Simple Adjustable Power Supply Circuit using Two Transistors

This Simple Adjustable Power Supply Circuit using Two Transistors is a simple power supply circuit.

It can change the output voltage by using two transistors like BD140 and MJ2955 to control the voltage.

What is a Adjustable Power Supply Circuit:

Adjustable power supply circuit is an electric circuit which uses two transistors to change and control output voltage.

Many time in electronic work we need one fixed voltage and this circuit help in that.

Mostly it gives steady DC voltage at output.

Circuit Working:

Parts List:

BD140 transistor control the voltage.

Potentiometer is connected to MJ2955 emitter and it changes base voltage of transistor.

BD140 try to keep same voltage between base and emitter.

When we turn potentiometer BD140 base voltage changes so output voltage also change.

Both BD140 and MJ2955 work together to control output voltage.

BD140 collector connect to MJ2955 base and this make push-pull type setup.

This setup give stable DC voltage at output.

MJ2955 emitter give the adjustable output voltage.

Voltage across potentiometer control BD140 base-emitter voltage and output voltage.

By turning potentiometer we can change output voltage but only inside one limit.

This circuit gives controlled DC output between MJ2955 emitter and positive line.

Potentiometer setting decide how much voltage comes out.

Formulas:

Below is formulas for emitter follower transistor circuit to get output voltage:

Estimated Output Voltage:

We can find output voltage Vout by subtracting base-emitter voltage Vbe from input voltage Vin.

This happen because in silicon transistor emitter voltage is almost same like base voltage but little low which is around 0.7V less.

Formula:

Vout = Vin – Vbe

where:

• Vout is voltage at output terminal.
• Vin is input voltage we give to circuit.
• Vbe is voltage drop between base and emitter which is normally around 0.7V for silicon transistor.

Another way to write:

Vout = Av × Vin

Where Av is gain and value is between 0.8 and 0.999

where:

• Vout is output voltage.
• Vin is input voltage.
• Av is voltage gain of emitter follower circuit which is less than 1.

Important Note:

These are easy formulas but they do not count things like transistor bias or load resistance.

To get exact output voltage we need more full calculation.

How to Build:

To make Simple Adjustable Power Supply Circuit using Two Transistors follow below steps:

Connections:

• First put 25V AC transformer on breadboard.
• Then connect 1N4007 diode to change AC to DC.
• Add 1000uF capacitor to smooth the DC voltage.
• Connect BD140 transistor emitter to positive side of filtered DC which is around 30V.
• Use 10k potentiometer and connect BD140 base to MJ2955 emitter through it.
• Also connect BD140 base to +25V line using 1k resistor.
• Connect BD140 collector to ground using another 1k resistor.
• Connect BD140 collector to MJ2955 base.
• MJ2955 collector connects to ground.
• Output voltage we take from MJ2955 emitter and positive line.

Adjustment:

• After switching ON check output voltage between MJ2955 emitter and positive wire using multimeter.
• To change voltage turn the potentiometer.

Testing and Troubleshooting:

• Use multimeter to check output voltage always.
• Watch for problem like voltage going up down or too much heat.
• If something is wrong then check and fix circuit problem.

Important Things:

• Check transistor pins again and be sure they are correct.
• Diode and capacitor polarity must be correct which is very important.
• Look at soldering it should be safe with no short circuit.
• Start with low input voltage first for safety.

Conclusion:

Simple Adjustable Power Supply Circuit using Two Transistors is just a basic idea.

Actual design may change based on parts or datasheet.

If anyone is confused ask experienced person or check tools or manuals.

Always be careful and safe while working with electrical parts. handling electrical components.

References:

Datasheet MJ2955

Datasheet BD140