DIY laboratory power supply. Homemade power supply: diagrams, instructions

Hello dear friends. Now I will tell you about a good and cheap power source (also a charger for a car), which you can assemble with your own hands. To assemble this circuit, you will need a list of parts, now I will list them for you: a power step-down transformer, a diode bridge, a high-capacity electrolyte capacitor and a smaller capacitor, two resistors (one variable and the other constant), microcircuit bank and three powerful transistors. The most important thing is that all these parts can be found in an old tube TV; in general, you don’t need to spend money on buying scarce radio components - this is a big plus of this scheme. The second significant advantage is that such a simple circuit is capable of delivering current up to 22 Amperes at 13 volts. You can see for yourself what great advantages it is: it’s lightweight, and without spending a lot of money, you can turn such a mono circuit into a laboratory power supply, a power supply for experiments (regulated), for powering powerful devices, and so on. See the power supply-charger diagram below.

Now I’ll tell you about each detail in more detail. Let's start with the power transformer. The power transformer is designed to convert voltage of one frequency. They can be up or down. A step-up transformer increases the voltage, and a step-down transformer lowers it, which means that since our transformer lowers the voltage according to the circuit, it is a step-down transformer. The transformer consists of a primary, secondary winding and magnetic circuit. The magnetic core consists of individual pressed sheets of electrical steel. The primary winding consists of many turns with a smaller cross-section of wire and is characterized by high resistance in relation to the secondary winding (when you are looking for a 220 volt winding, measure the resistance; where there is more, that is the mains winding).

The secondary consists of the least number of turns and the cross-section of the wire is larger - this is necessary in order to remove more current. Beginners may ask why pins 15, 13 and 10,11 are connected to secondary ones. This must be done for a higher output voltage of the transformer. You can simply wind more wire on the point - the voltage will rise. And if your transformer does not have enough voltage, then you can connect two transformers to the network and connect the secondary ones in series, but then it is better to take transformers of the same power, since a transformer of lower power will heat up more. You can independently rewind the transformer to the voltage and current you need - but more on that in another article. In general, this is what the transformer looks like, as described above. You can get it from a tube TV, it will be 150 watts. 150/10=15 A, at 10 volts such a transformer will give you 15 amperes, and at 150 volts - 150./150=1 only one ampere. Calculate for yourself what current you need.

The diode bridge is assembled using a bridge circuit. A diode bridge using a bridge circuit is twice as good at removing network ripples as a single half-wave rectifier, so diode bridges are installed in power supplies using a bridge circuit so that the equipment that powers the network through the diode bridge does not fail, even if the ULF produces a characteristic sound. Any capacitors, but with a current of at least 15-20 Amps, or buy a diode bridge on the market and a current of at least 20 Amps. A 47,000 microfarad capacitor electrolyte removes ripples just like a diode bridge, only the capacitor removes these ripples better and, accordingly, the larger the capacitance of the capacitor, the more ripples it can remove. You can make electrolytic capacitors yourself: take a half-liter jar and pour electrolyte, lower 2 plates (one copper and the other iron), you get an anode and a cathode and can be connected to the network. The capacitance of the capacitor will directly depend on the amount of electrolyte (or rather, charged electrolyte) and the size of the plates (or rather, how quickly we can charge the electrolyte and discharge it, because with a larger area of ​​the plates we will charge the liquid faster). By the way, with a very large capacitance, you can dispense with the stabilizer, since the capacitor will actually act as a voltage stabilizer and filter.

Chip KREN8b will stabilize the current to 1 Ampere. This microcircuit in this power supply can be compared to a pre-amplifier in the ULF, since the main amplification occurs in transistors T1, T2, T3. We must place all transistors on radiators. With resistor R1 we regulate the current (up to 1 Ampere), which is stabilized by the microcircuit and supplied to the base of the transistor. Accordingly, we regulate the gain of all three transistors at once (the maximum current to the base of one transistor is 0.33 A, since 1/3 = 0.333333 A). The positive charge is amplified both through the microcircuit (to control the gain of the transistors) and through the transistors (we supply the transistors with a positive charge, and from the microcircuit we control the gain).

If we connect three more transistors in parallel with these three and connect another one in parallel with the KRNE microcircuit, then we can get a current twice as high as with this working standard circuit. I recommend it if you need high currents, but the transformer must be powerful enough. With my method, the output current should be 40 A at 13 volts, which means 40 * 13 = 520 watts. The transformer should have a capacity of half a kilowatt. Resistor R2 is needed to limit the current to prevent a short circuit. Then we next install an electrolyte capacitor to smooth out pulsations at the final stage, and it would not hurt to also install a capacitor of a smaller capacity in order to smooth out pulsations of higher frequencies. Also, if you have a lot of interference in your network, I recommend installing a throttle, which will remove all high-frequency RF interference. Install the throttle in series, in the open circuit in front of the microcircuit, to the plus, of course.

