Those beginners who are just starting to study electronics are in a hurry to build something supernatural, like microbugs for wiretapping, a laser cutter from a DVD drive, and so on... and so on... What about assembling a power supply with an adjustable output voltage? This power supply is an essential item in every electronics enthusiast's workshop.
Where to start assembling the power supply?
First, you need to decide on the required characteristics that the future power supply will satisfy. The main parameters of the power supply are the maximum current ( Imax), which it can supply to the load (powered device) and the output voltage ( U out), which will be at the output of the power supply. It’s also worth deciding what kind of power supply we need: adjustable or unregulated.
Adjustable power supply is a power supply whose output voltage can be changed, for example, from 3 to 12 volts. If we need 5 volts - we turned the regulator knob - we got 5 volts at the output, we need 3 volts - we turned it again - we got 3 volts at the output.
An unregulated power supply is a power supply with a fixed output voltage - it cannot be changed. For example, the well-known and widely used “Electronics” power supply D2-27 is unregulated and has an output voltage of 12 volts. Also unregulated power supplies are all kinds of chargers for cell phones, adapters for modems and routers. All of them, as a rule, are designed for one output voltage: 5, 9, 10 or 12 volts.
It is clear that for a novice radio amateur it is the regulated power supply that is of greatest interest. It can power a huge number of both homemade and industrial devices designed for different supply voltages.
Next you need to decide on the power supply circuit. The circuit should be simple, easy to repeat by beginning radio amateurs. Here it is better to stick to a circuit with a conventional power transformer. Why? Because finding a suitable transformer is quite easy both in radio markets and in old consumer electronics. Making a switching power supply is more difficult. For a switching power supply, it is necessary to produce quite a lot of winding parts, such as a high-frequency transformer, filter chokes, etc. Also, switching power supplies contain more electronic components than conventional power supplies with a power transformer.
So, the circuit of the regulated power supply proposed for repetition is shown in the picture (click to enlarge).
Power supply parameters:
Output voltage ( U out) – from 3.3...9 V;
Maximum load current ( Imax) – 0.5 A;
The maximum amplitude of output voltage ripple is 30 mV;
Overcurrent protection;
Protection against overvoltage at the output;
High efficiency.
It is possible to modify the power supply to increase the output voltage.
The circuit diagram of the power supply consists of three parts: a transformer, a rectifier and a stabilizer.
Transformer. Transformer T1 reduces the alternating mains voltage (220-250 volts), which is supplied to the primary winding of the transformer (I), to a voltage of 12-20 volts, which is removed from the secondary winding of the transformer (II). Also, “part-time”, the transformer serves as a galvanic isolation between the electrical network and the powered device. This is a very important function. If the transformer suddenly fails for any reason (voltage surge, etc.), then the mains voltage will not be able to reach the secondary winding and, therefore, the powered device. As you know, the primary and secondary windings of a transformer are reliably isolated from each other. This circumstance reduces the risk of electric shock.
Rectifier. From the secondary winding of power transformer T1, a reduced alternating voltage of 12-20 volts is supplied to the rectifier. This is already a classic. The rectifier consists of a diode bridge VD1, which rectifies alternating voltage from the secondary winding of the transformer (II). To smooth out voltage ripples, after the rectifier bridge there is an electrolytic capacitor C3 with a capacity of 2200 microfarads.
Adjustable pulse stabilizer.
The pulse stabilizer circuit is assembled on a fairly well-known and affordable DC/DC converter microcircuit - MC34063.
To make it clear. The MC34063 chip is a specialized PWM controller designed for pulsed DC/DC converters. This chip is the core of the adjustable switching regulator used in this power supply.
The MC34063 chip is equipped with a protection unit against overload and short circuit in the load circuit. The output transistor built into the microcircuit is capable of delivering up to 1.5 amperes of current to the load. Based on a specialized microcircuit, the MC34063 can be assembled as step-up ( Step-Up), and downward ( Step-Down) DC/DC converters. It is also possible to build adjustable pulse stabilizers.
Features of pulse stabilizers.
By the way, switching stabilizers have a higher efficiency compared to stabilizers based on KR142EN series microcircuits ( CRANKS), LM78xx, LM317, etc. And although power supplies based on these microcircuits are very simple to assemble, they are less economical and require the installation of a cooling radiator.
