Relays
Physics: Cambridge International GCSE Supplement, topic 4.13.3
Circuit Components: Relays
A relay is a switch that is operated by an electromagnet.
How they work
- When a small current passes through the electromagnet, the soft iron arm is attracted to the electromagnet. This arm pushes against a switch to close it.
- When the current stops, the arm moves back and the switch opens again.
- The switch in a relay can be normally open or normally closed.
/high-voltage%20-%20Edited-min,w_800.jpg)
Using relays
- In a relay, the circuit containing the switch is separate from the circuit containing the electromagnet.
- The circuit with the electromagnet can have a small electric current. However, this circuit can switch a circuit with a dangerously high electric current on or off.
- Relays can be used alongside light dependent resistors (LDRs) and thermistors to create environment sensitive switches.
/4.3.3.5%20-%20Light-sensitive-switch-picture-min,w_800.jpg)
Making a light sensitive switch
- A light dependent resistor (LDR) is connected in the same circuit as the electromagnet.
- When light shines on the LDR, its resistance falls and the electric current through the electromagnet increases.
- The increase in current turns the electromagnet on and activates the switch in the other circuit.
/fire-alarm-min,w_800.jpg)
Making a temperature dependent alarm
- A thermistor is connected in the same circuit as the electromagnet.
- When the thermistor heats up, its resistance falls and the electric current through the electromagnet increases.
- The increase in current turns the electromagnet, and the circuit containing the alarm on.