Voltage and current regulated laboratory power supply

Schematic and assembly of a home-built power supply with voltage and current regulation

In an amateur radio or home handyman’s lab doing electronic repair, a laboratory power supply is an absolute necessity. It has to deliver a regulated voltage from 0 to 12 V (or preferably up to 30 V) with a current of at least 1.5 amps (5 amps is better). It is also useful to have overload protection and a maximum current limit. Such a power supply can be assembled with your own hands.

Types of power supplies

All power supplies can be divided into two large classes:

  • pulsed;
  • transformer.

These terms are not very accurate – a transformer power supply can have both linear and pulsed voltage regulator, and a pulsed PSU contains a transformer.

Each type has its own advantages and disadvantages based on the principle of operation. A transformer power supply with a linear voltage regulator distributes power between the load and the regulating element (usually a powerful transistor) and is a voltage divider. One arm is the regulating element and the other arm is the load.

When the voltage on the load decreases (e.g. due to an increase in current consumption), the transistor opens slightly and keeps this voltage constant. When the load voltage increases, the opposite process occurs – the transistor closes. This is how the stabilization process works.

Schematic and assembling a home-built power supply with voltage and current regulation

the output voltage is free of high-frequency parasitic components (power supply interference is minimal).

A current equal to the load current is constantly flowing through the control transistor – a lot of power is wasted;

Pulsed power supply operates on a different principle. Here the energy is distributed over time. Key transistors have only two states – they are either fully open or fully closed. The duration of the open position determines the average current through the transformer primary and the average voltage across the filter output capacitors (and therefore the load). This process is conveniently controlled by pulse width modulation (PWM), where the conversion frequency remains constant and only the pulse length changes.

The ideal pulse source of stabilized voltage has zero resistance, no voltage drop in the open position, and no current in the closed position. Therefore, no energy is dissipated on the transistors. In practice, things are not so rosy. There are no perfect transistors, so in the open state a certain voltage falls on them (resistance is not zero), and in the closed state there is a leakage current (resistance is not infinite).

But the main losses that reduce the efficiency occur for another reason. Transistor keys do not change from one state to the opposite state instantaneously. It takes time, which depends on the speed of the element. During the transition a through current flows through the transistor, the voltage drops across the transistor and therefore power is released. These losses are called switching losses, their value depends on the frequency of conversion.

Schematic and assembling a home-built power supply with voltage and current regulation

But still, the efficiency of such a source is higher than that of a linear source. And this is the main advantage of this circuit. Another advantage is the smaller size and weight of the power supply. This is achieved due to the fact that the transformation is carried out at a fairly high frequency – up to several tens of kilohertz. Therefore, the heaviest and most bulky element (power transformer) is light and compact. The main disadvantage is the complexity of the circuit.

Usually linear voltage sources are used for currents up to 2 A. Closer to currents of 3 A and above, the advantages of pulsers begin to outweigh.

The main nodes of the regulated power supply

The transformer power supply is in most cases made according to the following structural scheme.

Running lights on LEDs

Schematic and assembling a home-built power supply with voltage and current regulation

The step-down transformer reduces the line voltage to the required level. The resulting AC voltage is converted into a pulsed voltage with a rectifier. The choice of circuit depends on the circuit of the secondary windings of the transformer. Most often a half-wave bridge circuit is used. Less often a single half-period circuit is used, because it does not allow to fully use the transformer’s power, and the level of ripple is higher. If the secondary winding has a derived midpoint, a two-half-period circuit can be built with two diodes instead of four.

Schematic and assembling a home-built power supply with voltage and current regulation

If the transformer is three-phase (and there is a three-phase circuit to power the primary), the rectifier can be built with a three-phase circuit. In this case, the level of ripple is the lowest and the transformer power is used most fully.

After the rectifier, a filter is installed, which smooths the surge voltage to a constant voltage. Usually, the filter consists of an oxide capacitor, in parallel with a ceramic capacitor of low capacity. Its purpose is to compensate the structural inductance of the oxide capacitor, which is made in the form of a rolled up strip of foil. As a result, the resulting parasitic inductance of such a coil impairs the filtering properties at high frequencies.