The MC34063 chip does not require a cooling radiator. It is worth noting that this chip can often be found in devices that operate autonomously or use backup power. The use of a switching stabilizer increases the efficiency of the device, and, consequently, reduces power consumption from the battery or battery. Due to this, the autonomous operating time of the device from a backup power source increases.
I think it’s now clear why a pulse stabilizer is good.
Parts and electronic components.
Now a little about the parts that will be required to assemble the power supply.
Power transformers TS-10-3M1 and TP114-163M
A TS-10-3M1 transformer with an output voltage of about 15 volts is also suitable. You can find a suitable transformer in radio parts stores and radio markets, the main thing is that it meets the specified parameters.
Chip MC34063 . The MC34063 is available in DIP-8 (PDIP-8) for conventional through-hole mount and SO-8 (SOIC-8) for surface mount. Naturally, in the SOIC-8 package the chip is smaller in size, and the distance between the pins is about 1.27 mm. Therefore, it is more difficult to make a printed circuit board for a microcircuit in the SOIC-8 package, especially for those who have only recently begun to master printed circuit board manufacturing technology. Therefore, it is better to take the MC34063 chip in a DIP package, which is larger in size, and the distance between the pins in such a package is 2.5 mm. It will be easier to make a printed circuit board for a DIP-8 package.
Chokes. Chokes L1 and L2 can be made independently. To do this, you will need two ring magnetic cores made of 2000HM ferrite, size K17.5 x 8.2 x 5 mm. The standard size is deciphered as follows: 17.5 mm. – outer diameter of the ring; 8.2 mm. - inner diameter; a 5 mm. – height of the ring magnetic circuit. To wind the choke you will need a PEV-2 wire with a cross section of 0.56 mm. 40 turns of such wire must be wound on each ring. The turns of the wire should be distributed evenly over the ferrite ring. Before winding, the ferrite rings must be wrapped in varnished cloth. If you don’t have varnished fabric at hand, you can wrap the ring with three layers of tape. It is worth remembering that ferrite rings may already be painted - covered with a layer of paint. In this case, there is no need to wrap the rings with varnished cloth.
In addition to homemade chokes, you can also use ready-made ones. In this case, the process of assembling the power supply will speed up. For example, as chokes L1, L2 you can use the following surface-mount inductors (SMD - inductor).
As you can see, on the top of their case the inductance value is indicated - 331, which stands for 330 microhenry (330 μH). Also, ready-made chokes with radial leads for conventional installation in holes are suitable as L1, L2. This is what they look like.
The amount of inductance on them is marked either with a color code or with a number. For the power supply, inductances marked 331 (i.e. 330 μH) are suitable. Taking into account the tolerance of ±20%, which is allowed for elements of household electrical equipment, chokes with an inductance of 264 - 396 μH are also suitable. Any inductor or inductor is designed for a certain direct current. As a rule, its maximum value ( I DC max) is indicated in the datasheet for the throttle itself. But this value is not indicated on the body itself. In this case, you can approximately determine the value of the maximum permissible current through the inductor based on the cross-section of the wire with which it is wound. As already mentioned, to independently manufacture chokes L1, L2, you need a wire with a cross-section of 0.56 mm.
Throttle L3 is homemade. To make it, you need a magnetic core made of ferrite. 400HH or 600HH with a diameter of 10 mm. You can find this in antique radios. There it is used as a magnetic antenna. You need to break off a piece 11 mm long from the magnetic circuit. This is quite easy to do; ferrite breaks easily. You can simply tightly clamp the required section with pliers and break off the excess magnetic circuit. You can also clamp the magnetic core in a vice, and then sharply hit the magnetic core. If you fail to carefully break the magnetic circuit the first time, you can repeat the operation.
Then the resulting piece of magnetic circuit must be wrapped with a layer of paper tape or varnished cloth. Next, we wind 6 turns of PEV-2 wire folded in half with a cross-section of 0.56 mm onto the magnetic circuit. To prevent the wire from unwinding, wrap it with tape on top. Those wire leads from which winding of the inductor began are subsequently soldered into the circuit in the place where the points are shown in image L3. These points indicate the beginning of winding the coils with wire.
