0-30V Variable Power Supply at 3A

This is a 0-30V Variable Power Supply circuit diagram. I recommended this. Because you can adjust an output voltage of 0V to 30V, at 3A current, and overload protection.

Also, It is a high-efficiency regulator using an IC regulator, UA723, and power transistor, TIP3055. So, it is a small laboratory DC Power Supply at an inexpensive cost.

0-30V Variable Power Supply at 3A

Best for who is finding circuits: 0-12V and 0-24V variable power supply circuit diagram. Because It is high current, normal parts, a new circuit design.

Why should make 0-30V variable Power Supply

  • Modern design—I used to show you the old circuit below, also using LM723. But it powers a current just 2A. And It uses 2N3055 power transistor. So, It is difficult to make. But this use TIP3055 instead. It is slim so easy to install on a heatsink.
  • Output voltage—voltage output adjustable: 0-30VDC
  • Current maximum—all range of output voltage it can power current up to 3A
  • Inexpensive—if you compare with old circuits. this is cheaper than them.
  • Need part—you should use a transformer: 12-0-12V 4A for full current up to 3A.

How does it work

The 0-30v variable power supply circuit diagram
The 0-30V 3A variable power supply circuit diagram

As Figure 1 circuit diagram. The order of operation is interesting.

Oops! Here is too difficult. Look at…
0-30v variable power supply. or

LM350 voltage regulator

Here is step by step a process.

  • Reduce AC voltage
    The AC main to a transformer reduces the AC high to AC low voltage. See below, how to set the voltage as you want.
  • Rectifier AC to DC
    Low AC voltage flows through the bridge diodes(D1 through D4) set. They rectify  AC to DC pulse voltage.
  • Unregulated DCV
    C1 is a filter capacitor to the pulsating voltage into a steady direct current (DC). Before, send it to…
  • DC regulator circuit
    The voltage comes to IC1 and some parts. They keep the output voltage is constant. Even the load will draw a current too many, the voltage is still stable.

    If you want to read more detail about LM723 circuits you can visit. Datasheet!

    But the current out of pin 11 of IC1  is very weak. They need increasing current.
  • Increase current
    Two transistors Q1 and Q2 are connected in Darlington mode, so a lot of gains.
  • Adjustable voltage
    You rotate VR1 to change the output voltage between 0V and 30V.
  • Special voltage selector
    Adjusting VR1 at low voltage is difficult. Such as 0-12V, 0-10V, and more.

    But this is special, you can select a range of voltage in 2 steps, low and high, with SWITCH in a circuit.

    Low: 0-12V and High: 0-30V. If you cannot imagine, see below!

Overload Protection

Detect overload
Sometimes it is too much current that the transistor and other parts tolerate it.

You look at parallel resistors(R1 to R5) that are reduced to 0.2 ohms to detect overload current.

If the current exceeds 3A. The voltage across these resistors more than 0.6V.
Also, The voltage at the base-emitter of TR3 is 0.6V, making it works.

Between its collector-emitter likes, a switch closed.
It also connects with the base-emitter of Q1. It makes Q1 reduce working, the current is lower.

Then, Q2–power transistor stops working too. Thus, the output is low current, this circuit is safe.

Let’s build them

First of all, get the electronics components.

Parts you will needs

We may buy them at electronics stores near you or online to get parts.

All parts on 0-30V variable power supply Circuit kits

Q1: BD140, 80V 1.5A PNP Transistor
Q2: TIP3055, 60V 15A, high-Speed Switching Silicon transistor
Q3: CS9012, 40V 0.5A PNP Bipolar Transistors
D1-D4: 1N5401, 100V 3A Rectifier Diodes

Electrolytic capacitors
C1: 4700uF 50V
C3: 100uF 50V

Ceramic Capacitors
C2: 680pF 50V
C4: 0.1uF 50V
0.5W 5% Resistors
R1-R5: 1 ohms
R6: 50 ohms
R7, R9, R12, R13: 100K,
R8: 2.7K,
R10: 10K,
R11: 5K,
VR1: 5K, Potentiometer

T1:12V-CT-12V, 4A transformer
Heatsink, PCB, Wires, and more…

Here are KITS that I bought from a local store. If you want to save time, it can be ordered through Amazon. READ MORE!

0-30V-3A-adjustable-power-supply at Amazon

Where buy electronics components


Then, Assemble the components on PCB.

Placing components on PCB of 30V 3A variable power supply

It starts with the lowest parts first, to be beautiful. Easy assembly To start with, followed by a diode, resistor, and respectively height and so on.

As the image below soldering lowest electronic components.

Soldering a lowest components on PCB first!

Be careful!
Some components have polarity like electrolytic capacitors, diodes, transistors, etc.

They require correct placing. If they are the wrong polarity. They will get destroyed from reverse voltage.

The components layout and wiring of 30V 3A power supply

The components layout and wiring of this 0-30v Variable Power Supply project.

We assembled all parts of 3A power supply PCB

We assembled all parts completely on PCB.

This project has high AC voltage, So we must be careful

This project uses AC power so we must be careful about high voltage.

0-30V 3A Laboratory DC Power Supply is complete.
Finally, we have made this project so successful.


The connecting “12” and “0” will be through a selector switch.

Selector switch to choose, high or low voltage rage

For 2 selector;

  • Use voltage 0-12V at “0”
  • Use voltage output 0-30V at “12,12” the voltage minimum is 0.3V to maximum is 33 volts.
    This way makes the transistor, not overheat.

The output we should connect a 3A fuse in series before to use.


We test this project. As the video step by step.

  1. Apply the AC power then turn on power switch ON.
  2. Select Switch to 0-12V Before then Rotate the knob VR1 fully the left.
  3. Measure voltage at the output point, then rotate the VR1 slowly to right, the output voltage starts is 0.3V or 300mV to 16VDC maximum.
  4. Toggle switch to “12-24V” then measure the voltage again 330mV to 32 volts.
  5. Setting the output is 12V, then Apply a 12V 50W as the load.
  6. Measure the current output when load 12V is 3A maximum.
  7. Temperature measurement of transistor operation.

How to solve circuit not works

If you made this project but it did not work. You do not worry. It is not a problem. It is a process of your electronic learning. You should have fun solving it.

Here are some basic guidelines for you.

No load, No power to this circuit. And Check and Check again with slowly. Surely, something is wrong.

If you check all is correct. You try to power to the circuit again. Then, measure the voltage at.

  • TP A (Test point)—It should read voltage about 33VDC. If it is not like this.
    You should check the unregulated voltage again. Which include a transformer, bridge diodes, and C1.
  • TP B—Also voltmeter should read about 33VDC. But it is zero or lower voltage. Show that parallel resistor, R1-R5 are broken.

Check the circuit again. even pins of IC1.

Equivalent Transistors

If you Cannot find a CS9012 transistor you may use an equivalent transistor is BC327 PNP transistor. But Different pinout positions between the Emitter and collector.

CS9012 datasheet : http://www.mouser.com/ds/2/149/SS9012-117759.pdf
Ice = 500mA max, Vce = 40max

BC327 Datasheet : http://www.mouser.com/ds/2/149/BC327-30422.pdf
Ice = 800mA, Vce = 40Vmax

Replace a CS9012 with the BC327 equivalent transistors

If you want the better circuits, look here!

0-50V 3A Variable DC Power Supply

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I always try to make Electronics Learning Easy.

Apichet Garaipoom

I love the electronic circuits. I will collect a lot of circuit electronics to teach my son and are useful for everyone.
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