A universal power supply, with which you can get all the voltages that may be needed in amateur radio and just everyday activities, should be in every home. And of course, the power supply must have good power - provide an output current not of 0.5 A, like cheap Chinese adapters, but several amperes to connect even lead batteries from a car for charging, or electric motors. Of course, I want the voltage range to also matter. Most circuits are limited to 12 volts, at best 20. But sometimes you need both 24 and 36 V. Is it difficult to create such a power supply yourself? No, because the circuit will only need a dozen parts. Here is a very simple, universal power supply with adjustable supply voltage. The maximum output voltage is 36 V - it is adjustable in the range from 1.2 to (vcc - 3) volts.

Regulated power supply circuit

Transistor Q1 is a high-power PNP Darlington, used to increase the current of the LM317 IC. The LM317L itself, without a heatsink, can supply 100 mA, which is enough to drive a transistor. Elements D1 and D2 are protective diodes, because when the circuit is turned on, the charge of the capacitors can damage the transistor or stabilizer.

To eliminate high-frequency noise, we install 100 nF capacitors in parallel with electrolytic capacitors, because electrolytic ones have large ESR and ESL values ​​and cannot clearly eliminate high-frequency noise. Here is a sample PCB design for this circuit.

Notes

• Transistor Q1 needs a heatsink and preferably a small fan.
• The maximum output power of the circuit is 125 watts.
• R1 - 2 W, other resistors - 0.25 watt.
• All capacitors are 50V.
• RV1 - 5 kOhm regulator.
• A transformer is required for 36 V 5 A. With a power of 150 watts and above.
• The terminals for connecting the output wires are the same as for speakers in amplifiers, screw type.

Making a laboratory power supply with your own hands is not difficult if you have the skills to use a soldering iron and you understand electrical circuits. Depending on the parameters of the source, you can use it to charge batteries, connect almost any household equipment, and use it for experiments and experiments in the design of electronic devices. The main thing during installation is the use of proven circuits and build quality. The more reliable the case and connections, the more convenient it is to work with the power source. It is desirable to have adjustments and devices for monitoring output current and voltage.

The simplest homemade power supply

If you do not have skills in making electrical appliances, then it is better to start with the simplest ones, gradually moving to complex designs. Composition of the simplest constant voltage source:

1. Transformer with two windings (primary - for connecting to the network, secondary - for connecting consumers).
2. One or four diodes for AC rectification.
3. Electrolytic capacitor for cutting off the variable component of the output signal.
4. Connecting wires.

If you use one semiconductor diode in the circuit, you will get a half-wave rectifier. If you use a diode assembly or a bridge circuit, then the power supply is called full-wave. The difference is in the output signal - in the second case there is less ripple.

Such a homemade power supply is good only in cases where it is necessary to connect devices with the same operating voltage. So, if you are designing automotive electronics or repairing them, it is better to choose a transformer with an output voltage of 12-14 volts. The output voltage depends on the number of turns of the secondary winding, and the current strength depends on the cross-section of the wire used (the greater the thickness, the greater the current).

How to make bipolar power supply?

Such a source is necessary to ensure the operation of some microcircuits (for example, power amplifiers and low frequencies). A bipolar power supply has the following feature: its output has a negative pole, a positive pole and a common pole. To implement such a circuit, it is necessary to use a transformer, the secondary winding of which has a middle terminal (and the value of the alternating voltage between the middle and extreme ones must be the same). If there is no transformer that satisfies this condition, you can upgrade any one whose network winding is designed for 220 volts.

Remove the secondary winding, but first measure the voltage on it. Count the number of turns and divide by the voltage. The resulting number is the number of turns required to produce 1 volt. If you need to get a bipolar power supply with a voltage of 12 volts, you will need to wind two identical windings. Connect the beginning of one to the end of the second and connect this middle point to the common wire. The two terminals of the transformer must be connected to the diode assembly. The difference from a unipolar source is that you need to use 2 electrolytic capacitors connected in series, the middle point is connected to the device body.

Voltage regulation in a unipolar power supply

The task may not seem very simple, but you can make a regulated power supply by assembling a circuit from one or two semiconductor transistors. But you will need to install at least a voltmeter at the output to monitor the voltage. For this purpose, a dial indicator with an acceptable measurement range can be used. You can purchase a cheap digital multimeter and customize it to suit your needs. To do this, you will need to disassemble it, set the desired switch position using soldering (with a voltage range of 1-15 volts, it is required that the device can measure voltages up to 20 volts).

