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:

An Adjustable Power Supply Circuit using Two Transistors is a type of electronic circuit that allows for the regulation and adjustment of output voltage using two transistors.

This circuit is often designed to provide a variable DC voltage output, and it can be useful in various electronics and electrical projects where a specific voltage level is required.

Circuit Working:

Parts List:

The BD140 transistor acts as a voltage regulator.

Its base voltage is controlled by the potentiometer connected to the emitter of the MJ2955 transistor.

The BD140 transistor tries to maintain a constant voltage drop between its base and emitter.

As the potentiometer is adjusted, the base voltage of BD140 changes which in turn, adjusts the output voltage.

The MJ2955 transistor in conjunction with BD140 helps in regulating the output voltage.

The collector of BD140 is connected to the base of MJ2955 forming a complementary push pull configuration.

This arrangement helps in providing a stable output voltage.

The adjustable output is taken from the emitter of the MJ2955 transistor.

The voltage across the potentiometer controls the base emitter voltage of BD140 and consequently, the output voltage.

This circuit allows you to vary the output voltage within certain limits using the potentiometer.

It provides a regulated DC output voltage between the emitter of MJ2955 and the positive line with the voltage range determined by the potentiometer setting.

Formula:

Formula for emitter follower transistor circuit to get the output voltage:

Estimated Output Voltage: The base emitter voltage Vbe less the input voltage Vin yields an approximate value for the output voltage Vout.

This is due to the fact that, in silicon transistors the emitter voltage normally follows the base voltage with a tiny decrease of about 0.7V.

out = V_in – V_be

where,

• out: output voltage, is the voltage that is present at the circuits output terminal.
• V_in: or input voltage, is the voltage that is applied to the emitter follower circuits input terminal.
• V_be: is the voltage drop across the transistors base and emitter terminals in a circuit.

V_out = A_v * V_in (where 0.8 < Av < 0.999)

where,

• V_out: voltage output, is the voltage that is present at the circuits output terminal.
• V_in: input voltage, is the voltage that is applied to the emitter follower circuits input terminal.
• A_v: This is the emitter follower circuits voltage gain, a voltage gain of one would be ideal for an emitter follower indicating that the output and input voltages are equal.

Crucial Information:

This is a simplified analysis that ignores elements such as load resistance and transistor biasing conditions.

Use more sophisticated analysis techniques or SPICE simulation for accurate output voltage calculations.

How to Build:

Building the simple adjustable power supply circuit using two transistors involves below mentioned connections:

• Connect the 25V AC transformer to the breadboard.
• Connect the 1N4007 diode to rectify the AC voltage.
• Connect the 1000uF capacitor for smoothing the rectified DC voltage.
• Connect the emitter of the BD140 transistor to the positive side of the filtered DC voltage around 30V.
• Connect the base of the BD140 to the emitter of the MJ2955 through a 10k potentiometer.
• Connect the base of BD140 to the positive 25V line through a 1k resistor.
• Connect the collector of BD140 to ground through a 1k resistor.
• Connect the collector of BD140 to the base of the MJ2955 transistor.
• Connect the collector of MJ2955 to the ground.
• The adjustable output is taken from the emitter of MJ2955 and the positive line.

Adjustment:

• Power up the circuit and use a multimeter to measure the output voltage across the emitter of MJ2955 and the positive line.
• Adjust the potentiometer to vary the output voltage within the desired range.

Testing and Troubleshooting:

• Verify the output voltage using a multimeter.
• Check for any abnormalities such as excessive heat or voltage fluctuations.
• Troubleshoot and make adjustments as needed.

Important Considerations:

• Double check the transistor pin configurations and ensure they are connected correctly.
• Pay attention to the polarity of capacitors and diodes.
• Ensure that soldered connections are secure and free from short circuits.
• Start with a lower input voltage when testing the circuit to avoid damage.

Conclusion

Remember that this guide provides a general overview, and actual assembly may vary based on specific component datasheets or modifications.

If you are unsure about any part of the circuit or its assembly seek advice from experienced individuals or consult relevant resources.

Always prioritize safety and handle electronic components with care.

References

Datasheet MJ2955

Datasheet BD140