3.7 - Relays & Capacitors

A relay is a switch operated by an electromagnet

• A relay is an electrically operated switch that allows a low-current, low-potential-difference circuit to control a separate high-current, high-potential-difference circuit.
• The two circuits are electrically isolated, protecting both the user and the low-current circuit components from high currents and voltages.
• Example: a car's starter motor needs a very high current, but the driver only turns a key in a safe, low-current circuit isolated by a relay.

How a relay works

1. Closing the switch in the low-current circuit sends current through the relay coil (an electromagnet), generating a magnetic field.
2. The magnetic field attracts an iron contact, which closes the switch in the high-current circuit.
3. Opening the low-current switch turns off the electromagnet, so the high-current switch springs open and breaks the high-current circuit again.

Capacitors store charge

• A capacitor is a component that stores electric charge.
• Connecting a capacitor to a power supply (such as a battery) causes charge to build up on it.

Charging

• The charging current decreases as the capacitor charges.
• As more charge is stored, the potential difference across the capacitor increases.
• When the potential difference across the capacitor equals the battery's, current stops and the capacitor is fully charged.

Discharging

• When the battery is removed and the circuit is completed, the capacitor discharges.
• The discharging current decreases over time in the same way as the charging current, but flows in the opposite direction.

Uses of capacitors

Capacitors store only a small amount of charge and can't provide a steady current, but they can release charge very quickly (e.g. in a camera flash).

Loudspeakers, microphones and motors convert energy

• Loudspeaker - Converts electrical energy into sound energy by turning electrical signals into vibrations of a cone.
• Microphone - Converts sound energy into electrical energy when sound waves vibrate a diaphragm, generating electrical signals.
• Motor - Converts electrical energy into kinetic energy in a rotating coil, often wasting some energy as thermal energy and sound.