The regulated power supply can be connected to any electrical device. First, you only need to set the required voltage value so as not to damage the devices. The voltage is changed using a variable resistor. You have the right to choose its design yourself. It could even be a slide-type device, the main thing is to comply with the nominal resistance. To make the power supply convenient to use, you can install a variable resistor paired with a switch. This will get rid of the extra toggle switch and make it easier to turn off the equipment.

Voltage regulation in a bipolar source

This design will be more complicated, but it can be implemented quite quickly if all the necessary elements are available. Not everyone can make a simple laboratory power supply, and even a bipolar one with voltage regulation. The circuit is complicated by the fact that it requires the installation of not only a semiconductor transistor operating in switch mode, but also an operational amplifier and zener diodes. When soldering semiconductors, be careful: try not to heat them too much, because their permissible temperature range is extremely small. When overheated, the germanium and silicon crystals are destroyed, causing the device to stop functioning.

When making a laboratory power supply with your own hands, remember one important detail: the transistors must be mounted on an aluminum radiator. The more powerful the power source, the larger the radiator area should be. Pay special attention to the quality of soldering and wires. For low-power devices, thin wires can be used. But if the output current is large, then it is necessary to use wires with thick insulation and a large cross-sectional area. Your safety and ease of use of the device depend on the reliability of switching. Even a short circuit in the secondary circuit can cause a fire, so when manufacturing the power supply, care should be taken to protect it.

Retro style voltage regulation

Yes, this is exactly what you can call making adjustments in this way. To implement it, you need to rewind the secondary winding of the transformer and make several conclusions depending on what voltage step and range you need. For example, a 30V 10A lab power supply in 1 volt increments would have 30 pins. A switch must be installed between the rectifier and the transformer. It is unlikely that you will be able to find one with 30 positions, and if you do find it, its dimensions will be very large. It is clearly not suitable for installation in a small case, so it is better to use standard voltages for manufacturing - 5, 9, 12, 18, 24, 30 volts. This is quite enough for convenient use of the device in the home workshop.

To manufacture and calculate the secondary winding of the transformer you need to do the following:

1. Determine what voltage is collected by one turn of the winding. For convenience, wind 10 turns, connect the transformer to the network and measure the voltage. Divide the resulting value by 10.
2. Wind the secondary winding, having first disconnected the transformer from the network. If it turns out that one turn collects 0.5 V, then to get 5 V you need to tap from the 10th turn. And using a similar scheme, you make taps for the remaining standard voltage values.

Anyone can make such a laboratory power supply with their own hands, and most importantly, there is no need to solder a circuit with transistors. Connect the secondary winding leads to a switch so that the voltage values ​​change from lower to higher. The central terminal of the switch is connected to the rectifier, the lower terminal of the transformer according to the diagram is supplied to the device body.

Features of switching power supplies

Such circuits are used in almost all modern devices - in phone chargers, in power supplies for computers and televisions, etc. Making a laboratory power supply, especially a switching one, turns out to be problematic: too many nuances need to be taken into account. Firstly, the circuit is relatively complex and the principle of operation is not simple. Secondly, most of the device operates under high voltage, which is equal to that flowing in the network. Look at the main components of such a power supply (using the example of a computer):

1. A network rectification unit designed to convert 220 volt alternating current into direct current.
2. An inverter that converts DC voltage into high frequency square wave signals. This also includes a special pulse-type transformer, which reduces the voltage to power the PC components.
3. Control responsible for the correct operation of all elements of the power supply.
4. An amplification stage designed to amplify PWM controller signals.
5. Block for stabilization and rectification of output pulse voltage.

Similar components and elements are present in all switching power supplies.

Computer power supply

The cost of even a new power supply that is installed in computers is quite low. But you get a ready-made design; you don’t even have to make a chassis. One drawback is that the output has only standard voltage values ​​(12 and 5 volts). But for a home laboratory this is quite enough. A laboratory power supply made from ATX is popular because it does not require major modifications. And the simpler the design, the better. But there are also “diseases” with such devices, but they can be cured quite simply.

Electrolytic capacitors often fail. Electrolyte leaks out of them, this can be seen even with the naked eye: a layer of this solution appears on the printed circuit board. It is gel-like or liquid, and over time it hardens and becomes hard. To repair a laboratory power supply from a computer power supply, you need to install new electrolytic capacitors. The second failure, which is much less common, is the breakdown of one or more semiconductor diodes. The symptom is a failure of the fuse mounted on the printed circuit board. To repair, you need to ring all the diodes installed in the bridge circuit.

