0-50V 3A Variable power supply

This regulator is 0-50V variable power supply circuit. Which you can adjust output voltage in a wide range between 0V to 50V, and a current at least 3A.  Also, the important is the overload protection. Since it will immediately cut off when a load short circuit.

0-50V 3A Variable power supplyThe output is a voltage regulation circuit. Because they use LM723 ICs to provide the best performance. Thus, it is suitable for a  power supply circuit in the various experiments.

Now, we have this project that covers almost all applications.

Because it can adjust the output voltage between 0V to 50V. Because we use 2SC5200 transistor to replace the 2N3055 old transistor.  2N3055, though very ancient but it still has many uses we like it.

And also apply current to 3A which better than DC Variable Supply 0-50V, 2A. Hope you will like it.

Technical information

  • Uses the transformer, size voltage: 24V – 18V – CT – 18V – 24V , at current : 3A  to 4A
  • There is an overcurrent protection to prevent damage to the circuit
  • Adjustable Voltage between 0V to 50 VDC (Depending upon  the AC supply voltage of Input)
    You can apply it to 48v power supply circuit diagram.
  • Can apply maximum current to 3A

How it works

Figure 1 is the circuit diagram of this project.

First of all, the AC-voltage from the transformer-T1 flows through diode D1-D4 to the rectifier to DC voltage.

Second, capacitor-C1  filters that pulsed DC voltage to smooth, before sent through R1, R2 into TR5.

0-50V 3A power supply
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0-50V 3A variable power supply circuit

And there are IC1 is used to control a voltage at OUT point by adjusting VR1 can adjust voltage between 0-50 volts.

0-30V Variable Power Supply at 3A’ »

Two resistors R1 and R2 limit the overload current. They work together with two transistors TR1, TR2.

When the current exceeded, TR1 and TR2 will work. Then, there is an output voltage at lead E  of TR2 through LED1 into 2 pins of IC1.  LED1 is an alarm display.

Thus the circuit stops to provide voltage to the output.

We need to use normal load current that under lower than 3A.
After that, press the switch-SW1 to reset the system to restart working circuit again.

How to builds

components layout of 0-50V power supply
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This project has quite many components. You should experience a modest electronic. You can use the perforated PCB. As components layouts in Figure 2

Figure 2 The components layout and wiring

If you want to build this circuit. I have designed this with www.easyeda.com

Schematic of 0-50V, 3A variable DC power supply
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First, Schematic diagram of 0-50V, 3A variable power supply

Second, Actual-size of Single-sided Copper PCB layout in this project.

Copper PCB layout of 50V 3A Variable power supply

Print it on 300dpi.

Component layout of 0-50V 3A Variable DC power supply

Third, the components layout of the variable power supply circuit

Parts lists

You may buy components at electronics stores here.


  1. IC1 = LM723—High precision voltage regulator
  2. TR1 = BF423—PNP transistor
  3. TR2,TR3 = BF422—NPN transistor
  4. TR4 = BD140—PNP transistors
  5. TR5 = 2SC5200—NPN transistors
  6. ZD1 = 36V 0.5W Zener diode
  7. D1-D4 1N5401—3A 100V Diode
  8. LED1,LED2—LED 3 mm as you want


  1. R1,R2 = 0.1 ohms 2W = 2 pcs.
  2. R3 = 56 ohms 0.5W = 1 pcs.
  3. R4,R10 = 100K 0.25W = 2 pcs.
  4. R5,R8,R20 = 10K 0.25W = 3 pcs.
  5. R6,R22,R23 = 1K 0.25W = 3 pcs.
  6. R7 = 15K 0.25W = 1 pcs.
  7. R9 = 2.7K 0.25W  = 1 pcs.
  8. R11,R12 = 5.6K 0.5W = 2 pcs.
  9. R13,R17 = 8.2K 0.5W = 2 pcs.
  10. R14,R15,R16 = 10K 0.5W = 3 pcs.
  11. R19 = 330 ohms = 1 pcs.
  12. VR1 = 10K potentiometer  = 1 pcs.

Ceramic capacitors
C6 = 470pF 50V = 1pcs.

Mylar Capacitors
C2,C7 = 0.1uF 100V = 2 pcs.

