Active devices in electronic circuits

Active devices in electronic circuits are components that can control the flow of electric current. These devices require an external energy source to function and are capable of amplifying or switching electronic signals. One such active device is the SCR (Silicon Controlled Rectifier).

What is an SCR?
- An SCR, also known as a thyristor, is a four-layer solid-state semiconductor device that is used to control high-power AC and DC circuits.
- It is a three-terminal device with a gate (G), an anode (A), and a cathode (K).
- The SCR acts as an electronic switch that can control the flow of current in a circuit.

Working principle of an SCR:
- The SCR allows current to flow from anode to cathode when a small current is applied to the gate terminal.
- Once the SCR is triggered, it remains conducting even after the gate current is removed until the current flowing through it drops below a certain threshold level.
- The SCR can be turned off by reducing the anode current to zero, also known as natural commutation, or by applying a reverse bias voltage to the anode terminal.

Applications of SCR:
- SCR is widely used in various electronic circuits and systems, including power control, motor control, lighting control, and voltage regulation.
- It is commonly used in dimmer switches, electric motor drives, power supplies, and battery chargers.
- SCR devices are capable of handling high currents and voltages, making them ideal for controlling electrical power in industrial and domestic applications.

Conclusion:
- Among the given options, the SCR is the only active device used in electronic circuits.
- Transformers are passive devices that transfer electrical energy between two or more circuits.
- Electric heaters are resistive loads that convert electrical energy into heat.
- Loudspeakers are transducers that convert electrical signals into sound waves.
- Therefore, the correct answer is option 'C', SCR.