Simple Variable power supply circuit

If you are a beginner and you want a power supply that the output voltage can be adjusted. Then a variable power supply circuit is what you are looking for. But there are many circuits in this category.

So, I try to compile most of them into this list. That you can choose the most suitable for your usages and learning.

Simple Variable power supply circuit 0-30V 2A

I think we should learn from the transistor circuits first before the IC in order to understand the basics. But if you looking for a circuit suitable for practical use, the IC circuits should be given priority.

What is a variable power supply circuit?

It is a power supply with an adjustable output voltage or current.

For example, you are experimenting with a circuit that requires a 12V power supply, so you are starting to look for a 12V power supply circuit. But you also want to know if it works better with a 15V or 9V power supply.

Surely, it would not be convenient to have 3 power supplies with 9V, 12V, and 15V just for testing. Unless you are a power supply collector like me. LOL

Would it be better if you can do all of that with just one variable power supply?


Of course, it will keep its main feature is that it maintains the same constant voltage as same as a normal fixed output power supply. For example, when you set the voltage to 9V, it will always stay at a constant 9V no matter how heavy the load is.

1# Variable power supply circuit

This circuit can output 0-30V 2A. It may be best suited for you. Because it has a few parts, small in size, and also very cheap.

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

Block Diagram Simple 0-30V Adjustable Voltage Power Supply

This is a Block Diagram of a 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 in 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 a Zener diode and a 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 has 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.

How Variable power supply circuit work

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 the unregulated power supply(full-wave rectifier) Section.

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

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

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

There is 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.

Adjustment and usage

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

In real usage that although it is 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 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

Recommended: 7805 regulator circuit datasheet & pinout

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
Q1: BD139, 80V 1.5A, TO-225 NPN Transistor
Q2: 2N3055 Power NPN transistor
Q3: 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
FuseHolder, Wire, Solder, case,

Note: You may Q1 is BC549 or BD139 (better).

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

2# 0-20V Variable Power supply circuit diagram

This is a simple 0-20V variable Power supply circuit diagram at a maximum output current of about 1A.

Which uses common components that are easy to buy in any store, and uses transistors as the main and control the output voltage with a potentiometer.

DC Supply adjustable Voltage 0-20V at 1A

The 0-20V variable Power supply circuit diagram

It also is an adjustable power supply for model railways and general projects.

My friend comes to see me. He brings a lot of electronic PCBs. He asks me, what do we do with them? It is a great idea for recycling.

I like to make a DC power supply project. Let to make a simple 0-20V variable Power supply circuit. He says okay, good idea.

He will apply them with the old power amplifier, about 40 years ago. Though very ancient, some parts still have many uses. We like it.

How it works

In the circuit above, When 220V/120VAC main comes to the circuit, pass through ON/OFF switch-S1 and Fuse-F1.

Then, the transformer T1 changes 220VAC to about 24VAC.

After that, The rectifier bridge, D1 through D4, rectifies the AC into pulsating DC.

The capacitor C1 acts as a storage capacitor, it will filter DC to smooth up as the unregulated power supply.

Now we have the 36V DC voltage at C1. Then this flows through to R1 and both Zener diodes ZD1 and ZD2, in series, each Zener is 10V. They are reference voltage. Using R1 limits current to ZD1 and ZD2 to keep the voltage constant at about 20V.

Next, this voltage comes to the base of Darlington regulator Q1, Q2. Both transistors will increase current up, so there are the voltage constant across R4 or output to load.

Why we add these components:

  • Potentiometer VR1, adjust the output voltage from 0V to 20V
  • Capacitor C2, the voltage drop across VR1 to be stable up
  • Both Capacitors C3, C4, reduce a noise signal (transient noise) in a circuit.
  • Diode D5, protect a feedback voltage that may kill other components

He uses all the transistors from it. You can change a transistor number instead Such as TIP41, TIP31, 2SD313, H1061, 2SC1061, MJE3055, and more. But its key feature is at least 3A, 40V, NPN transistors.

The DC adjustable power supply has an output of 0V to 20V at 1A with a transformer that is rated at 2A; or 0.7A, for a transformer that is rated at 1A.

3# Variable power supply 1A, 0-30V

I am going to show you an old circuit that is interesting. No ICs, no Zener diode. But it is also a steady voltage.

This is a simple transistor variable power supply project, 0-30V 1A.

Transistor Variable Power supply

Why should we make it?

It is a small circuit, cheap, and buys easy.

