Simple 5V and 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.

Small 12V switching power supply circuit

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.

What is more?

See higher current transistor switching power supply circuit here.

5V switching regulator circuit using transistor BC337

This is 5V switching regulator circuit using a transistor, decreases the size voltage or Step down voltage converter circuit. Make voltage output there is the size voltage a little more input at from circuit picture will decrease volt 6-18V from be left 5V. It gives current get 100mA. in working in switching forms The strategy works as follows.

Step down voltage converter 5V using transistor BC337

The old circuit


The new circuit diagram

As soon as we enter the voltage to the transistor circuit Q1 and Q2 are connected as the astable multivibrator, the circuit will generate the frequency output at the collector of Q2 before sending to the base pin of Q3. Q3, which will act like the on-off switch, Q4 works by applying a voltage to determine the collector pin, But must pass before the R6 base of Q4.

When base pin of Q4 has low voltage will cause Q4 to begin working, C3 will charge through coil- L1. The output voltage will increase up continuously, but when have voltage more than 5 volts will results to Q5 working since have a voltage bias at pin base in way of ZD1. The transistor Q1 is stopped, the pin base is equivalent to the ground there. The circuit shutdown frequency out, Q4 will stop working.

Because the voltage at the base pin of Q4 through R5 output so not have voltage., Q5 so stop working. Then Q1 is working again. And the behavior, it would go like the first one. Cause the loop to do this. However, there are very high, thus making has output 5 volts to appear at it.


The components layout (non PCB)

Components list
Resistors size ¼W +5%
R1, R4, R7: 4.7K
R2, R3: 47K
R5: 1K
R6: 100 Ohms
C1: 0.0015uF 50V Polyester
C2: 0.01uF 50V, Polyester
C3: 470uF 16V, Electrolytic
Q1, Q2, Q3, Q5: BC548, 45V 100mA NPN Transistor
Q4: BC337, 45V 800mA PNP Transistor
D1: 1N4001, 1A 50V Diode
ZD1: 5.1V 500mW, Zener diode
Other components
L1-transformer toroidal core diameter 2.5-3.0 cm.
Copper wire size 0.4 mm.


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 to you.

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

Related Posts


I always try to make Electronics Learning Easy.


  1. 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

  2. Jame

    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?

  3. marvin

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

  4. 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.

Leave a Reply

Your email address will not be published. Required fields are marked *

This site uses Akismet to reduce spam. Learn how your comment data is processed.