Electrolytic capacitor

  1. C1 = 2200uF 63V  = 1 pcs.
  2. C3 = 47uF 50V = 1 pcs.
  3. C4,C5 = 10uF 50V  = 2 pcs.
  4. C8 = 100uF 63V = 1 pcs.
    SW1—Reset switch normally open pushbutton = 1 pcs.
    PCB,wires,heatsink,nut,screw,and other.

0-50V 3A Variable Power Supply Project

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Mounting transistor to the heatsink

When the load uses too much current. The 2SC5200 is also too heat.

Thus, we must mount the large size heatsink to save 2SC5200.  We can install the 2SC5200 transistor to the heatsink as Figure 3.

It is a proper way of mounting the transistor in a heatsink. Using hex nut and metal screw to mount them together by use mica insulator to protect the short circuit.

The mounting the transistor on the heatsink
Figure 3 The mounting the transistor on the heatsink

All Transistors pinout in this project

Transistors pinout 2SC5200 BD140 BF423 BF422
There are 2SC5200, BD140, BF423  and BF422. You may mistake them.

Connecting the transformer

Let’s see in Figure 2.

  • If you want to use output voltage are 0-50VDC. You connect 18,24 terminal.
  • The output 0-30VDC can connect 0,24 terminal.
  • The output 0-20VDC, can connect 0,18 terminal.

connecting wrong polarity voltage is low

But I cannot buy the transformer in Figure 2. So, use 2 transformers to connected with 0-18V and 0-24V in series.

18V+24V = 42V AC.

If connecting is the wrong polarity, the output voltage will be low. As Figure 6, the output is 5VAC only.

It means that you connect the transformer wrong. Must return a new polarity wire. We will read voltage about 43V.

Eventually, we will have a successful circuit

Testing and Repair

To begin with, connect the transformer as shown in Figure 2. Then, use a coil of a transformer to match our needing. Next, measure the voltage at OUT.

Be careful!! Check and Check again!
Check for error before power on the circuit.

After that, use a voltmeter to measure the voltage at the output. Hold it!
Next, supply the AC main to the transformer. While we read voltage the output 0-50V, begin to adjust the VR1.

As Video below.

The repairing
If this project is not working. Please check and check it again. Often, caused by putting the wrong device position and soldering. Make sure that all the various solder points.

Then, you measure a voltage across C5, it should be 36V. It is a supply voltage of IC1. If it is not same this.  You check the voltage across C1,  about 58V. If not have this voltage. Please check wires transformer, diodes D1-D4, and C1 again.

And then, come back to check components, DC regulator of IC1 again.

  • R1—If it fails. No current to Q3 so no voltage at C5.
  • Q3—Check a pinout of Q3 again. Some friend put it wrong.
  • ZD1—If you put it reverse polarity. The circuit may do not works.

High voltage and cannot adjust

First, check the voltage at pin 11. Now it is high voltage. You may remove R11, R12. Then, measure a voltage at the output. It should be low voltage or zero.

But, it is still a high voltage. It may be Q4 not correct, its lead is switched.

Next, if it works.  Check R4, R23, R22, R21, R20, and VR1. They may be an error.

Answer the Question- FAQ

The transformer and potentiometer
Mr balasaheb desai: You can use 15V 0 15V transformer, you not use CT or 0 terminal. I think you will have the maximum output voltage of about 40V. Yes, you can use 20K pot. But, you should add 20K resistor in parallel with this 20K pot. See in the picture below, the total resistance is 10K the same in the circuit.

Can adapt to 0-70V variable power supply?

I think it can use for 0-70V variable power supply. Because 2SC5200 can use max the voltage at 230V in a datasheet. But I am not sure in real using. Please test it with being careful.

And you need use voltage of the transformer is 55V AC input. When the rectifier convert is about 76V.

Can increase current up to 5A?

Some friend asks me. Can increase current up to 5A?
Yes, you can. According to the principle. We should do the following:

  • Change the transformer size to 5A
  • Change or increase the capacitor size double the original. May add 2,200μF or change to 4,700μF. The voltage is the same.
  • Parallel another power transistor.  Should wear a shock absorber for it Because each transistor property may not be the same.Parallel power transistor to increase current
  • RA, RB is a shock absorber,  1.2Ω 5W.  And change R3 to 100Ω.
  • Warning I haven’t tried this method yet. Therefore cannot confirm
    I believe in you. You love challenges. Happy to try doing it. Although there is a risk that It will not work.

Keep reading: ‘0-30V 0-5A regulated variable power supply’ »

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