1. Electromagnetic relay
Simple description of the relay

2. Description Electrically controlled switch Current through coil
Magnetises soft iron core
Soft iron armature moves
Contacts join
Come in many different sizes…speed of operation….number of contacts to be made maximum current

3. Normally Open [NO] Relay
No current flowing through the coil
Contacts are not made
Coil must be energised for switched circuit to work
Self explanatory!

4. Normally Closed [NC] Relay
No current flowing through the coil
Contacts are made
Coil must be energised for switched circuit to stop working
Self explanatory!

5. In a circuit Low current flows through coil Attracts armature
Switched circuit connected
Lamp lights
BUT Coil is an inductive load
Add protection diode
Prevent high voltage spike
The diode is reverse biased…when the switch is opened dI/dt is big so the voltage across the coil is big [damage the coil by driving big current] the diode allows the gradual change in current flow so dI/dt is small.

6. www…search for
Reed relay
Electromagnetic relay
Magnetic switch