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Simple Adjustable Power Supply Circuit using Two Transistors

The described circuit is an simple adjustable power supply that uses two transistors BD140 and MJ2955 to regulate the output voltage.

What is a Adjustable Power Supply Circuit:

The adjustable power supply circuit is an electrical circuit that uses two transistors to control and modify the output voltage.

In many electrical and electronic activities when a particular voltage level is required this circuit is useful.

It is commonly used to supply an output of constant DC voltage.

Circuit Working:

Parts List:

Component TypeComponent DetailsQuantity
Resistor1k 1/4 W CFR1
Potentiometer10k 1
CapacitorElectrolytic 1000µF 50V1
SemiconductorsTransistor MJ29551
Transistor BD1401
Diode 1N40071
Transformer 0-25V 1 Amp1

The voltage is regulated by the BD140 transistor.

The potentiometer connected to the MJ2955 transistors emitter controls its base voltage.

Maintaining a steady voltage drop between its base and emitter is the aim of the BD140 transistor.

Adjusting the potentiometer causes the BD140s base voltage to fluctuate, which modifies the output voltage.

The output voltage is controlled by the BD140 and the MJ2955 transistor.

A similar push pull arrangement is created when the base of the MJ2955 and the collector of the BD140 are joined.

This configuration provides with an steady output voltage.

The MJ2955 transistors emitter provides the adjustable output.

The output voltage and base emitter voltage of the BD140 are controlled by the voltage across the potentiometer.

With this circuit one can use the potentiometer to change the output voltage within certain limits.

Between the emitter of the MJ2955 and the positive line it produces a controlled DC output voltage, the potentiometers setting controls the voltage range.

Formulas:

Following are the formulas for emitter follower transistor circuit to get the output voltage:

Estimated Output Voltage:

One can figure out the output voltage Vout by deducting the input voltage Vin from the base emitter voltage Vbe.

This is because silicon transistors generally have an emitter voltage that follows the base voltage with a small decrease of about 0.7V.

out = Vin – Vbe

where,

Vout = Av * Vin (where 0.8 < Av < 0.999)

where,

Crucial Information:

This is a simple calculation that misses things like transistor biasing conditions and load resistance.

For accurate output voltage calculations use SPICE simulation or advanced methods of analysis.

How to Build:

To build a simple adjustable power supply circuit using two transistors involves below mentioned connections:

Adjustment:

Testing and Troubleshooting:

Important Considerations:

Conclusion:

Keep in mind that this tutorial provides an overall view, actual assembly may differ based on the datasheets or changes of individual components.

Seek advice from experienced individuals or refer to appropriate tools if you have any questions concerning any component of the circuit or its installation.

Always put safety first and take care while handling electrical components.

References:

Datasheet MJ2955

Datasheet BD140

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