Additions.
Depending on your needs, you can make certain changes to the design.
For example, instead of a VD3 zener diode type 1N5348 (stabilization voltage - 11 volts), you can install a protective diode - a suppressor - in the circuit 1.5KE10CA.
A suppressor is a powerful protective diode, its functions are similar to a zener diode, however, its main role in electronic circuits is protective. The purpose of the suppressor is to suppress high-voltage pulse noise. The suppressor has a high speed and is able to extinguish powerful impulses.
Unlike the 1N5348 zener diode, the 1.5KE10CA suppressor has a high response speed, which will undoubtedly affect the performance of the protection.
In technical literature and among radio amateurs, a suppressor can be called differently: protective diode, limiting zener diode, TVS diode, voltage limiter, limiting diode. Suppressors can often be found in switching power supplies - there they serve as protection against overvoltage of the powered circuit in the event of faults in the switching power supply.
You can learn about the purpose and parameters of protective diodes from the article about suppressor.
Suppressor 1.5KE10 C A has a letter WITH in the name and is bidirectional - the polarity of its installation in the circuit does not matter.
If there is a need for a power supply with a fixed output voltage, then the variable resistor R2 is not installed, but replaced with a wire jumper. The required output voltage is selected using a constant resistor R3. Its resistance is calculated using the formula:
Uout = 1.25 * (1+R4/R3)
After the transformations, we obtain a formula that is more convenient for calculations:
R3 = (1.25 * R4)/(U out – 1.25)
If you use this formula, then for U out = 12 volts you will need a resistor R3 with a resistance of about 0.42 kOhm (420 Ohm). When calculating, the value of R4 is taken in kilo-ohms (3.6 kOhm). The result for resistor R3 is also obtained in kilo-ohms.
To more accurately set the output voltage U out, you can install a trimming resistor instead of R2 and set the required voltage using the voltmeter more accurately.
It should be taken into account that a zener diode or suppressor should be installed with a stabilization voltage 1...2 volts higher than the calculated output voltage ( U out) power supply. So, for a power supply with a maximum output voltage equal to, for example, 5 volts, a 1.5KE suppressor should be installed 6V8 CA or similar.
Manufacturing of printed circuit board.
A printed circuit board for a power supply can be made in different ways. Two methods for making printed circuit boards at home have already been discussed on the pages of the site.
The fastest and most comfortable way is to make a printed circuit board using a printed circuit board marker. Marker used Edding 792. He showed himself at his best. By the way, the signet for this power supply was made with just this marker.
The second method is suitable for those who have a lot of patience and a steady hand. This is a technology for making a printed circuit board using a correction pencil. This is a fairly simple and affordable technology that will be useful to those who could not find a marker for printed circuit boards, but do not know how to make boards with LUT or do not have a suitable printer.
The third method is similar to the second, only it uses tsaponlak - How to make a printed circuit board using tsaponlak?
In general, there is plenty to choose from.
Setting up and checking the power supply.
To check the functionality of the power supply, you first need to turn it on, of course. If there are no sparks, smoke or pops (this is quite possible), then the power supply is most likely working. At first, keep some distance from him. If you made a mistake when installing electrolytic capacitors or set them to a lower operating voltage, they can “pop” and explode. This is accompanied by electrolyte splashing in all directions through the protective valve on the body. So take your time. You can read more about electrolytic capacitors. Don’t be lazy to read this – it will come in handy more than once.
Attention! The power transformer is under high voltage during operation! Don't put your fingers near it! Don't forget about safety rules. If you need to change something in the circuit, then first completely disconnect the power supply from the mains, and then do it. There is no other way - be careful!
At the end of this whole story, I want to show you a finished power supply that I made with my own hands.
Yes, it does not yet have a housing, a voltmeter and other “goodies” that make it easier to work with such a device. But, despite this, it works and has already managed to burn out an awesome three-color flashing LED because of its stupid owner, who loves to twist the voltage regulator recklessly. I wish you, novice radio amateurs, to collect something similar!
Details
Diode bridge at the input 1n4007 or a ready-made diode assembly designed for a current of at least 1 A and a reverse voltage of 1000 V.Resistor R1 is at least two watts, or 5 watts 24 kOhm, resistor R2 R3 R4 with a power of 0.25 watts.
