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:

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 = V_in – V_be

where,

• out is the output voltage that is present at the circuits output terminal.
• V_in is the input 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 present at the circuits output terminal.
• V_in is the voltage that is applied to the emitter follower circuits input terminal.
• A_v is the emitter follower circuits voltage gain

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:

• 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:

• After turning on the circuit measure the output voltage across the MJ2955s emitter and the positive wire with a multimeter.
• To change the output voltage within the specified range adjust the potentiometer.

Testing and Troubleshooting:

• Make the use of a multimeter to confirm the output voltage.
• Look for any problems if any such as high heat or variations in voltage.
• Detect issues and fix them if necessary.

Important Considerations:

• Make sure the transistor pin configurations are right by checking them twice.
• Diode and capacitor polarity should be observed.
• Check for short circuits and make sure soldered connections are safe.
• While testing the circuit start with a lower input voltage to prevent harm.

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