How it works

Look at in the circuit includes 2SC1061 and small transistors as a main. It will control the current up to 1 A.

0-30V 1A transistor variable regulator circuit

Circuit diagram of Variable power supply 0-30V 1A using transistors

Also, another a few components. You can adjust the output voltage with VR1.

Thus, It is suitable for learning a basic variable regulator.

What is more? 

The details

The IC (integrated circuit) is so famous in this electronic age. Because they are small, easy, and maybe cheaper. But someone may not like it.

They like the transistor circuits. Because when it is broken. We can repair it by changing only a few parts. So it so saves money.

But if the IC circuit. We need to change the IC.

Perhaps it is expensive or we cannot buy it at the local stores.

This circuit is the series regulator mode so it has high efficiency.

First of all, the AC-main comes to the circuit. It is flowing to the DC unregulated section. They include T1,D1,D2,D3,D4 and C1. You can read more about DC unregulated power supply.
Now the voltage across C1 is about 33VDC.

Then, the little current comes to the base of Q1 through R1—limiting current resistor—. Since Q1 and Q3 are The Darlington-pair emitter-follower transistors. It makes Q1 and Q3 are working with high current.

Some current passes R5 to D5. The voltage across it is the constant voltage, 0.6V.

Even a little current flows through R3, VR1, and R4. They are voltage divider circuits.

VR1, adjust the output voltage. When we adjust VR1 to control a bias current of the transistor Q2-BC337. It will control the bias current to the base of driver transistor-Q1.

To drive Q1(power transistor) runs in the full current, with an output voltage as we adjust, 0V to 30V.

You may like it

The Shopping lists

0.25W Resistors
R1, R2: 10K
R3, R4: 100 ohms
C1: 2,200µF 50V Electrolytic
C2,C3: 100µF 50V Electrolytic
Q1, Q2: BC337, 50V 800mA NPN Transistor
Q3: 2SC1061 or TIP41 or H1061 or MJE3055, 50V 4A NPN Transistor
D1-D5: 1N4002, 100V 1A Diode
VR1: 10K Potentiometer  
T1: 24V 1A transformer

Making this project

First, assemble all the components into the PCB layout below. Although it is a small circuit. But always be careful.


PCB layout of Simple Variable power supply 0-30V 1A

Second, Enter the power into the circuit. Then uses a voltmeter to measure the voltage output.

Third, adjust the VR1, then look at the meter should change as we adjust it.

Fourth, Test to connect the load to the circuit, The output voltage should be a constant voltage and does not change.

Do not forget. The Q3-C1061 must hole with a proper heat sink.

Other reads: The meaning of variable power supply on Wikipedia

0-30V Variable Power Supply at 3A

0-30V Variable Power Supply at 3A

Let’s create a 0-30V 3A Laboratory DC Power Supply. The high-efficiency variable regulator that new circuit design using UA723 and TIP3055. Read more: 0-30V Power Supply

0-50V 3A Variable Power supply

0-50V 3A Variable power supply

Let’s build a 0-50V Variable power supply circuit, 3A. can adjust output 0V to 50V with overcurrent protection at 3 A. Easy Linear supply. Read more: 0-50V power supply

LM317 Power Supply (My First Circuit)

LM317 variable power supply

This is First Variable DC Power Supply 1.2V to 30V 1A using LM317. using a few parts and build easy with PCB layout and can fine adjust voltage output. Read more LM317 Power Supply


I always try to make Electronics Learning Easy.

Get Ebook: Simple Electronics Vol.4

38 thoughts on “Simple Variable power supply circuit”

  1. #Variable power supply 1A, 0-30V

    Hi Van Boni..Yes The Transformer will Take The Current it will need!!! You will then have a 2-1 or 1A of Headroom

  2. #0-20V Variable power supply circuit

    Capacitors comes in different voltage values in 1000uf capacitance, say like 1000uf,16v , 1000uf 36v , ……..upto 250v I’ve seen till now . They may be available more than 250V also!.

  3. #0-20V Variable power supply circuit

    Here the maximum voltage is 24 v as per the transformer output so you can use capacitor voltage value more than 24v. (The voltage mentioned on the capacitor means the maximum voltage that capacitor can bear)

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

  5. 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.

    • Hello Mahar Falcutila,

      Thank you for visiting and interested in this project.
      It is 24V 1A transformer.

      If your project is successful. Don’t forget to share it with me, thank you.

  6. 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.


      • 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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