Electrolytic capacitor on the high side 400 volts 47 uF.
Output 35 volts 470 – 1000 uF. Film filter capacitors designed for a voltage of at least 250 V 0.1 - 0.33 µF. Capacitor C5 – 1 nF. Ceramic, ceramic capacitor C6 220 nF, film capacitor C7 220 nF 400 V. Transistor VT1 VT2 N IRF840, transformer from an old computer power supply, diode bridge at the output full of four ultra-fast HER308 diodes or other similar ones.
In the archive you can download the circuit and board:
(downloads: 1555)
The printed circuit board is made on a piece of foil-coated single-sided fiberglass laminate using the LUT method. For ease of connecting power and connecting output voltage, the board has screw terminal blocks.
12 V switching power supply circuit
The advantage of this circuit is that this circuit is very popular of its kind and is repeated by many radio amateurs as their first switching power supply and efficiency and times more, not to mention size. The circuit is powered from a mains voltage of 220 volts; at the input there is a filter which consists of a choke and two film capacitors designed for a voltage of at least 250 - 300 volts with a capacity of 0.1 to 0.33 μF; they can be taken from a computer power supply.
In my case there is no filter, but it is advisable to install it. Next, the voltage is supplied to a diode bridge designed for a reverse voltage of at least 400 Volts and a current of at least 1 Ampere. You can also supply a ready-made diode assembly. Next in the diagram there is a smoothing capacitor with an operating voltage of 400 V, since the amplitude value of the mains voltage is around 300 V. The capacitance of this capacitor is selected as follows, 1 μF per 1 Watt of power, since I am not going to pump large currents out of this block, then in my case, the capacitor is 47 uF, although such a circuit can pump out hundreds of watts. The power supply for the microcircuit is taken from the alternating voltage, here a power source is arranged, resistor R1, which provides current damping, it is advisable to set it to a more powerful one of at least two watts since it is heated, then the voltage is rectified by just one diode and goes to a smoothing capacitor and then to the microcircuit. Pin 1 of the microcircuit is plus power and pin 4 is minus power.
You can assemble a separate power source for it and supply it with 15 V according to the polarity. In our case, the microcircuit operates at a frequency of 47 - 48 kHz. For this frequency, an RC circuit is organized consisting of a 15 kohm resistor R2 and a 1 nF film or ceramic capacitor. With this arrangement of parts, the microcircuit will work correctly and produce rectangular pulses at its outputs, which are supplied to the gates of powerful field switches through resistors R3 R4, their ratings can deviate from 10 to 40 Ohms. Transistors must be installed N channel, in my case they are IRF840 with a drain-source operating voltage of 500 V and a maximum drain current at a temperature of 25 degrees of 8 A and a maximum power dissipation of 125 Watts. Next in the circuit there is a pulse transformer, after it there is a full-fledged rectifier made of four diodes of the HER308 brand, ordinary diodes will not work here since they will not be able to operate at high frequencies, so we install ultra-fast diodes and after the bridge the voltage is already supplied to the output capacitor 35 Volt 1000 μF , it is possible and 470 uF, especially large capacitances in switching power supplies are not required.
Let's return to the transformer, it can be found on the boards of computer power supplies, it is not difficult to identify it; in the photo you can see the largest one, and that is what we need. To rewind such a transformer, you need to loosen the glue that glues the halves of the ferrite together; to do this, take a soldering iron or a soldering iron and slowly warm up the transformer, you can put it in boiling water for a few minutes and carefully separate the halves of the core. We wind up all the basic windings, and we will wind our own. Based on the fact that I need to get a voltage of around 12-14 Volts at the output, the primary winding of the transformer contains 47 turns of 0.6 mm wire in two cores, we make insulation between the windings with ordinary tape, the secondary winding contains 4 turns of the same wire in 7 cores . It is IMPORTANT to wind in one direction, insulate each layer with tape, marking the beginning and end of the windings, otherwise nothing will work, and if it does, then the unit will not be able to deliver all the power.
Block check
Well, now let's test our power supply, since my version is completely working, I immediately connect it to the network without a safety lamp.Let's check the output voltage as we see it is around 12 - 13 V and does not fluctuate much due to voltage drops in the network.
