Simple 12V transistor switching power supply

This is Simple 12V transistor switching power supply circuit. One of my friends asked. Can we build the switching DC power supply circuit with 2 transistors? The applications IC that Sometimes is difficult. A Some number IC is hard to find and very expensive. When considering Applications for some time.

We do not need to use a high current. and have sufficient space to install circuits. The selection uses the transistors. Therefore as an alternative to saving, and extremely value for money.

How it works
In Figure 1 it is the design of the switching regulator, which is a step-down dc converter converts the voltage from 15-20 volts to a voltage of 12 volts.

Simple 12V transistor switching power supply circuit
Figure 1 The switching DC regulator from a step down converter circuit

The important components are T1 act as switching and have common components. The main is L1, D1 transistors, NPN+PNP Acted to feedback to each other. Increased frequency generators or to Makes the switch to run continuously. Only in this circuit of the coil is not up because there is a voltage detector with C2, R4 and Zener diode D2(C12) to control the voltage

When we apply DC current to this circuit the transistor T2 causes bias. Because the current flow through R5 to bias makes T2(BC550) into the current conduction Impact to the T1.(2N2905). It makes T1 has current flow through the emitter pin to base pin.

Then, the current pass through the base of Q2(BC550). This case therefore equivalent to Q2(BC550) a unit to control bias T1. It makes Q1(2N2905) apply current to the collector to the coil L1.

While the base of Q2 has a fixed reference Zener diode to keep the voltage constant
across emitter and ground or the output. We need 12V fixed regulator so we use a 12V Zener diode.

When the output voltage rises to 12 volts. As a result, to Makes up with the emitter voltage of T2. It is the same point. As a result, T2 turns off. Stop conduction. And, providing T1 stop conduct current, too.

Now the coil will start providing current., To accumulate at C1 with a rectifier D1(BTA43). And when compared an electrical potential to ground. To the left of L1 negative voltage and the current of L1 has gone. Working new round will occur.

If we consider this system. Found that the performance of this circuits by more than 90% of quite.


We do not need to design a PCB for this circuit. If you do not want to design own PCB. Or use universal PCB Board that difficult.
I would recommend a DC Converter Power Supply Buck Step Down Regulator In:4-40V Out:1.5-35V. It is a DIY Kits available for use. I hope this guide is helpful for you.


Although the circuits are is not the same. It can be switching DC regulator as well.

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Hi !

Great job you’ve done. Can you explain to me the purpose of R3,R4,C2. Its not very clear to me.

Thank You


what’s function of R3, R4 & C2? i tried remove them and then the circuit still work well.
is it step-down circuit or just linear circuit?


is bta43 typographical error? can i use schottkey bat43 for the diode? or is there any alternative for bta43? thanks!

YK Chan

This is my feedback after tried out the 12V output switching regulator physically. I replace the inductor value from 220uF to 220uH.

(1) it is a linear 12V regulator at quiescent state, when resistive loaded it become a relaxation oscillator and not a switching regulator.

(2) a switching regulator has switching format change to compensate load variance that this design is missing, such as PWM, PPM or the like.

This has prompted me to build test my own self reg. with a complementary pair.

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