Next is the stabilizer. It can be either linear or pulse. The pulse one is more complicated and negates all the advantages of a transformer PSU in the output current range up to 2…3 amps. If you need an output current higher than this value, it’s easier to make the whole power supply with a pulse circuit, so usually a linear regulator is used here.

The output filter is made on the basis of an oxide capacitor of relatively low capacitance.

Schematic and assembling a home-built power supply with voltage and current regulation

Pulsed power supplies are built on a different principle. Because the current consumption has a sharply non-sinusoidal nature, a filter is installed at the input. It does not affect the performance of the unit in any way, so many industrial manufacturers of PSUs of Economy class do not install it. It is also possible not to install it in a simple self-made source, but it will lead to the fact that microcontroller devices powered from the same 220 volt mains will start to malfunction or work unpredictably.

Then the mains voltage is rectified and smoothed. Inverter on transistor keys in the transformer primary circuit creates pulses with an amplitude of 220 volts and high frequency – up to several tens of kilohertz, as opposed to 50 hertz in the mains. This makes the power transformer compact and lightweight. The secondary voltage is rectified and filtered. Due to the high frequency conversion, smaller capacitors can be used here, which has a positive effect on the dimensions of the device. Also in high-frequency voltage filters it becomes reasonable to use chokes – small-sized inductances effectively smooth out HF pulsations.

Quick heating soldering iron with his own hands

Voltage regulation and current limitation is performed by feedback circuits, to which the voltage from the source output is fed. If the voltage begins to decrease due to an increase in load, the control circuit increases the interval of the open state of the keys without decreasing the frequency (pulse width control method). If the voltage has to be reduced (also to limit the output current), the open time of the keys is reduced.

How to choose components

For a transformer power supply, first of all, the transformer is chosen. In most cases it is taken from what is available. This unit must deliver the required current at the maximum voltage. The combination of these parameters is provided by the dimensional capacity of the transformer. For industrial devices the parameters can be found in a reference book. For occasional transformers the power can be determined from the dimensions of the core (in centimeters).

Voltage and current regulated power supply

Today I would like to tell you about my new home-built device – the voltage and current regulated power supply which is the dream of all amateur radio beginners and experienced radio amateurs. The device can be used as a laboratory unit for powering various home-made products, and as a battery charger for charging car batteries. The power supply has a stabilized voltage regulator and current limiting system, protection against polarity reversal of the battery terminals with light indication, as well as an automatic fan speed regulator, which changes the speed depending on the heat of the radiator. This diagram shows a voltage and current regulated power supply circuit designed for a current of up to 10A. You can connect any transformer or switching power supply from 12 to 30V to this circuit. For those who like it powerful, you will also find in this article a diagram for currents up to 25A. I will not rush things. Read the article carefully to the end.

A schematic of a power supply with voltage and current regulation 1.2. 30V 10A

Schematic of a voltage and current regulated power supply 1.2V … 30V 10A

The LM317 adjustable voltage regulator allows smooth regulation of voltages in the range of 1.2V to 30V. The voltage regulation is done with a variable resistor P1. The transistor T1 MJE13009 serves as a high current switch.

The current limiting system is made on the T2 IRFP260 field-effect transistor, allows to limit the current from 0 to 10A, the current is controlled by a variable resistor P2, which allows to use this power supply as a charger for charging car batteries. A powerful resistor R6 with a resistance of 0.1 Ohm 20W serves as a shunt. It is no problem to buy it in China from Ali Express. If you don’t want to wait too long, you can connect several resistors in parallel to make one powerful resistor. Note that a special formula is used when the resistors are connected in parallel.