Methods for protecting power supplies

The easiest way to protect yourself is to install fuses. You can use such a laboratory power supply with protection without fear that a fire will occur due to a short circuit. To implement this solution, you will need to install two fuses in the power supply circuit of the mains winding. They need to be taken at a voltage of 220 volts and a current of about 5 amperes for low-power devices. Suitable fuses must be installed at the output of the power supply. For example, when protecting a 12-volt output circuit, you can use fuses used in cars. The current value is selected based on the maximum power of the consumer.

But this is the age of high technology, and making protection using fuses is not very profitable from an economic point of view. It is necessary to replace the elements after each accidental touching of the power wires. As an option, install self-restoring fuses instead of conventional fuse links. But they have a small resource: they can serve faithfully for several years, or they can fail after 30-50 outages. But a 5A laboratory power supply, if assembled correctly, functions correctly and does not require additional protection devices. The elements cannot be called reliable; household appliances often become unusable due to the failure of such fuses. It is much more effective to use a relay circuit or a thyristor circuit. Triacs can also be used as an emergency shutdown device.

How to make a front panel?

Most of the work is designing the enclosure rather than assembling the electrical circuit. You will have to arm yourself with a drill, files, and if painting is necessary, you will also have to master painting. You can make a homemade power supply based on the case from some device. But if you can purchase sheet aluminum, you can create a beautiful chassis that will serve you for many years. To begin, draw a sketch in which you arrange all the structural elements. Pay special attention to the design of the front panel. It can be made of thin aluminum, only reinforced from the inside - screwed to aluminum corners, which are used to give greater rigidity to the structure.

The front panel must have holes for installing measuring instruments, LEDs (or incandescent lamps), terminals connected to the output of the power supply, and sockets for installing fuses (if this protection option is selected). If the appearance of the front panel is not very attractive, then it needs to be painted. To do this, degrease and clean the entire surface until shiny. Before starting painting, make all the necessary holes. Apply 2-3 layers of primer to the heated surface and let dry. Next, apply the same number of layers of paint. Varnish should be used as a finishing coat. As a result, a powerful laboratory power supply, thanks to the paint and the resulting shine, will look beautiful and attractive and will fit into the interior of any workshop.

How to make a chassis for a power supply?

Only a design that is completely made independently will look beautiful. But you can use anything as a material: from sheet aluminum to personal computer cases. You just need to carefully think through the entire design so that unforeseen situations do not arise. If the output stages require additional cooling, install a cooler for this purpose. It can work both constantly when the device is turned on, and in automatic mode. To implement the latter, it is best to use a simple microcontroller and a temperature sensor. The sensor monitors the temperature of the radiator, and the microcontroller contains the value at which it is necessary to turn on the air blowing. Even a 10A laboratory power supply, whose power is quite large, will work stably with such a cooling system.

Airflow requires air from outside, so you will need to install a cooler and radiator on the rear wall of the power supply. To ensure chassis rigidity, use aluminum corners, from which you first form a “skeleton”, and then install the casing on it - plates made of the same aluminum. If possible, connect the corners by welding, this will increase strength. The lower part of the chassis must be strong, since the power transformer is mounted on it. The higher the power, the larger the dimensions of the transformer, the greater its weight. As an example, we can compare a 30V 5A laboratory power supply and a similar design, but at 5 volts and a current of about 1 A. The latter will have much smaller dimensions and light weight.

There must be a layer of insulation between the electronic components and the housing. You need to do this exclusively for yourself, so that in the event of an accidental break in the wire inside the unit, it does not short out to the housing. Before installing the sheathing on the “skeleton”, insulate it. You can stick thick cardboard or thick adhesive tape. The main thing is that the material does not conduct electricity. With this modification, security is improved. But the transformer can produce an unpleasant hum, which can be eliminated by fixing and gluing the core plates, as well as installing rubber pads between the body and chassis. But you will get the maximum effect only by combining these solutions.

Summing up

In conclusion, it is worth mentioning that all installation and testing work is carried out in the presence of life-threatening voltage. Therefore, you need to think about yourself; be sure to install automatic switches in the room, paired with protective shutdown devices. Even if you touch the phase, you will not receive an electric shock, since the protection will work.

When working with switching power supplies for computers, follow safety precautions. The electrolytic capacitors in their design remain energized for a long time after switching off. For this reason, before starting repairs, discharge the capacitors by connecting their leads. Just don’t be alarmed by the spark; it will not harm you or the devices.

When making a laboratory power supply with your own hands, pay attention to all the little things. After all, the main thing for you is to ensure stable, safe and convenient operation. And this can only be achieved if all the little details are carefully thought out, not only in the electrical circuit, but also in the device body. Monitoring devices will not be superfluous in the design, so install them to have an idea of, for example, what current the device you assembled in your home laboratory consumes.

How to make a full-fledged power supply yourself with an adjustable voltage range of 2.5-24 volts is very simple; anyone can repeat it without any amateur radio experience.