As a load, a 12 V car lamp with a power of 50 Watts flows a current of 4 A. If such a unit is supplemented with current and voltage regulation, and an input electrolyte of a larger capacity is supplied, then you can safely assemble a car charger and a laboratory power supply.
Before starting the power supply, you need to check the entire installation and connect it to the network through a 100-watt incandescent safety lamp; if the lamp burns at full intensity, then look for errors when installing the snot; the flux has not been washed off or some component is faulty, etc. When assembled correctly, the lamp should be slightly flash and go out, this tells us that the input capacitor is charged and there are no errors in the installation. Therefore, before installing components on the board, they must be checked, even if they are new. Another important point after startup is that the voltage on the microcircuit between pins 1 and 4 must be at least 15 V. If this is not the case, you need to select the value of resistor R2.
So the next device has been assembled, now the question arises: what to power it from? Batteries? Batteries? No! The power supply is what we will talk about.
Its circuit is very simple and reliable, it has short-circuit protection and smooth adjustment of the output voltage.
A rectifier is assembled on the diode bridge and capacitor C2, circuit C1 VD1 R3 is a reference voltage stabilizer, circuit R4 VT1 VT2 is a current amplifier for power transistor VT3, protection is assembled on transistor VT4 and R2, and resistor R1 is used for adjustment.
I took the transformer from an old charger from a screwdriver, at the output I got 16V 2A
As for the diode bridge (at least 3 amperes), I took it from an old ATX block as well as electrolytes, a zener diode, and resistors.
I used a 13V zener diode, but the Soviet D814D is also suitable.
The transistors were taken from an old Soviet TV; transistors VT2, VT3 can be replaced with one component, for example KT827.
Resistor R2 is a wirewound with a power of 7 Watts and R1 (variable) I took nichrome for adjustment without jumps, but in its absence you can use a regular one.
It consists of two parts: the first one contains the stabilizer and protection, and the second one contains the power part.
All parts are mounted on the main board (except for power transistors), transistors VT2, VT3 are soldered onto the second board, we attach them to the radiator using thermal paste, there is no need to insulate the housing (collectors). The circuit was repeated many times and does not need adjustment. Photos of two blocks are shown below with a large 2A radiator and a small 0.6A.
Indication
Voltmeter: for it we need a 10k resistor and a 4.7k variable resistor and I took an indicator m68501, but you can use another one. From resistors we will assemble a divider, a 10k resistor will prevent the head from burning out, and with a 4.7k resistor we will set the maximum deviation of the needle.
After the divider is assembled and the indication is working, you need to calibrate it; to do this, open the indicator and glue clean paper onto the old scale and cut it along the contour; it is most convenient to cut the paper with a blade.
When everything is glued and dry, we connect the multimeter in parallel to our indicator, and all this to the power supply, mark 0 and increase the voltage to volts, mark, etc.
Ammeter: for it we take a resistor of 0.27 ohm!!! and variable at 50k, The connection diagram is below, using a 50k resistor we will set the maximum deviation of the arrow.
The graduation is the same, only the connection changes, see below; a 12 V halogen light bulb is ideal as a load.
List of radioelements
| Designation | Type | Denomination | Quantity | Note | Shop | My notepad |
|---|---|---|---|---|---|---|
| VT1 | Bipolar transistor | KT315B | 1 | To notepad | ||
| VT2, VT4 | Bipolar transistor | KT815B | 2 | To notepad | ||
| VT3 | Bipolar transistor | KT805BM | 1 | To notepad | ||
| VD1 | Zener diode | D814D | 1 | To notepad | ||
| VDS1 | Diode bridge | 1 | To notepad | |||
| C1 | 100uF 25V | 1 | To notepad | |||
| C2, C4 | Electrolytic capacitor | 2200uF 25V | 2 | To notepad | ||
| R2 | Resistor | 0.45 Ohm | 1 | To notepad | ||
| R3 | Resistor | 1 kOhm | 1 | To notepad | ||
| R4 | Resistor |
The 12 volt DC power supply consists of three main parts:
- A step-down transformer from a conventional input alternating voltage of 220 V. At its output there will be the same sinusoidal voltage, only reduced to approximately 16 volts at idle - without load.
