Simple Variable power supply circuit 0-30V 2A

If you are beginners. You want a simple variable power supply. That can give 0-30V 2A output.

This circuit may be best for you. Because it has a few parts, small, and cheapest. Than others circuit that same power.

The features of this handset are the DC voltage continuously from 0-30 volts. And, apply current up to 2 amperes. And can put easily in a large pro box.

Simple Variable power supply circuit 0-30V 2A

How it works

The power supply is a circuit that is easy to understand. Because there are quite clear components.

I like to study it. If you like me. Let’s see. Is it really easy to understand?

Block Diagram

See the image below.

This is a Block Diagram of Simple Adjustable Power supply circuit. Let me explain to you how it works step by step.

Unregulated Power supply

It will change AC main into lower DC voltage, about 36V 2A. Almost all of the power supply will use it.

I believe that you have used and understood its work well.

Recommended: Working principle of unregulated power supply 

Adjustable Reference Voltage

The electricity is divided into 2 ways.

The first way:
Some little current flows through Adjustable Reference Voltage.
It determines the output voltage level of the output constant.
This circuit consists of Zener diode and Variable resistor.

Related: Learn Fixed voltage Regulator Working principle

Power Transistors

The Second way:
The most current will flows through the power transistor. It works like a large bridge for higher current through.

And it have the controlled current lead. To control the max output current, 2A.

Read Also: Simple Fixed DC regulator

Overload Protection

When the current exceeds 2A. The power transistor is working hard. There is high heat. Until it may be damaged. And also damage other devices.

We should therefore have the overload protection section.

Inside there is the resistor to check higher current and transistor to cut of the contrlled current of power transistor down.

See the Circuit Diagram

First, we apply the AC line to the circuit to SW1 on/off to the transformer T1 and fuse F1 to the protection circuit when too much power source.

Beginning Variable power supply circuit 0-30v 2A
Figure 1 Beginning Variable power supply circuit 0-30v 2A

Here is unregulated power supply(full-wave rectifier) Section.

Second, T1 reduce AC voltage 220V into 24V 0 24V.

Then, the current through to both diode D1, D2 to rectifiers to DC voltage.

Next, C1 filter current to DC voltage about 36VDC and 2A max.

There are LED1 to show power on and R1 limit current to a safe value.

Read next: Limiting current using Resistor

Next, the current comes to the regulated section.

The reference voltage
R2-100ohms and ZD1-30V are connected together as the 30V steady DC regulators.

The variable resistors VR1 is used to adjust the output voltage between 0V to 30V. 

What is more? the constant voltage will control base of power transistor, below!

There are transistors Q1, Q2 in Darlington mode. To drive or increase the output current up to 2A.

Short-circuit protection

Also, there is the short-Circuit protection include Q3, R3. How it works?

overload protection section

Here is a step by step process.

  • First of all, load uses too many currents.
  • Second, the voltage across R3 is more than 0.6V and B-E of Q2 too.
  • So, Between C-E is like a closed switch. To stop Q1 and Q2 run.
  • And C-E of Q2. It is an open switch. The current can via it lower too.
  • Thus, the output is low current to protect this circuit.

Note: In my opinion, this overload part is not the best. But better than none.

How it builds

The circuit assembly is very simple Because the devices are less Can assemble down on the perforated board.

While the power transistor Q3 – 2N3055 is working, will hot so we always use a heat sink on it.

Should use low wattage soldering iron does not exceed 30 watts. To soldering the legs of transistors and diodes.

We should use pliers with leg devices Because these devices cannot be very hot.

Adjustment and usage

The prototype works well, have ripple voltage at output lower than 1 mV and the output voltage will drop lower than 0.1 volts

In real usage that although it off, LED power ON will still light up for a moment. Because the LED gets current from C1 that does not discharge fully.

If you want to stop LED immediately. You may move R1 and LED1 to across the secondary of T1.

