positive types 78xx .

The fixed 3-pin regulator is useful in applications where a regulated output is not required, making the output power supply simple but very flexible since the output voltage depends only on the selected regulator. They are called 3-pin voltage regulators because they only have three terminals to connect to and that accordingly Entrance , General And Exit .

The input voltage for the regulator will be the yellow + 12 V wire from the power supply (or a separate transformer power supply), which is connected between the input and common terminals. Stabilized +9 volts are taken through the output and common, as shown.

Voltage regulator circuit

So, let's say we want to get +9V output voltage from our desktop power supply, then all we need to do is connect the +9V voltage regulator to the yellow +12V wire. Since the power supply has already done the rectification and smoothing to +12V output, the only additional components required are a capacitor at the input and another at the output.

These additional capacitors contribute to the stability of the regulator and can range from 100 to 330 nF. An additional 100uF output capacitor helps smooth out the characteristic ripple for good transient response. This large capacitor placed at the output of the power supply circuit is usually called a "smoothing capacitor".

These series regulators 78xx produce a maximum output current of about 1.5 A at fixed stabilized voltages of 5, 6, 8, 9, 12, 15, 18 and 24 V, respectively. But what if we want the output voltage to be +9V, but only have a 7805, +5V regulator? The +5V output of the 7805 refers to the ground, Gnd or 0V terminal.

If we were to increase this voltage at pin 2 from 4V to 4V, the output would also increase by another 4V, provided the input voltage is sufficient. Then, by placing a small 4V (closest preferred value is 4.3V) Zener diode between pin 2 of the regulator and ground, we can force the 7805 5V regulator to generate a +9V output voltage, as shown in the figure.

Increasing output voltage

So how does it work. A 4.3V zener diode requires a reverse bias current of about 5mA to maintain output with the regulator drawing about 0.5mA. This full 5.5mA current is supplied through resistor "R1" from output pin 3.

So the resistor value required for the 7805 regulator would be R = 5V/5.5mA = 910 ohms. The feedback diode D1 connected across the input and output terminals is for protection and prevents the regulator from reverse biasing when the input supply voltage is turned off and the output supply voltage remains on or active for a short period of time due to large inductance. load such as a solenoid or motor.

We can then use 3-pin voltage regulators and a suitable zener diode to obtain different fixed output voltages from our previous power supply ranging from +5V to +12V. But we can improve this design by replacing the DC voltage regulator with an AC voltage regulator such as LM317T .

AC voltage source

The LM317T is a fully adjustable 3-pin positive voltage regulator capable of delivering 1.5A output voltages ranging from 1.25V to just over 30V. By using the ratio of two resistances, one fixed and the other variable (or both fixed), we can set the output voltage at the desired level with a corresponding input voltage ranging from 3 to 40 volts.

The LM317T AC Voltage Regulator also has built-in current limiting and thermal shutdown features, making it short circuit tolerant and ideal for any low voltage or home benchtop power supply.

The output voltage of LM317T is determined by the ratio of two feedback resistors R1 and R2, which form a potential divider network at the output terminal as shown below.

LM317T AC Voltage Regulator

The voltage across feedback resistor R1 is a constant reference voltage of 1.25 V, V ref, created between the output and adjustment terminals. The adjustment terminal current is 100 μA constant current. Since the reference voltage through resistor R1 is constant, constant current will flow through the other resistor R2, resulting in an output voltage of:

Then, any current flowing through R1 also flows through R2 (ignoring the very small current at the regulation terminal), with the sum of the voltage drops across R1 and R2 equaling the output voltage Vout. Obviously, the input voltage Vin must be at least 2.5 V greater than the required output voltage to power the regulator.

In addition, the LM317T has very good load regulation, provided the minimum load current is greater than 10mA. So, to maintain a constant reference voltage of 1.25V, the minimum value of feedback resistor R1 should be 1.25V/10mA = 120 ohms, and this value can vary from 120 ohms to 1000 ohms with typical values ​​of R1 being approximately 220 ohms to 240 ohms. for good stability.