- Rectifier in the form of a diode bridge. It “cuts off” the lower half-sine waves and puts them up, that is, the resulting voltage varies from 0 to the same 16 volts, but in the positive region.
- A high-capacity electrolytic capacitor that smooths out the half-sine voltage, making it approach a straight line at 16 volts. This smoothing is better, the larger the capacitor capacity.
The simplest thing you need to obtain a constant voltage capable of powering devices designed for 12 volts - light bulbs, LED strips and other low-voltage equipment.
A step-down transformer can be taken from an old computer power supply or simply bought in a store so as not to bother with windings and rewinding. However, in order to ultimately reach the desired 12 volts of voltage with a working load, you need to take a transformer that lowers the volts to 16.
For the bridge, you can take four 1N4001 rectifier diodes, designed for the voltage range we need or similar.
The capacitor must have a capacity of at least 480 µF. For good output voltage quality, you can use more, 1,000 µF or higher, but this is not at all necessary to power lighting devices. The operating voltage range of the capacitor is needed, say, up to 25 volts.
Device layout
If we want to make a decent device that we won’t be ashamed to attach later as a permanent power supply, say, for a chain of LEDs, we need to start with a transformer, a board for mounting electronic components and a box where all this will be fixed and connected. When choosing a box, it is important to consider that the electrical circuits heat up during operation. Therefore, it is good to find a box that is suitable in size and with holes for ventilation. You can buy it in a store or take a case from a computer power supply. The latter option may be cumbersome, but as a simplification you can leave the existing transformer in it, even along with the cooling fan.
On the transformer we are interested in the low-voltage winding. If it reduces the voltage from 220 V to 16 V, this is an ideal case. If not, you'll have to rewind it. After rewinding and checking the voltage at the output of the transformer, it can be mounted on the circuit board. And immediately think about how the circuit board will be attached inside the box. It has mounting holes for this.
Further installation steps will take place on this mounting board, which means that it must be sufficient in area, length and allow the possible installation of radiators on diodes, transistors or a microcircuit, which must still fit into the selected box.
We assemble the diode bridge on the circuit board, you should get such a diamond of four diodes. Moreover, the left and right pairs consist equally of diodes connected in series, and both pairs are parallel to each other. One end of each diode is marked with a stripe - this is indicated by a plus. First we solder the diodes in pairs to each other. In series - this means the plus of the first is connected to the minus of the second. The free ends of the pair will also turn out - plus and minus. Connecting pairs in parallel means soldering both pluses of the pairs and both minuses. Now we have the output contacts of the bridge - plus and minus. Or they can be called poles - upper and lower.
The remaining two poles - left and right - are used as input contacts, they are supplied with alternating voltage from the secondary winding of the step-down transformer. And the diodes will supply a pulsating voltage of constant sign to the bridge outputs.
If you now connect a capacitor in parallel with the output of the bridge, observing the polarity - to the plus of the bridge - plus of the capacitor, it will begin to smooth out the voltage, and as well as its capacitance is large. 1,000 uF will be enough, and even 470 uF is used.
Attention! An electrolytic capacitor is an unsafe device. If it is connected incorrectly, if voltage is applied to it outside the operating range, or if it is overheated, it may explode. At the same time, all its internal contents scatter around the area - tatters of the case, metal foil and splashes of electrolyte. Which is very dangerous.
Well, here we have the simplest (if not primitive) power supply for devices with a voltage of 12 V DC, that is, direct current.
Problems with a simple power supply with a load
The resistance drawn on the diagram is the equivalent of the load. The load must be such that the current supplying it, with an applied voltage of 12 V, does not exceed 1 A. You can calculate the load power and resistance using the formulas.
Where does the resistance R = 12 Ohm, and the power P = 12 watts come from? This means that if the power is more than 12 watts and the resistance is less than 12 ohms, then our circuit will begin to work with overload, will get very hot and will quickly burn out. There are several ways to solve the problem:
- Stabilize the output voltage so that when the load resistance changes, the current does not exceed the maximum permissible value or when there are sudden current surges in the load network - for example, when some devices are turned on - the peak current values are cut to the nominal value. Such phenomena occur when the power supply powers radio-electronic devices - radios, etc.
