0-50V 3A Variable Power Supply

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.
The 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.
2
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
See circuit diagram of this project below.
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.
3
<The-circuit-diagram-of-0-50V-3A-Variable-DC-power-supply.jpg>
And there are IC1 is used to control a voltage at OUT point by adjusting
VR1 can adjust voltage between 0-50 volts.
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
4
This project has quite many components. You should experience a

modest electronic. You can use the perforated PCB. As components
layouts in Fig
<The-components-layout-and-wiring.jpg>
See, Schematic diagram of 0-50V, 3A variable power supply

5
Then, See the actual-size of Single-sided Copper PCB layout in this

project.
How to make a PCB. It is easy and fast. I use laser printer it on 300dpi.
6
Third, the components layout of the variable power supply circuit.
Parts lists
Semiconductors
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

7
8. LED1,LED2—LED 3 mm as you want
Resistors
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
8
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.
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 Fig.
9
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.
All Transistors pinout in this project
See:
10
There are
2SC5200, BD140, BF423 and BF422. You may mistake them.
Connecting the transformer
First, you should 3A up transformer for full output current. Then, you can
choose the voltage of transformer list:
● The output 0-20VDC, you use 18 transformer.
● The output 0-30VDC, you use 24V transformer.
● If you want to use output voltage are 0-50VDC. You use 42V
transformer. But sometime we cannot find this transformer. We may
use 2 transformers connect together in series.
11
<Connecting-2-transformer-42V-out.png>
Use 2 transformers to connected with 0-18V and 0-24V in series.
18V+24V = 42V AC.

12
If connecting is the wrong polarity, the output voltage will be low. As Fig,
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 above. Then, use a coil
of a transformer to match our needing. Next, measure the voltage at
OUT.
13
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.
Video: 0-50V 3A Variable DC power supply.mov
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.
14
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.
15
Can modify to 0-70V 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

16
● 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.
17
● 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.

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0-50V 3A Variable power supply

Because it can adjust the output voltage between 0V to 50V. Because we

And also apply current to 3A which better than DC Variable Supply

○ Uses the transformer, size voltage: 24V – 18V – CT – 18V – 24V

○ There is an overcurrent protection to prevent damage to the

○ Adjustable Voltage between 0V to 50 VDC (Depending upon the

○ Can apply maximum current to 3A

Second, capacitor-C1 filters that pulsed DC voltage to smooth, before

Thus the circuit stops to provide voltage to the output.

This project has quite many components. You should experience a

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

Then, See the actual-size of Single-sided Copper PCB layout in this

Third, the components layout of the variable power supply circuit.

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.

C6 = 470pF 50V = 1pcs.

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

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.

Mounting transistor to the heatsink

It is a proper way of mounting the transistor in a heatsink. Using hex nut

All Transistors pinout in this project

Connecting the transformer

● The output 0-20VDC, you use 18 transformer.

● The output 0-30VDC, you use 24V transformer.

Use 2 transformers to connected with 0-18V and 0-24V in series.

18V+24V = 42V AC.

Eventually, we will have a successful circuit

Testing and Repair

Be careful!! Check and Check again!

Check for error before power on the circuit.

Video: 0-50V 3A Variable DC power supply.mov

Then, you measure a voltage across C5, it should be 36V. It is a supply

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.

High voltage and cannot adjust

Answer the Question- FAQ

Can modify to 0-70V power supply?