Parallel connection of resistors formula

The total resistance of the resistors is divided by the number of resistors. How to determine the total resistance, of the same resistors? You just take the resistance of one resistor and divide by the number of resistors. For example, I have 4 resistors, the resistance of each resistor is 1 Ohm and the power dissipated is 10 W, so the total resistance of all the resistors is 1 Ohm, if you connect them in parallel, you get the total resistance of the four resistors 0.25 Ohm 40 W. The power of all resistors is summed up. This way you can make a resistor of any power. In the pictures and in the video you can see in my PSU an assembly of 4 1 Ohm resistors of 10W with a total resistance of 0.25 Ohm and a power of 40W. I did this because I didn’t have a powerful 0.1 Ohm 20W resistor handy at the time and I didn’t have one in the store either. But, lo and behold, it turned out that this circuit current control works fine even with 0.25 Ohm resistor. I got interested and decided to do a series of experiments with resistors which came in a couple weeks from China, with 0.1 Ohm, 0.25 Ohm, 0.5 Ohm resistance, and I came to the conclusion that with any of these resistors the current regulation works fine. That is, you can put resistors with any resistance in the range from 0.1 Ohm to 0.5 Ohm into this circuit, which makes this circuit available for assembly to novice radio amateurs. It is not always possible to find resistors with the right resistance and power in the store. I tried to replace the resistor with a piece of nichrome spiral from the electric stove, all the same on the work of the current regulator, it did not affect, the only drawback is that the coil is strongly heated, and it had to be cast into the concrete.

Three useful circuits for the garage and not only

The circuit has built-in protection against polarity reversal. When the power supply is properly connected to the battery, the green LED Led1 lights up. In case of incorrect connection, the red LED Led2 lights up, indicating a connection error. The system works correctly only when the power supply is switched off. That is, first connect the battery, when the green LED lights up, plug the power supply into the mains.

The automatic fan speed regulator is designed to reduce the noise level occurring during operation of the power supply. The voltage regulator L7812CV maintains a constant voltage of 12V which goes to the divider consisting of thermistor R8 mounted on the heatsink and trimmer resistor P3. The voltage from the divider goes to the base of transistor T3. During operation of the power supply from the high load heatsink, the resistance of thermistor R8 set in the heatsink becomes less than the resistance of trimmer resistor P3, the voltage at the base of the transistor increases and transistor opens, thereby increasing the speed of the fan. The sensitivity of the regulator is adjusted with the trimmer resistor P3.

In this regulator circuit it is possible to connect different models of voltmeters and ammeters, both dial and electronic. With analog classics marked on the scheme by the letters V voltmeter and A ammeter everything is clear to connect according to the scheme. It is better to buy an ammeter with built-in shunt, it is much more compact and cheaper. Accuracy class voltmeter and ammeter with Ali Express should be 2.5 these devices work fine. But with the Chinese electronic will have to tinker. At the moment there are two models of Chinese Universal Measuring Instruments (UMI). The first model with a blue wire with built-in shunt is more accurate less glitchy, recently it is difficult to find on Ali Express. The second model with a yellow wire and built-in shunt is not accurate and very glitchy with jumping readings of the ammeter from 0 to 0.25A at idle with no load. I do not understand why they sell it at all? If you are going to install an electronic MCCB, then you have to break the section of the circuit marked with a red cross in the schematic. Otherwise, in this scheme, the electronic MCCB will not work properly.

Voltage and current regulated power supply

This scheme is for those who like powerful power supplies. As promised up to 25A.

Schematic of a power supply with voltage and current regulation 1.2 30V 25A

Scheme of power supply with voltage and current regulation 1.2 … 30V 25A

I added additional powerful transistor T2 TIP35C which can withstand up to 25A current and resistor R3 200 Ohm. Diode bridge replaced with a more powerful one. The IRFP250 transistor can handle 30A and the IRFP260 49A.

This picture shows the power supply PCB with 10A voltage and current regulation.

Circuit board of power supply with voltage and current regulation 1.2. 30V 10A

Voltage and Current Controlled 1.2…30V 10A Power Supply PCB

This figure shows the power supply PCB with voltage and current adjustment for 25A.

Circuit board of power supply with voltage and current regulation 1.2. 30V 25A

The printed circuit board for a 1.2V…30V…25A Current and Voltage Adjustment Power Supply

The LM317 voltage regulator, TIP35C, IRFP250, 260 transistors are mounted on a heatsink with insulating pads and thermowells. Transistor MJE13009 mounted on a radiator without insulation, otherwise from the strong heat and poor heat dissipation through the thermal pad will overheat and fail. Install the voltage regulator L7812CV and the transistor BD139 on different heat sinks. Insert the thermistor into the drilled hole in the heatsink and fix it with Poxypol or Epoxy resin. While installing the thermistor, use a multimeter to check for electrical contact between the thermistor and the heat sink. The variable resistors, as well as the LEDs, if necessary, can be wired and placed outside of the board.