- Use special protection circuits that would turn off the power supply if the load current exceeds.
- Use more powerful power supplies or power supplies with more power reserves.
The figure below shows the development of the previous simple circuit by including a 12-volt stabilizer LM7812 at the output of the microcircuit.
This is already better, but the maximum load current of such a stabilized power supply unit should still not exceed 1 A.
High Power Power Supply
The power supply can be made more powerful by adding several powerful stages using TIP2955 Darlington transistors to the circuit. One stage will provide an increase in load current of 5 A, six composite transistors connected in parallel will provide a load current of 30 A.
A circuit with this kind of power output requires adequate cooling. Transistors must be provided with heat sinks. You may also need an additional cooling fan. In addition, you can protect yourself with fuses (not shown in the diagram).
The figure shows the connection of one composite Darlington transistor, which makes it possible to increase the output current to 5 amperes. You can increase it further by connecting new cascades in parallel with the specified one.
Attention! One of the main disasters in electrical circuits is a sudden short circuit in the load. In this case, as a rule, a current of gigantic power arises, which burns everything in its path. In this case, it is difficult to come up with such a powerful power supply that can withstand this. Then protection circuits are used, ranging from fuses to complex circuits with automatic shutdown on integrated circuits.
I present the simplest miniature switching power supply that can be successfully replicated by a beginning radio amateur. It is reliable, operates in a wide range of supply voltages, and has compact dimensions.
The power supply has a relatively low power, within 2 watts, but it is literally indestructible, not afraid of even long-term short circuits.
The circuit is simpler than even the simplest switching power supplies, which include chargers for mobile phones.
The power supply is a low-power switching power supply of the self-oscillator type, assembled with just one transistor. The autogenerator is powered from the network through a current-limiting resistor R1 and a half-wave rectifier in the form of a diode VD1.
A pulse transformer has three windings, a collector or primary winding, a base winding and a secondary winding.
An important point is the winding of the transformer, and the beginning of the windings are indicated on the printed circuit board and on the diagram, so there should be no problems. I didn’t do any calculations, but the number of turns of the windings was borrowed from a transformer for charging cell phones, since the circuit diagram is almost the same, the number of windings is the same. The primary winding is wound first, which consists of 200 turns, the wire diameter is from 0.08 to 0.1 mm, then insulation is installed and the base winding, which contains from 5 to 10 turns, is wound with the same wire. We wind the output winding on top, the number of turns depends on what voltage you need, according to my conservative calculations it turns out to be about 1 volt per turn.
The core for the transformer can be found in non-working power supplies from mobile phones, LED drivers and other low-power power sources, which are usually built on the basis of single-ended circuits that include the required transformer.
One point - the block is single-cycle and there must be a non-magnetic gap between the halves of the core, such a gap is found in cores from cell phone chargers. The gap is relatively small (half a millimeter is enough). If you do not find transformers with a gap, it can be made artificially by placing one layer of office paper between the halves of the core.
The finished transformer is assembled back, the halves of the core are pulled together with, say, tape or tightly glued together with superglue.
The circuit does not have output voltage stabilization and short circuit protection units, but oddly enough, it is not afraid of any short circuits. During short circuits, the current in the primary circuit naturally increases, but it is limited by the previously mentioned resistor, and all excess is dissipated on the resistor in the form of heat, so that the block can be safely shorted, even for a long time. This solution reduces the efficiency of the power source as a whole, but makes it literally indestructible, unlike the same chargers for mobile phones.
A resistor of the specified value limits the input current to 14.5 mA, according to Ohm's law, knowing the voltage in the network, you can easily calculate the power, which is around 3.3 watts, this is the input power, taking into account the efficiency of the converter, the output power will be 20 percent -30 less than that. You can increase the power; to do this, it is enough to reduce the resistance of the specified resistor.
The power transistor is a low-power, high-voltage reverse conduction bipolar transistor; switches like MJE13001, 13003, 13005 are suitable; there is no point in installing more powerful ones, the first option is quite sufficient.
A rectifier based on a pulse diode is installed at the output of the circuit; to reduce losses, I advise you to use a Schottky diode rated for a current of 1A. Next is a filter capacitor, an LED power indicator and a pair of resistors.
