Home Top Ad

Transistor Relay Driver

Share:

The Transistor relay driver circuit helps control high-power devices using a low-power signal. It is widely used in home automation, industrial controls, and automotive electronics. Its primary function is to use a small input signal to activate a relay (5V, 6V, or 12V), which can then switch on bigger loads like motors, lights, or other high-current devices.

Components List

The following components are required to make this project.

Designation Parts Quantity
RLY1 5V/6V/12V Relay X1
Q1 2N3904 NPN Transistor X1
R1 4.7kΩ 1/4W Resistor X1
R2 2.2kΩ 1/4W Resistor X1
D1, D2 1N4002 Diode X1

Circuit Diagram

The schematic of a transistor relay driver circuit is shown below.

Circuit Explanation

The input signal is applied to the base of the transistor (Q1) through a resistor (R1). This resistor limits the base current of the transistor Q1, protecting it from excessive current.

The 2N3904 is an NPN transistor. When the input signal is high (≥ 4V), the transistor gets biased and allows current to flow from the collector to the emitter.

A resistor (R2) is connected between the base of the transistor and ground. It ensures that the transistor turns off when there is no input signal by pulling the base to ground.

The relay coil is connected between the VCC and the collector of the transistor Q1. When the transistor is ON, the current flows through the relay coil, energizing it and changing the state of the relay contacts (NO, NC, and COM).

NO (Normally Open), NC (Normally Closed), and COM (Common) are the relay contacts. When the relay coil is energized, the NO and COM contacts close, while the NC and COM contacts open. When the relay coil is not energized, the NC and COM contacts are closed, while the NO and COM contacts are open.

A diode (D1) is connected in parallel with the relay coil. It is a Flyback Diode that protects the transistor from the high voltage spike generated when the relay coil is de-energized. Another, diode (D2) is connected across the emitter and the ground to provide additional protection for the transistor against voltage spikes.

No comments

If you have any doubts or questions, please let me know. Don't add links as it goes to spam. Share your valuable feedback. Thanks