We will make it from an old computer power supply, TX or ATX, it doesn’t matter, fortunately, over the years of the PC Era, every home has already accumulated a sufficient amount of old computer hardware and a power supply unit is probably also there, so the cost of homemade products will be insignificant, and for some masters it will be zero rubles .

I got this AT block for modification.

Look what is written on the case.

There are many options for modifying a standard computer power supply, but they are all based on a change in the wiring of the IC chip - TL494CN (its analogues DBL494, KA7500, IR3M02, A494, MV3759, M1114EU, MPC494C, etc.).

Let's look at several options execution of computer power supply circuits, perhaps one of them will be yours and dealing with the wiring will become much easier.

Scheme No. 1.

Let's get to work.
First you need to disassemble the power supply housing, unscrew the four bolts, remove the cover and look inside.

In my case, a KA7500 chip was found on the board, which means we can begin to study the wiring and the location of unnecessary parts that need to be removed.

Let's disconnect the power and fan, solder or cut out the output wires so that they don't interfere with our understanding of the circuit, leave only the necessary ones, one yellow (+12v), black (common) and green* (start ON) if there is one.

This is done because our modified unit will produce not +12 volts, but up to +24 volts, and without replacement, the capacitors will simply explode during the first test at 24v, after a few minutes of operation. When selecting a new electrolyte, it is not advisable to reduce the capacity; increasing it is always recommended.

The most important part of the job.
We will remove all unnecessary parts in the IC494 harness and solder other nominal parts so that the result is a harness like this (Fig. No. 1).

We will only need these legs of the microcircuit No. 1, 2, 3, 4, 15 and 16, do not pay attention to the rest.

Explanation of symbols.

Some resistors that are already soldered into the wiring diagram can be suitable without replacing them, for example, we need to put a resistor at R=2.7k connected to the “common”, but there is already R=3k connected to the “common”, this suits us quite well and we leave it there unchanged (example in Fig. No. 2, green resistors do not change).

Thus, we review and redo all the circuits on the six legs of the microcircuit.

This was the most difficult point in the rework.

We make voltage and current regulators.

Voltage and current control.
To control we need a voltmeter (0-30v) and an ammeter (0-6A).

IMPORTANT- inside the device there is a Current resistor (Current sensor), which we need according to the diagram (Fig. No. 1), therefore, if you use an ammeter, then you do not need to install an additional Current resistor; you need to install it without an ammeter. Usually a homemade RC is made, a wire D = 0.5-0.6 mm is wound around a 2-watt MLT resistance, turn to turn for the entire length, solder the ends to the resistance terminals, that's all.

Everyone will make the body of the device for themselves.
You can leave it completely metal by cutting holes for regulators and control devices. I used laminate scraps, they are easier to drill and cut.

A good laboratory power supply is quite expensive and not all radio amateurs can afford it.
Nevertheless, at home you can assemble a power supply with good characteristics, which will cope well with providing power to various amateur radio designs, and can also serve as a charger for various batteries.
Such power supplies are assembled by radio amateurs, usually from , which are available and cheap everywhere.

In this article, little attention is paid to the conversion of the ATX itself, since converting a computer power supply for a radio amateur of average qualification into a laboratory one, or for some other purpose, is usually not difficult, but beginning radio amateurs have many questions about this. Basically, what parts in the power supply need to be removed, what parts should be left, what should be added in order to turn such a power supply into an adjustable one, and so on.

Especially for such radio amateurs, in this article I want to talk in detail about converting ATX computer power supplies into regulated power supplies, which can be used both as a laboratory power supply and as a charger.

For the modification, we will need a working ATX power supply, which is made on a TL494 PWM controller or its analogues.
The power supply circuits on such controllers, in principle, do not differ much from each other and are all basically similar. The power of the power supply should not be less than that which you plan to remove from the converted unit in the future.

Let's look at a typical ATX power supply circuit with a power of 250 W. For Codegen power supplies, the circuit is almost no different from this one.

The circuits of all such power supplies consist of a high-voltage and low-voltage part. In the picture of the power supply printed circuit board (below) from the side of the tracks, the high-voltage part is separated from the low-voltage part by a wide empty strip (without tracks), and is located on the right (it is smaller in size). We will not touch it, but will work only with the low-voltage part.
This is my board and using its example I will show you an option for converting an ATX power supply.

