Scheme of a simple power supply up to 12 W. Powerful DIY laboratory unit

This power supply, based on the LM317 chip, does not require any special knowledge for assembly, and after proper installation from serviceable parts, does not require adjustment. Despite its apparent simplicity, this unit is a reliable power source for digital devices and has built-in protection against overheating and overcurrent. The microcircuit inside itself has over twenty transistors and is a high-tech device, although from the outside it looks like an ordinary transistor.

The circuit's power supply is designed for voltages up to 40 volts alternating current, and at the output you can get from 1.2 to 30 volts of constant, stabilized voltage. Adjustment from minimum to maximum with a potentiometer occurs very smoothly, without jumps or dips. Output current up to 1.5 amperes. If the current consumption is not planned to exceed 250 milliamps, then a radiator is not needed. When consuming a larger load, place the microcircuit on a heat-conducting paste to a radiator with a total dissipation area of ​​350 - 400 or more square millimeters. The selection of a power transformer must be calculated based on the fact that the voltage at the input to the power supply should be 10 - 15% greater than what you plan to receive at the output. It is better to take the power of the supply transformer with a good margin, in order to avoid excessive overheating, and be sure to install a fuse at its input, selected according to the power, to protect against possible troubles.
To us, to make this desired device, you will need the following details:

• Chip LM317 or LM317T.
• Almost any rectifier assembly or four separate diodes with a current of at least 1 ampere each.
• Capacitor C1 from 1000 μF and higher with a voltage of 50 volts, it serves to smooth out voltage surges in the supply network and the larger its capacitance, the more stable the output voltage will be.
• C2 and C4 – 0.047 uF. There is a number 104 on the capacitor cap.
• C3 – 1 µF or more with a voltage of 50 volts. This capacitor can also be used with a larger capacity to increase the stability of the output voltage.
• D5 and D6 - diodes, for example 1N4007, or any others with a current of 1 ampere or more.
• R1 – potentiometer for 10 Kom. Any type, but always good, otherwise output voltage will "jump".
• R2 – 220 Ohm, power 0.25 – 0.5 watts.

Assembling an adjustable stabilized power supply

Checking the power supply

Switching power supplies are often used by radio amateurs in homemade designs. With relatively small dimensions they can provide high output power. Using pulse circuit It became possible to obtain output power from several hundred to several thousand watts. Moreover, the dimensions of the pulse transformer itself are no larger than a matchbox.

Switching power supplies - operating principle and features

The main feature of pulsed power supplies is their increased operating frequency, which is hundreds of times higher than the network frequency of 50 Hz. At high frequencies with a minimum number of turns in the windings, high voltage can be obtained. For example, to obtain 12 Volts of output voltage at a current of 1 Ampere (in the case of a mains transformer), you need to wind 5 turns of wire with a cross-section of approximately 0.6–0.7 mm.

If we talk about a pulse transformer, the master circuit of which operates at a frequency of 65 kHz, then to obtain 12 Volts with a current of 1A, it is enough to wind only 3 turns with a wire of 0.25–0.3 mm. This is why many electronics manufacturers use pulse block nutrition.

However, despite the fact that such blocks are much cheaper, more compact, have high power and light weight, they have electronic filling, and therefore are less reliable when compared with a network transformer. It is very simple to prove their unreliability - take any switching power supply without protection and short-circuit the output terminals. At best, the unit will fail, at worst, it will explode and no fuse will save the unit.

Practice shows that the fuse in a switching power supply burns out last, first of all the power switches and the master oscillator fly out, then all parts of the circuit one by one.

Switching power supplies have a number of protections both at the input and output, but they do not always save. In order to limit the current surge when starting the circuit, almost all SMPS with a power of more than 50 Watts use a thermistor, which is located at the input of the circuits.

Let's now look at the TOP 3 best schemes switching power supplies that you can assemble with your own hands.

Simple DIY switching power supply

Let's look at how to make the simplest miniature switching power supply. Any novice radio amateur can create a device according to the presented scheme. It is not only compact, but also operates over a wide range of supply voltages.

A homemade switching power supply has a relatively low power, within 2 Watts, but it is literally indestructible and is not afraid of even long-term short circuits.

