12V Variable Power Supply Circuit Diagram

Introduction

A power supply is an essential component in any electronic circuit. It provides the necessary electrical energy required to power various devices or components. In this article, we will discuss a 12V variable power supply circuit diagram using 2N3055, BC547, and BC557 transistors. These transistors, along with a 15V zener diode, play a crucial role in regulating the output voltage and current of the power supply. Before diving into the circuit diagram and its specifications, let’s understand the functioning of these components.

The low cost 12v variable power supply shown below has an output voltage range of 1.5 V to 15 V at a maximum current of 500 mA. Regulation is better than 2% for output currents not exceeding 350 mA. Voltage adjustment is effected by a potentiometer and an acoustic overload indication is provided.

Circuit Diagram of 12V Variable Power Supply

This project can be designed using a few basic components. The circuit diagram of this project is shown below.

**Simple Variable 12V Power Supply Circuit Diagram**

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2N3055 Pinout

Components List of 12V Variable Power Supply

Following is the list of all components used in this project:

2N3055 Transistor x 1
BC547 Transistor x 2
BC547 Transistor x 2
15V Zener Diode x 1
1N4002 Diode x 4
2200uf/25v Capacitor x 1
22uf/25v Capacitor x 1
2.2uf/25v Capacitor x 1
220nf Capacitor x 1
1 Ohm/5W Resister x 1
1k Resister x 2
220 Ohms Resister x 1
10k Resister x 5
10k Potentiometer x 1
24v Buzzer x 1

Explanation of 12V Variable Power Supply

Diode D1 establishes a reference voltage of 15 V. Transistor T4 compares the voltage at the wiper of P1 with the output voltage. When this is 0.65 V higher than the set voltage, transistor T4, and hence transistor T2 is switched on. This removes the base current from the darlington power stage T3-T5 and bypasses it directly to ground. In this manner the output voltage of the supply is held at 0.65 V higher than the reference potential at the base of T4., which is derived from a 15 V zener diode D1.

The a.c. voltage at the 18 V, 1A winding of the transformer is rectified by a bridge rectifier and smoothed by capacitor C1. All the load current passes through the current sensing resistor R1. With R1 equal to 1.5 ohms, transistor T1 starts to conduct when the load currents exceeds about 500 mA.. When this happens, a simple acoustic overload alarm (Bz1) is actuated. Note that the exact level of actuation depends on the electrical specifications of the buzzer, which should be a 24 V, self oscillating type.

This 12v variable power supply is, in principle, not short-circuit protected although the use of a generous heat sink for T5 will enable the transistor to withstand the maximum dissipation of about 30 W for the few seconds that lapse before the supply is switched off.

Working of Major Components:

2N3055 transistor: The 2N3055 transistor is a popular power transistor widely used in various electronic applications. It is an NPN bipolar junction transistor (BJT) capable of handling high currents and voltages.
BC547 transistor: The BC547 transistor is a general-purpose NPN transistor used in low-power applications such as amplification and switching purposes. It exhibits decent performance at low frequencies and low voltages.
BC557 transistor: Similar to the BC547 transistor, the BC557 transistor is a general-purpose PNP transistor used in low-power applications. It is suitable for amplification and switching purposes in low voltage and low current circuits.
15V Zener Diode: A zener diode is a special type of diode designed to operate in the reverse-biased breakdown region, known as the zener breakdown region.

Conclusion of 12V Variable Power Supply

In conclusion, the 12V variable power supply circuit using 2N3055, BC547, and BC557 transistors provides a versatile and adjustable power supply solution for various electronic applications. The 2N3055 transistor, with its high current and voltage handling capabilities, acts as the main driver for the power supply, while the BC547 and BC557 transistors assist in regulating the output voltage.

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