In one of the previous articles, we had already discussed How a transistor acts as a switch? Here is another example of bipolar junction transistor BJT switching. In this case, we will study a high voltage switching circuit i.e. how a transistor act as a switch for cutting the flow of current when a high voltage flows. Today BJT switches are common in many electronic circuits and control systems. The main application of the transistor acting as a switch is in the automation field. This example article explains how a transistor can control a DC motor. Animated working of the transistor switch is also given for better understanding.
Circuit Diagram of High Voltage Switching Circuit
- Transistor (BC548)
- Resistor (1kΩ)
- Relay (6V)
- The example external circuit contains a DC motor. It gets power from the Vcc via the Normally Closed (NC) terminal of the relay.
- Resistor R1 is a current-limiting resistor that protects the transistor BC548 from overcurrent.
- Consider no high voltage is applied to the transistor, then the transistor becomes OFF and no current will flow through it. Hence the relay remains in an OFF state.
- Power to the DC motor is fed from the Normally Closed (NC) terminal of the relay. So the motor will rotate when the relay is in an OFF state.
- The application of a high voltage at the base of BC548 causes the turning ON of the transistor and the relay coil to energize.
- Thus the NC terminal is disconnected from the motor, it cuts the flow of current through the DC motor.
Transistor is more than a switch, a good tool to control different components. Here we have seen how to make a DC motor control circuit using a transistor.