Circuit diagram of a simple switching power supply

Transformer of a simple switching power supply

Let's go first primary winding, which consists of 200 turns, wire cross-section from 0.08 to 0.1 mm. Then we put insulation and use the same wire to wind the base winding, which contains from 5 to 10 turns.

We wind the output winding on top, the number of turns depends on what voltage is needed. On average, it turns out to be about 1 Volt per turn.

Video about testing this power supply:

Do-it-yourself stabilized switching power supply on SG3525

Let's take a step-by-step look at how to make a stabilized power supply using the SG3525 chip. Let's immediately talk about the advantages of this scheme. The first and most important thing is stabilization of the output voltage. There is also a soft start, protection against short circuit and self-recording.

Don't think that this will make the scheme more complicated. On the contrary, everything becomes simpler, safer and cheaper. For example, if you install a driver at the output of a microcircuit, then it needs a harness.

We calculate the resistance of the resistor:

R = U squared/P
R = 24 squared/1
R = 576/1 = 560 Ohm.

Calculation of voltage stabilization:

U out = 2 + U stab1 + U stab2
U out = 2 + 11 + 11 = 24V
Possible error +- 0.5 V.

You may ask, why not increase the fee and make everything normal? The answer is as follows: this was done so that it would be cheaper to order the board in production, since boards larger than 100 square meters. mm are much more expensive.

Well, now it’s time to assemble the circuit. Everything is standard here. We solder without any problems. We wind the transformer and install it.

Check the output voltage. If it is present, then you can already connect it to the network.

Now let's check the most important thing - stabilization. To do this, take a 24V lamp with a power of 100W and connect it to the load.

Video about this switching power supply:

The 12 volt DC power supply consists of three main parts:

• Step-down transformer from a regular input AC voltage 220 V. At its output there will be the same sinusoidal voltage, only reduced to approximately 16 volts 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.
• Electrolytic capacitor large capacity, which smooths out the voltage half-sine waves, making them 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 quality The output voltage can be higher, 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 mounting board electronic components and boxes where all this will be secured 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 plate, which means it must be of sufficient area, length and allow possible installation radiators for diodes, transistors or a microcircuit that 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:

1. Stabilize the output voltage so that when the load resistance changes, the current does not exceed the maximum permissible value or during sudden surges in current in the load network - for example, when some devices are turned on - peak current values ​​were cut to nominal. Such phenomena occur when the power supply powers radio-electronic devices - radios, etc.
2. Use special protection circuits that would turn off the power supply if the load current exceeds.
3. 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.

It's better, but maximum current the load of such a stabilized power supply 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 powerful block nutrition that can withstand it. Then protection schemes are used, starting from fuses and ending complex circuits with automatic shutdown on integrated circuits.

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 simple circuits with all the detailed explanations for beginners. So, there are 4 ways for you to choose from.

We'll tell you how to do it adjustable block power 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 trim resistor at 10 kilo-ohms. 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 resistors rang from it feedback. 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.

Required homemade regulator 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 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? This transistor we throw it away 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. The only thing that depends on this resistor is minimum voltage, to which he can lower it. 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

Simple option homemade device for powering regulated devices. 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

Connect to the output of the device electronic load. 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 tutorial on block conversion ATX power supply In a desktop power supply, one very good 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 a variable output voltage from a few volts to some maximum value, all with currents of about 1.5 amperes.

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 much 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 chain to the output of the yellow wire +12 Volts. 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. Zener diode 4.3 V requires reverse current offsets about 5 mA to maintain output with the regulator drawing about 0.5 mA. This full current 5.5 mA 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 DC 100 µA. Because reference voltage through resistor R1 is constant, constant current will flow through another resistor R2, resulting in an output voltage:

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

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.

When you assemble any electronic homemade product, you need a power supply to test it. There is a wide variety on the market ready-made solutions. Beautifully designed, have many functions. There are also many kits for self-made. I'm not even talking about the Chinese with their trading platforms. I bought step-down converter module boards on Aliexpress, so I decided to make them on it. The voltage is regulated, there is enough current. The unit is based on a module from China, as well as radio components that were in my workshop (they had been lying around for a long time and were waiting in the wings). The unit regulates from 1.5 volts to the maximum (it all depends on the rectifier used to the adjustment board.

Description of components

Power supply assembly