I am going to show you an old circuit that 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.

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.

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.

If you want higher current and efficiency. 

• See 0-30V 3A Variable power supply circuit.

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.

• Learn how DC regulated power supply circuit works

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 a voltage divider circuit.

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.

Recommend: 0-20V 1A Adjustable DC Supply

The Shopping lists

0.25W Resistors
R1, R2: 10K
R3, R4: 100 ohms
Capacitors
C1: 2,200µF 50V Electrolytic
C2,C3: 100µF 50V Electrolytic
Semiconductors
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
Others
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, does not change.

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

Here are a few related posts you may find helpful, too:

• Simple 0-30V 2A Variable DC Supply for beginner
• How Op-amp 741 adjustable regulator circuit works
• 0-50V 3A Power supply project

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