The low-voltage part of the circuit we are considering consists of a TL494 PWM controller, an operational amplifier circuit that controls the output voltages of the power supply, and if they do not match, it gives a signal to the 4th leg of the PWM controller to turn off the power supply.
Instead of an operational amplifier, transistors can be installed on the power supply board, which in principle perform the same function.
Next comes the rectifier part, which consists of various output voltages, 12 volts, +5 volts, -5 volts, +3.3 volts, of which for our purposes only a +12 volt rectifier will be needed (yellow output wires).
The remaining rectifiers and accompanying parts will need to be removed, except for the “duty” rectifier, which we will need to power the PWM controller and cooler.
The duty rectifier provides two voltages. Typically this is 5 volts and the second voltage can be around 10-20 volts (usually around 12).
We will use a second rectifier to power the PWM. A fan (cooler) is also connected to it.
If this output voltage is significantly higher than 12 volts, then the fan will need to be connected to this source through an additional resistor, as will be later in the circuits under consideration.
In the diagram below, I marked the high-voltage part with a green line, the “standby” rectifiers with a blue line, and everything else that needs to be removed with red.

So, we unsolder everything that is marked in red, and in our 12 volt rectifier we change the standard electrolytes (16 volts) to higher voltage ones, which will correspond to the future output voltage of our power supply. It will also be necessary to unsolder the 12th leg of the PWM controller and the middle part of the winding of the matching transformer - resistor R25 and diode D73 (if they are in the circuit) in the circuit, and instead of them, solder a jumper into the board, which is drawn in the diagram with a blue line (you can simply close diode and resistor without soldering them). In some circuits this circuit may not exist.

Next, in the PWM harness on its first leg, we leave only one resistor, which goes to the +12 volt rectifier.
On the second and third legs of the PWM, we leave only the Master RC chain (in the diagram R48 C28).
On the fourth leg of the PWM we leave only one resistor (in the diagram it is designated as R49. Yes, in many other circuits between the 4th leg and the 13-14 legs of the PWM there is usually an electrolytic capacitor, we don’t touch it (if any) either, since it is intended for a soft start of the power supply. My board simply did not have it, so I installed it.
Its capacity in standard circuits is 1-10 μF.
Then we free the 13-14 legs from all connections, except for the connection with the capacitor, and also free the 15th and 16th legs of the PWM.

After all the operations performed, we should get the following.

This is what it looks like on my board (in the picture below).
Here I rewound the group stabilization choke with a 1.3-1.6 mm wire in one layer on the original core. It fit somewhere around 20 turns, but you don’t have to do this and leave the one that was there. Everything works well with him too.
I also installed another load resistor on the board, which consists of two 1.2 kOhm 3W resistors connected in parallel, the total resistance was 560 Ohms.
The native load resistor is designed for 12 volts of output voltage and has a resistance of 270 Ohms. My output voltage will be about 40 volts, so I installed such a resistor.
It must be calculated (at the maximum output voltage of the power supply at idle) for a load current of 50-60 mA. Since operating the power supply completely without load is not desirable, that’s why it is placed in the circuit.

View of the board from the parts side.

Now what will we need to add to the prepared board of our power supply in order to turn it into an regulated power supply;

First of all, in order not to burn the power transistors, we will need to solve the problem of load current stabilization and short circuit protection.
On forums for remaking similar units, I came across such an interesting thing - when experimenting with the current stabilization mode, on the forum pro-radio, forum member DWD I cited the following quote, I will quote it in full:

“I once told you that I could not get the UPS to operate normally in current source mode with a low reference voltage at one of the inputs of the error amplifier of the PWM controller.
More than 50mV is normal, but less is not. In principle, 50mV is a guaranteed result, but in principle, you can get 25mV if you try. Anything less didn’t work. It does not work stably and is excited or confused by interference. This is when the signal voltage from the current sensor is positive.
But in the datasheet on the TL494 there is an option when negative voltage is removed from the current sensor.
I converted the circuit to this option and got an excellent result.
Here is a fragment of the diagram.

Actually, everything is standard, except for two points.
Firstly, is the best stability when stabilizing the load current with a negative signal from the current sensor an accident or a pattern?
The circuit works great with a reference voltage of 5mV!
With a positive signal from the current sensor, stable operation is obtained only at higher reference voltages (at least 25 mV).
With resistor values ​​of 10Ohm and 10KOhm, the current stabilized at 1.5A up to the output short circuit.
I need more current, so I installed a 30 Ohm resistor. Stabilization was achieved at a level of 12...13A at a reference voltage of 15mV.
Secondly (and most interestingly), I don’t have a current sensor as such...
Its role is played by a fragment of a track on the board 3 cm long and 1 cm wide. The track is covered with a thin layer of solder.
If you use this track at a length of 2cm as a sensor, then the current will stabilize at the level of 12-13A, and if at a length of 2.5cm, then at the level of 10A."

Since this result turned out to be better than the standard one, we will go the same way.

