Relays and Diodes

Overview

In this lesson, you meet components that make circuits more useful and more selective. A relay lets a small current control another circuit, while a diode only allows current in one direction.

What You Need to Know

• A relay is an electrically operated switch. A current in the coil creates a magnetic effect that opens or closes another circuit.
• A diode allows current in one direction only, so it behaves differently from a normal resistor.
• An LED is a diode that emits light when current passes through it in the correct direction.
• The current-voltage graph for a constant resistor is a straight line through the origin.
• The current-voltage graph for a filament lamp curves because the resistance changes as it heats up, and the graph for a diode shows current only after forward bias is reached.

How to Work Through It

1. Start by identifying what each symbol means in a circuit diagram.
2. Compare a simple switch, a relay, and a diode so you can see what makes each one useful.
3. Study the three current-voltage graphs side by side and explain why they are different.
4. Finish with short questions that ask which component would be used for control, protection, or one-way current.

Check Your Understanding

• Why is a diode different from an ordinary resistor when the current direction is reversed?
• What makes a relay useful in a control circuit?
• Can you match each current-voltage graph to the correct component and explain your choice?

Common Mistakes

• Mixing up a relay with a normal switch. A relay uses one circuit to control another.
• Assuming a diode behaves like a constant resistor. Its graph shows that it does not.
• Forgetting that LEDs and diodes must be connected the correct way round to work properly.

Next Steps

• Use the diode past-paper questions to practise graph interpretation.
• Carry the idea of circuit behaviour and safe design into the final lesson on electrical safety.