Simple constant voltage circuit using small transistors

If you maybe like to learn about a simple regulator circuit like me. I am showing you, tiny constant voltage circuit using two small transistors. You can use many Zener voltage regulators. But this circuit is special because putting the diode in a different position, and adjustable output voltage.

Simple constant voltage circuit using small transistors

How does the circuit work

In the circuit above, there are two transistors and a few components, Zener diode, three resistors, a capacitor, and a potentiometer.

It is easy to understand them.

First of all, the current flows through R1, VR1, and R2 to the ground.

The Q1-2N3904 get the bias current. It is working look like the amplifier transistor model in common-emitter.

Which makes the voltage across between collector and ground, equal the voltage across the base to ground, combined with the BE voltage(0.6 V) to VZD1 (6.2 V).

Then, VR1 adjusts the output voltage.

This circuit has a temperature coefficient is very little, almost zero.

The joint base-emitter of Q1 with a temperature coefficient of -2 mV per Degree Celsius. The zener didoe-ZD1 has the temperature coefficient of +2 mV per degree Celsius. To reduce the effect of temperature on the circuit.

When Q2 get a bias current from Q1.

The Q2 helps to drive the more output current. But this event makes the output voltage to dropdown.

However, the output of this regulator is your adjustable voltage of 6.8V to 15.6V.

The R3 and C1 are RC filters to reduce less ripple voltage.

But this circuit can deliver the output current just less than 100 mA current only.

It is suitable for the load that requires a low current.

Below is the circuit like this one.

The tiny constant voltage circuit using two transistors

Something is different, ZD1 voltage and number of the transistor. Which you can use them many NPN transistors such as 2SC1815, 2SC828, BC548, BC549 and more.

I hope you are satisfied with this circuit.

‘Keep reading: Simple step down 12v to 9v converter circuit’ »

Sharing is caring!

How to learn electronics with simple ways

Related Posts


I always try to make Electronics Learning Easy.

Leave a Reply

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

newest oldest most voted
Notify of

The circuit is perfect but still there is no resistors wattage.

Close Menu