Adjustable or Variable Power Supply Circuit Diagram (0-30V)

Here in this article, we will see the circuit diagram of an Adjustable or variable power supply. We can adjust the output voltage level as per our requirements. This circuit will provide a 0 to 30V DC power supply with a 2A current. So you can use this circuit for laboratory use, battery charging, amplifier circuits, testing purposes, etc. So, let's see the circuit diagram and the connection between different components.

Variable Power Supply Circuit Diagram

Here, you can see the circuit diagram of the 0-30V variable power supply.

List of Components Used

• A 230V/24V center-tapped step-down transformer
• 2 Pcs 1N4007 PN Junction Diode
• 1 Pcs 2200uF, 36V Electrolytic Capacitor
• 1 Pcs 220uF, 36V Electrolytic Capacitor
• 1 Pcs 3.3 Kiloohm Resistor
• 1 Pcs 100 ohm Resistor
• 1 Pcs 0.3ohm, 5 watt resistor
• 1 Pcs 10 Kiloohm Potentiometer
• 1 Pcs 30V Zener Diode(1N972)
• 2 Pcs BC548 Transistors
• 1 Pcs 2n3055 Transistor
• 1 Pcs LED

Connection Procedure

1. Connect two 1N4007 Diodes to the output 24V terminals of the transformer and make short their cathode terminals and this will be the positive output of the rectifier.
2. The 0V terminal of the transformer will be the negative terminal of the rectifier.
3. Connect the 2200uF, 36V Capacitor in parallel with the output of the rectifier circuit.
4. Connect the LED across the output of the rectifier circuit in series with the 3.3 Kilo Ohm resistor.
5. Connect the Zener diode in reverse bias across the output of the rectifier circuit through the series resistor of 100 ohms.
6. Connect the cathode terminal of the Zener diode to any one side terminal of the potentiometer.
7. Connect another side terminal of the potentiometer to the negative terminal of the rectifier output.
8. Connect the emitter terminal of the first BC548 transistor to the base terminal of the transistor 2n3055.
9. Connect the emitter terminal of transistor 2n3055 to the base terminal of the second BC548 transistor.
10. Connect the collector terminal of the 2n3055 transistor to the collector terminal of the first BC548 transistor and finally connect it to the positive terminal of the rectifier output.
11. Connect the Base terminal of the first BC548 transistor to the negative terminal of the rectifier output in series with a 220uF, 36V Transistor.
12. Connect the collector terminal of the second BC548 transistor to the middle terminal of the potentiometer.
    
13. Connect a 0.3ohm, 5w resistor between the base and emitter terminal of the second BC548 transistor.
14. Now get the output power supply across the emitter terminal of the second BC548 transistor and the negative terminal of the rectifier output.

Operation and Working Principle

Here, the step-down transformer steps down the 230V AC voltage into 24V-0V-24V AC. And the two PN junction diodes converts the AC power supply into a DC power supply. The capacitor 2200uF connected in parallel with the output of the rectifier circuit removes the ripples from the DC power supply. The LED is used for the indication purpose whether the circuit has power or not. The 30V Zener diode is used to maintain the maximum 30V output voltage. 

The transistor BC548 and the transistor 2n3055 are connected in the Darlington mode. This is because to achieve the 2A load current. And the second transistor BC548 is used to control or vary the output voltage. When we operate the potentiometer the base current of that transistor changes so the emitter current also changes, which causes to change in the voltage drop across the 0.3 ohm resistor connected between the base and emitter terminal, so the output voltage of the circuit also changes.

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