What are transistors and thyristors?

• NPN transistors are key components in electronics, often used as switches or to amplify signals from sensors.
• They have three legs: the base (middle), collector (positive voltage), and emitter (ground/negative voltage).
• Applying a small current or voltage to the base allows a larger current to flow from the collector to the emitter, turning the transistor on.
• Thyristors are semiconductor devices that act as switches, allowing a large current to flow from the anode to the cathode when a small current is applied to the gate.
• They have three legs: the gate (control), anode (positive), and cathode (negative).
• Once activated by a small current at the gate, the thyristor remains on until the power is turned off, making it a latching device.

A transistor is a key component in electronics, often used as a switch or to amplify a signal from a sensor. NPN (negative-positive-negative) transistors are the most used transistor.

An NPN transistor has three legs. In circuit diagrams or symbols, the legs of an NPN transistor can be identified as follows:

• Base (B): Usually located in the middle.

• Collector (C): Often connected to the positive voltage.

• Emitter (E): Typically connected to the ground or negative voltage.

• when a small current or voltage is applied to the base (acting as a switch), it allows a larger current to flow from the collector to the emitter

• this means that the transistor is “on” when there is a voltage of 0.6V at the base, allowing current to pass through the other two legs

1. Operation: When darkness falls on the light dependant resistor (LDR), a current flows through the 2.2kΩ resistor into the base of the transistor. This activates the transistor (switches on), allowing current to flow from the collector to the emitter, and the lamp lights up.
2. Protection: The 2.2kΩ resistor in the circuit protects the transistor from being damaged by too high a voltage or current. The transistor needs 0.6V on the base leg to switch on.

A thyristor is a semiconductor device that acts as a switch. It allows a large current to flow from the anode to the cathode when a small current is applied to the gate.

Once activated, the thyristor remains on until the power is turned off. This makes it a latching device, ideal for alarm circuits.

A thyristor has three legs:

1. Gate (G): the control leg that activates the thyristor
2. Anode (A): the positive leg
3. Cathode (C): the negative leg

It will conduct a forward current (in the direction of the triangle – from the anode to the cathode) once a small trigger of current is detected at the gate leg.

Once the thyristor starts to conduct, current continues to flow until the voltage between the anode and cathode pins is reduced to zero.

Thyristors are commonly used in alarm circuits, motor speed controls, light dimmers, and power switching applications.

They provide efficient control of high power and can handle large currents and voltages but can be sensitive to voltage spikes and require careful handling to avoid damage.

You can see an example of a thyristor in a circuit in How a thyristor acts as a latching switch.