How do you use transistors and thyristors in circuits?

• An NPN transistor is an electronic component used as a switch or amplifier. It has three legs: base (B), collector ©, and emitter (E).
• In circuit diagrams, the base is usually in the middle, the collector is connected to positive voltage, and the emitter is connected to ground or negative voltage.
• An NPN transistor acts as a switch when a small current or voltage is applied to the base. This allows a larger current to flow from the collector to the emitter.
• The transistor switches on with a Base voltage of around 0.6V to 0.8V, allowing current to flow from the collector to the emitter.
• A thyristor is an electronic component used in high-power applications. It has three legs: gate (G), anode (A), and cathode ©. When a small current is applied to the Gate, it allows a larger current to flow from the Anode to the Cathode.
• Once activated, the thyristor remains on until the power is turned off or the circuit is reset. Thyristors are commonly used in motor speed controls, light dimmers, and power switching applications.

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.

In circuits, transistors act as switches or amplifiers, controlling current flow based on the voltage at their base. They switch on with a base voltage of around 0.6V to 0.8V. Circuit diagrams often show transistors in switching circuits.

For an ideal NPN transistor, the switch-on voltage at the base is typically 0.6V (acceptable range is 0.6V to 0.8V). This means that when the base voltage reaches this level, the transistor turns on, allowing current to flow from the collector to the emitter.

How the transistor works as a switch in this circuit:

1. Overview: the transistor is part of a low voltage circuit (5V) and when activated it can switch on the relay, which in turns activates a separate higher voltage circuit containing a 24V lamp.
2. Base activation: when the PTM is pressed, it makes the PIC input pin go high. This then makes the output pin of the PIC activate (due to the flowchart programming) which creates the 0.6V needed to switch on the base leg of the transistor.
3. Transistor on: current can now flow from collector to emitter, activating the relay. Due to the electromagnet in the relay, it then switches on the higher voltage circuit. A diode is included across the relay to prevent back EMF (electromagnetic fields).
4. Lamp lights: now that the higher voltage circuit is activated, the 24V lamp can 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.

A thyristor acts as a latching switch, meaning once it is turned on by a small current at the gate, it remains on even if the gate current is removed. The thyristor will stay on until the power is turned off or the circuit is reset. This makes it useful in applications where you need a switch to stay on without continuous input, for example an alarm.

Consider a simple alarm circuit using a thyristor:

1. Operation: when the Push To Make (PTM) switch is triggered, a small current flows into the gate, activating the thyristor. This allows a larger current to flow from the anode to the cathode, sounding the buzzer
2. Latching: the buzzer continues to sound even if the PTM switch is no longer triggered, due to the latching nature of the thyristor. The alarm can only be reset by turning off the power or by pressing the 'reset' PTM switch