5V Switching Regulator Circuit using transistors

This is a 5V switching regulator circuit using a transistor. It will decrease the voltage, which is also called a step-down voltage converter circuit. It is a switching converter regulator circuit that makes the output voltage smaller than the input voltage applied to this circuit.

In the circuit, the input voltage has a wide range from 6V to 15V, giving us an output of 5V at 100mA of current. Because it works in a switching mode.

Let’s learn how it works, as follows:

How it works

5V 100mA Step down Switching regulator using transistors
5V 100mA Step down Switching regulator using transistors

Learn more: What is switching power supply vs linear, how does it work?

When we enter power into the circuit, the current will flow into the transistors Q1, Q2, R1, R2, R3, R4, C1, and C2, which are connected together as the astable multivibrator. They will produce a frequency pulse signal output at pin C of Q2.

The pulse signal flows through a base of Q3. Which will act like the on-off switch (between C and E). When Q3 works, pins C and E are like a switch closed. The Q4 PNP transistor begins work because the base current flows from GND, C-E of Q3, and R6 into B of Q4.

The high current from VCC flows C-E of Q4 to charge C3 through L1. The output voltage will increase continuously.

But when the output has a voltage of more than 5V. Part of the current can flow through the R7 and ZD1-Zener diodes and into pin B of Q5.

As a result, Q5 works between its C and E pins, like a switch on to stop the working of Q1 in the astable multivibrator circuit. They do not produce any frequencies.

This causes none of the base current to flow through R4 into pin B of Q3, between its C-E pins, like a switch-off.

So there is no base current for Q4, causing it to stop conducting current. As a result, the output voltage drops below 5V again.

This causes Q5 to no longer receive base bias current, so the astable multivibrator circuit returns to producing frequency. And this kind of work will repeat continuously. at high speed. So we have a constant output voltage of 5V at all times.

Read also: DC to DC Buck converter working principle

How to build

For this circuit, we try to assemble it on the breadboard. Or you may solder the device pins directly according to the component layout below.

The components layout (without PCB)

The output voltage is set with a Zener diode. You can try changing it to 3V or 6V; D1 may try using the number 1N4007; it has the same effect.

Increasing the load beyond the current it can supply. The voltage then drops to 4.68V.

When the circuit is completed. We tested the voltage when there was no load, and it was 5V.  But when we added load, it exceeded the current it could supply. The voltage level will drop to 4.68V, which is normal.

Recommended: 5V 3A Linear Power supply circuits You can build it!

Components list

Resistors size 0.25W +5%
R1, R4, R7: 4.7K
R2, R3: 47K
R5: 1K
R6: 100Ω

C1: 0.0015µF 50V Polyester
C2: 0.01µF 50V, Polyester
C3: 470µF 16V, Electrolytic

Q1, Q2, Q3, Q5: BC548 or BC549, 45V 100mA NPN Transistor
Q4: BC327, 45V 800mA PNP Transistor
D1: 1N5819, 40V 1A, Schottky Barrier Diode
ZD1: 5.1V 500mW (1N5231), Zener diode
Other components
L1-transformer toroidal core diameter 2.5-3.0 cm.
Copper wire size 0.4 mm.


The circuit is relatively simple, and the output voltage can be controlled to be constant. But it’s not very stable. When compared to circuits that use ready-made ICs. The switching regulator IC will have better performance and more convenience. However, this time we will gain some knowledge.

In my experience even though the IC is very good, but transistors will always be good helpers for ICs. We should mix the old and the new as perfectly as possible.
If you would like to experiment with a better-designed transistor circuit, Please go here: 12V 0.5A Switching regulator circuit. You also change the zener diode to 5V like this circuit.

Or it is better if you use MC34063 as 5V 2A buck converter


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