Then, add the diode-1N4001 series with LED to protects voltage backward which LED may be damaged as Figure 2

The LED will outage immediately when switched off
Figure 2: The LED will outage immediately when switched off

If need to use the dual power supply (Positive Ground and Negative terminal) For testing amplifier OCL.

We change the rectifier section to new, from a Full-wave into the bridge and use the two secondary coil; 12V 2A.

Then, apply the negative and positive of each channel to the ground as Figure 3

Making supply is 2 sets with bridge diode and capacitor
Figure 3: Making supply is 2 sets with a bridge diode and capacitor

The parts you need

0.25W Resistors, tolerance: 5%
R1: 3.3K, 0.25W resistor
R2: 100 ohms, 0.5W resistor
R3: 0.3 ohms 10W resistor
VR1: 10K Potentiometer
Electrolytic Capacitors
C1: 2,200µF  50V
C2: 220µF  50V
Semiconductors and others
Q2, Q3: 2N3055 Power NPN transistor
Q1: BC548,BC549, 45V 100mA NPN Transistor
D1, D2: 1N5402 3A 100V Diode
D3: 1N4002 1A 100V Diode
ZD1: 30V 1W Zener
T1: 117V/230V AC primary to 24V-0-24V,2A secondary transformer
S1: On-Off toggle switch
F1: 110V/220V, 500mA, slow-blow fuse

Be careful Components pinouts

Some parts of this have different pins. You need to put it in the correct way only!

Be careful component pinouts on power supply circuit
Be careful component pinouts on the power supply circuit

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I always try to make Electronics Learning Easy.


  1. Subodh

    where is the placement of VR1 10k pot. I can not see it in figure

  2. Hi, subodh
    Thanks for your feedback.
    Please look at the figure 1.
    VR1-10K ,

  3. Lily

    hi is this still uselful with 12 0 12 volts transformer?? can u make using 12 0 12 volt transformer.?

  4. spewkz

    hi, how can i change the output to 3 Amp in this schematic diagram?

  5. Efraem Salcedo

    This circuit is great. It works. Thanks!

  6. Aravind

    Hey! Plz explain me! How the current is amplified … Coz , its confusing me! Make it fast, or reply to my mail id. [email protected]

  7. Mamdouh

    I would like to send me some mini circuits

  8. Jed Frances Ararao

    a current limit function would be very nice..anyone can tell me how to add a current limiter function to this circuit?

  9. Hello, Jed Frances Ararao
    Thanks to your question. This is a simple circuit. It has basically a current limiting function. When the current is too much. There is a voltage across R3. Then, Q3 get a biased current. It works, the collector and emitter is low voltage. Thus, Q1 and Q2 stop. No current to load.

  10. Leon Spri

    Is it possible to change the output current to 1.5 A with small adjustments? And which are they?

  11. Andrew

    If the output current is always dependent on transformer secondary,
    then what is the usefulness of the output transistors

      • Join the discussion…ok thank u, but can u please explain further, more about how it look like a bridge, or like an analogy or illustration

  12. Victor G.

    Thank you for such an amazing explanation/tutorial!
    I only have one question. When connecting the transformer outputs in parallel, the voltage would be 24V, not 48V! Why does the diagram shows such connection? Shouldn’t the transformer be in series to output 48V? Thanks for your response 🙂

    • Hello Victor G.
      Thanks for your feedback. I am happy that you interested in this circuit.
      It is a full-wave rectifier unregulated power supply circuit mode.

      Yes, you are correct. My drawing is wrong. You are a great person.

      Thanks a lot back.


      • wasay

        Hi, kindly show me the correct drawing , i wannted to make a variable power supply of 0-30v and 2A current maximum.. i am a beginner and a bit confused that in book LM317 IC is used …can we make this circuit by using transistors?

        • Hello wasay,

          Thanks for your question. I am happy that you are interested in Electronics. Yes, when you are a beginner it has a lot of information. But you need to know. Do not worry about mistakes. It may be a step toward good learning.

          This circuit is easy to build. But my experience. It might not be the best circuit for us. It is better If you have tried to build many circuits.

          Have a good day.

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