First, you will need to unsolder the middle terminal of the secondary winding of the transformer (flexible braid) from the negative wire, or better without soldering it (if the signet allows) - cut the printed track on the board that connects it to the negative wire.
Next, you will need to solder a current sensor (shunt) between the cut of the track, which will connect the middle terminal of the winding to the negative wire.

It is best to take shunts from faulty (if you find them) pointer ampere-voltmeters (tseshek), or from Chinese pointer or digital instruments. They look something like this. A piece 1.5-2.0 cm long will be sufficient.

You can, of course, try to do as I wrote above. DWD, that is, if the path from the braid to the common wire is long enough, then try to use it as a current sensor, but I didn’t do this, I came across a board of a different design, like this one, where the two wire jumpers that connected the output are indicated by a red arrow braids with a common wire, and printed tracks ran between them.

Therefore, after removing unnecessary parts from the board, I removed these jumpers and in their place soldered a current sensor from a faulty Chinese "tseshka".
Then I soldered the rewound inductor in place, installed the electrolyte and load resistor.
This is what a piece of my board looks like, where I marked with a red arrow the installed current sensor (shunt) in place of the jumper wire.

Then you need to connect this shunt to the PWM using a separate wire. From the side of the braid - with the 15th PWM leg through a 10 Ohm resistor, and connect the 16th PWM leg to the common wire.
Using a 10 Ohm resistor, you can select the maximum output current of our power supply. On the diagram DWD The resistor is 30 ohms, but start with 10 ohms for now. Increasing the value of this resistor increases the maximum output current of the power supply.

As I said earlier, the output voltage of my power supply is about 40 volts. To do this, I rewound the transformer, but in principle you can not rewind it, but increase the output voltage in another way, but for me this method turned out to be more convenient.
I’ll tell you about all this a little later, but for now let’s continue and start installing the necessary additional parts on the board so that we have a working power supply or charger.

Let me remind you once again that if you did not have a capacitor on the board between the 4th and 13-14 legs of the PWM (as in my case), then it is advisable to add it to the circuit.
You will also need to install two variable resistors (3.3-47 kOhm) to adjust the output voltage (V) and current (I) and connect them to the circuit below. It is advisable to make the connection wires as short as possible.
Below I have given only part of the diagram that we need - such a diagram will be easier to understand.
In the diagram, newly installed parts are indicated in green.

Diagram of newly installed parts.

Let me give you a little explanation of the diagram;
- The topmost rectifier is the duty room.
- The values ​​of the variable resistors are shown as 3.3 and 10 kOhm - the values ​​are as found.
- The value of resistor R1 is indicated as 270 Ohms - it is selected according to the required current limitation. Start small and you may end up with a completely different value, for example 27 Ohms;
- I did not mark capacitor C3 as newly installed parts in the expectation that it might be present on the board;
- The orange line indicates elements that may have to be selected or added to the circuit during the process of setting up the power supply.

Next we deal with the remaining 12-volt rectifier.
Let's check what maximum voltage our power supply can produce.
To do this, we temporarily unsolder from the first leg of the PWM - a resistor that goes to the output of the rectifier (according to the diagram above at 24 kOhm), then you need to turn on the unit to the network, first connect it to the break of any network wire, and use a regular 75-95 incandescent lamp as a fuse Tue In this case, the power supply will give us the maximum voltage it is capable of.

Before connecting the power supply to the network, make sure that the electrolytic capacitors in the output rectifier are replaced with higher voltage ones!

All further switching on of the power supply should be carried out only with an incandescent lamp; it will protect the power supply from emergency situations in case of any errors. In this case, the lamp will simply light up, and the power transistors will remain intact.

Next we need to fix (limit) the maximum output voltage of our power supply.
To do this, we temporarily change the 24 kOhm resistor (according to the diagram above) from the first leg of the PWM to a tuning resistor, for example 100 kOhm, and set it to the maximum voltage we need. It is advisable to set it so that it is 10-15 percent less than the maximum voltage that our power supply is capable of delivering. Then solder a permanent resistor in place of the tuning resistor.

If you plan to use this power supply as a charger, then the standard diode assembly used in this rectifier can be left, since its reverse voltage is 40 volts and it is quite suitable for a charger.
Then the maximum output voltage of the future charger will need to be limited in the manner described above, around 15-16 volts. For a 12-volt battery charger, this is quite enough and there is no need to increase this threshold.
If you plan to use your converted power supply as an regulated power supply, where the output voltage will be more than 20 volts, then this assembly will no longer be suitable. It will need to be replaced with a higher voltage one with the appropriate load current.
I installed two assemblies on my board in parallel, 16 amperes and 200 volts each.
When designing a rectifier using such assemblies, the maximum output voltage of the future power supply can be from 16 to 30-32 volts. It all depends on the model of the power supply.
If, when checking the power supply for the maximum output voltage, the power supply produces a voltage less than planned, and someone needs more output voltage (40-50 volts for example), then instead of a diode assembly, you will need to assemble a diode bridge, unsolder the braid from its place and leave it hanging in the air, and connect the negative terminal of the diode bridge in place of the soldered braid.