The finished power supply starts working as soon as power is applied to the board. The only thing to adjust is the rotation speed of the fan. To do this, you have to set the voltage of the fan to about 1 volt when the radiator is cold, using the P3 trimmer. The fan will start spinning when the radiator temperature is about 45 degrees, the speed will rise in direct proportion to the temperature of the radiator. As the radiator cools, the fan speed will decrease. This is how the automatic fan speed controller works.

Voltage and current regulated power supply

So how do you use the power supply? It’s very simple. Turn the power on and set the voltage you want with the adjustable resistor P1. The knob of adjustable resistor P2 put in extreme right position corresponding to the maximum current. Connect the load to the power supply, if necessary, add voltage. If necessary, you can limit the current with the P2 resistor.

Universal protection scheme against undervoltage or overvoltage.

Voltage and Current Controlled Power Supply Load

How to charge the battery? Easy! When you connect the battery, the power supply must be unplugged. Set the knobs of resistors P1 and P2 to the leftmost position, minimum voltage and minimum current. Connect the battery to the power supply. The green LED should light up, it means that the battery is connected correctly. In the case of a connection error, the red LED will light up. After you are sure that the battery is connected correctly, plug the power supply into the mains. Use variable resistor P1 to set the voltage to 14.5V. Then use resistor P2 to set the current to 10% of the battery capacity, i.e. for a 60Ah battery the initial current should not exceed 6A.

Voltage and current regulated power supply start charging the battery

After setting the current there will be a voltage drop to about 13V. As the battery is charged, the voltage will gradually rise to 14.5V and the amperage will drop to 0.1A indicating that the battery is fully charged.

Voltage and current regulated power supply end of battery charging

What will happen to the power supply in the event of a short circuit? Nothing bad will happen. In the event of a short circuit, current limiting protection will trip. According to Ohm’s law: The greater the resistance of a circuit, the less current it will carry. Therefore, in the event of a short circuit, the maximum possible current will be present. The voltage will drop and the current will be the current that you have limited with resistor P2.

The parts to build a 10A voltage and current regulated power supply

  • Diode bridge KBPC2510, KBPC3510, KBPC5010
  • Capacitor C1 4700mf 50V
  • Adjustable voltage regulator LM317
  • Transistors T1 MJE13009, T2 IRFP250, IRFP260, T3 KT815, BD139
  • Resistors P1 5K, P2 1K, P3 10K
  • Rabitron 12V 5W 1N5349BRLG
  • Resistors R1, R2 200R 0.25W, R3 1K 5W, R4 100R 0.25W, R5 47R 0.25W, R6 0.1R 20W, R7 3K 0.25W
  • Thermal resistor R8 B57164-K 103-J resistance 10K
  • LEDs 5mm red and green, 3V supply voltage
  • Heat sink 100x63x33mm 1pc, heat sink KG-487-17 (HS 077-30) 2pc
  • Fan 70×70 mm

Components for assembling power supply with voltage and current regulation for 25A

  • Diode bridge KBPC2510, KBPC3510, KBPC5010
  • Capacitor C1 4700mf 50V
  • Adjustable voltage regulator LM317
  • Transistors T1 MJE13009, T2 TIP35C, T3 IRFP250, IRFP260, T4 KT815, BD139
  • Resistors P1 5K, P2 1K, P3 10K
  • Rabitron 12V 5W 1N5349BRLG
  • Resistors R1, R2, R3 200R 0.25W, R4 1K 5W, R5 100R 0.25W, R6 47R 0.25W, R7 0.1R 20W, R8 3K 0.25W
  • Thermal resistor R9 B57164-K 103-J resistance 10K
  • LEDs 5mm red and green, 3V supply voltage
  • Heat sink 100x63x33mm 1pc, heat sink KG-487-17 (HS 077-30) 2pc
  • Fan 70×70 mm

Friends, I wish you good luck and good mood! See you in new articles!

I recommend to see a video on how to make a power supply with voltage and current regulation

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