If we know the value of the required output voltage, Vout, and the feedback resistor R1 is, say, 240 ohms, then we can calculate the value of resistor R2 from the above equation. For example, our original output voltage of 9V will give a resistive value for R2:

R1. ((Vout / 1.25) -1) = 240. ((9 / 1.25) -1) = 1,488 Ohms

or 1500 ohms (1 kohms) to the nearest preferred value.

Of course, in practice, resistors R1 and R2 are usually replaced by a potentiometer to generate an alternating voltage source, or by several switched preset resistors if multiple fixed output voltages are required.

But in order to reduce the math required to calculate the value of resistor R2, each time we need a specific voltage, we can use standard resistance tables as shown below, which give us the output voltage of the regulators for different ratios of resistors R1 and R2 with using E24 resistance values,

Ratio of resistance R1 to R2

R2 value	Resistor R1 value
	150	180	220	240	270	330	370	390	470
100	2,08	1,94	1,82	1,77	1,71	1,63	1,59	1,57	1,52
120	2,25	2,08	1,93	1,88	1,81	1,70	1,66	1,63	1,57
150	2,50	2,29	2,10	2,03	1,94	1,82	1,76	1,73	1,65
180	2,75	2,50	2,27	2,19	2,08	1,93	1,86	1,83	1,73
220	3,08	2,78	2,50	2,40	2,27	2,08	1,99	1,96	1,84
240	3,25	2,92	2,61	2,50	2,36	2,16	2,06	2,02	1,89
270	3,50	3,13	2,78	2,66	2,50	2,27	2,16	2,12	1,97
330	4,00	3,54	3,13	2,97	2,78	2,50	2,36	2,31	2,13
370	4,33	3,82	3,35	3,18	2,96	2,65	2,50	2,44	2,23
390	4,50	3,96	3,47	3,28	3,06	2,73	2,57	2,50	2,29
470	5,17	4,51	3,92	3,70	3,43	3,03	2,84	2,76	2,50
560	5,92	5,14	4,43	4,17	3,84	3,37	3,14	3,04	2,74
680	6,92	5,97	5,11	4,79	4,40	3,83	3,55	3,43	3,06
820	8,08	6,94	5,91	5,52	5,05	4,36	4,02	3,88	3,43
1000	9,58	8,19	6,93	6,46	5,88	5,04	4,63	4,46	3,91
1200	11,25	9,58	8,07	7,50	6,81	5,80	5,30	5,10	4,44
1500	13,75	11,67	9,77	9,06	8,19	6,93	6,32	6,06	5,24

By changing resistor R2 for the 2k ohm potentiometer, we can control the output voltage range of our benchtop power supply from approximately 1.25 volts to a maximum output voltage of 10.75 (12-1.25) volts. Then our final modified AC power supply circuit is shown below.

AC power supply circuit

We can improve our basic voltage regulator circuit a little by connecting an ammeter and a voltmeter to the output terminals. These instruments will visually display the current and voltage output of the AC voltage regulator. If desired, a fast-blow fuse can also be included in the design to provide additional short circuit protection, as shown in the illustration.

Disadvantages of LM317T

One of the major disadvantages of using the LM317T as part of an AC power circuit to regulate voltage is that up to 2.5 volts is dropped or lost as heat through the regulator. So, for example, if the required output voltage must be +9 volts, then the input voltage must be as much as 12 volts or more if the output voltage is to remain stable under maximum load conditions. This voltage drop across the regulator is called "dropout". Also because of this voltage drop some form of heat sink is required to keep the regulator cool.

Fortunately, low-dropout AC voltage regulators are available, such as the National Semiconductor "LM2941T" low-dropout AC voltage regulator, which has a low cut-off voltage of only 0.9V at maximum load. This low voltage drop comes at a cost, as this device is only capable of delivering 1.0 amps with an AC output of 5 to 20 volts. However, we can use this device to produce an output voltage of about 11.1 V, just below the input voltage.

So to summarize, our desktop power supply that we made from an old PC power supply in the previous tutorial can be converted to provide a variable voltage source using an LM317T to regulate the voltage. By connecting the input of this device through the yellow +12V output wire of the power supply, we can have a fixed voltage of +5V, +12V and a variable output voltage ranging from 2 to 10 volts with a maximum output current of 1.5A.