Rectifier circuit with diode bridge.

With a diode bridge, the output voltage of the power supply will be twice as high.
Diodes KD213 (with any letter) are very suitable for a diode bridge, the output current with which can reach up to 10 amperes, KD2999A,B (up to 20 amperes) and KD2997A,B (up to 30 amperes). The last ones are best, of course.
They all look like this;

In this case, it will be necessary to think about attaching the diodes to the radiator and isolating them from each other.
But I took a different route - I simply rewound the transformer and did it as I said above. two diode assemblies in parallel, since there was space for this on the board. For me this path turned out to be easier.

Rewinding a transformer is not particularly difficult, and we’ll look at how to do it below.

First, we unsolder the transformer from the board and look at the board to see which pins the 12-volt windings are soldered to.

There are mainly two types. Just like in the photo.
Next you will need to disassemble the transformer. Of course, it will be easier to deal with smaller ones, but larger ones can also be dealt with.
To do this, you need to clean the core from visible varnish (glue) residues, take a small container, pour water into it, put the transformer there, put it on the stove, bring to a boil and “cook” our transformer for 20-30 minutes.

For smaller transformers this is quite enough (less is possible) and such a procedure will not harm the core and windings of the transformer at all.
Then, holding the transformer core with tweezers (you can do it right in the container), using a sharp knife we ​​try to disconnect the ferrite jumper from the W-shaped core.

This is done quite easily, since the varnish softens from this procedure.
Then, just as carefully, we try to free the frame from the W-shaped core. This is also quite easy to do.

Then we wind up the windings. First comes half of the primary winding, mostly about 20 turns. We wind it up and remember the direction of winding. The second end of this winding does not need to be unsoldered from the point of its connection with the other half of the primary, if this does not interfere with further work with the transformer.

Then we wind up all the secondary ones. Usually there are 4 turns of both halves of 12-volt windings at once, then 3+3 turns of 5-volt windings. We wind everything up, unsolder it from the terminals and wind a new winding.
The new winding will contain 10+10 turns. We wind it with a wire with a diameter of 1.2 - 1.5 mm, or a set of thinner wires (easier to wind) of the appropriate cross-section.
We solder the beginning of the winding to one of the terminals to which the 12-volt winding was soldered, we wind 10 turns, the direction of winding does not matter, we bring the tap to the “braid” and in the same direction as we started - we wind another 10 turns and the end solder to the remaining pin.
Next, we isolate the secondary and wind the second half of the primary onto it, which we wound earlier, in the same direction as it was wound earlier.
We assemble the transformer, solder it into the board and check the operation of the power supply.

If during the process of adjusting the voltage any extraneous noises, squeaks, or crackles occur, then to get rid of them, you will need to select the RC chain circled in the orange ellipse below in the figure.

In some cases, you can completely remove the resistor and select a capacitor, but in others you can’t do it without a resistor. You can try adding a capacitor, or the same RC circuit, between 3 and 15 PWM legs.
If this does not help, then you need to install additional capacitors (circled in orange), their ratings are approximately 0.01 uF. If this doesn’t help much, then install an additional 4.7 kOhm resistor from the second leg of the PWM to the middle terminal of the voltage regulator (not shown in the diagram).

Then you will need to load the power supply output, for example, with a 60-watt car lamp, and try to regulate the current with resistor “I”.
If the current adjustment limit is small, then you need to increase the value of the resistor that comes from the shunt (10 Ohms) and try to regulate the current again.
You should not install a tuning resistor instead of this one; change its value only by installing another resistor with a higher or lower value.

It may happen that when the current increases, the incandescent lamp in the network wire circuit will light up. Then you need to reduce the current, turn off the power supply and return the resistor value to the previous value.

Also, for voltage and current regulators, it is best to try to purchase SP5-35 regulators, which come with wire and rigid leads.

This is an analogue of multi-turn resistors (only one and a half turns), the axis of which is combined with a smooth and coarse regulator. At first it is regulated “Smoothly”, then when it reaches the limit, it begins to be regulated “Roughly”.
Adjustment with such resistors is very convenient, fast and accurate, much better than with a multi-turn. But if you can’t get them, then buy ordinary multi-turn ones, such as;

Well, it seems like I told you everything that I planned to complete on remaking the computer power supply, and I hope that everything is clear and intelligible.

If anyone has any questions about the design of the power supply, ask them on the forum.

Good luck with your design!