A relay is an electrically operated switch. Current flowing through the coil of the relay creates a magnetic field which attracts a lever and changes the switch contacts. The coil current can be on or off so relays have two switch positions and most have double throw (changeover) switch contacts as shown in the diagram.Relays allow one circuit to switch a second circuit which can be completely separate from the first.
For example a low voltage battery circuit can use a relay to switch a 230V AC mains circuit. There is no electrical connection inside the relay between the two circuits, the link is magnetic and mechanical.

RELAY CONSTRUCTION AND WORKING

Relay

The coil of a relay passes a relatively large current, typically 30mA for a 12V relay, but it can be as much as 100mA for relays designed to operate from lower voltages. Most ICs (chips) cannot provide this current and a transistor is usually used to amplify the small IC current to the larger value required for the relay coil. The maximum output current for the popular 555 timer IC is 200mA so these devices can supply relay coils directly without amplification.
Relays are remote control electrical switches that are controlled by another switch, such a computer as in power train control module.Relays allow a small current flow circuit to control a higher current circuit.Several designs of arrays are in use today,3-pin,4-pin,5-pin and 6-pin,single switches or dual switches.

All relays operate using the same basic principle.The example below uses a 4-pin relay.Relays have two circuits:
1)Control circuit(in green)
2)Load circuit(in red)
The control circuit has a small control coil and the load circuit has a switch. The coil controls the operation of the switch.

ON CONDITION
Current flowing through the control circuit coil creates a magnetic filed and causes the switch to close .The switch is a part of load circuit and is used to control an electrical circuit that is connected to it and hence current flows.This is energized state.

OFF CONDITION
When current flows through the control circuit, the relay becomes de-energized.The switch opens as there is no magnetic field and hence no current flows through the load circuit.

Most relays are designed for PCB mounting but you can solder wires directly to the pins providing you take care to avoid melting the plastic case of the relay.

CONNECTING A RELAY
Relay appears somewhat complex to connect in the circuit but is not so.
The supplier’s catalogue should show you the relay’s connections. The coil will be obvious and it may be connected either way round. Relay coils produce brief high voltage ‘spikes’ when they are switched off and this can destroy transistors and ICs in the circuit. To prevent damage you must connect a protection diode across the relay coil.

The relay’s switch connections are usually labelled COM, NC and NO:
COM = Common, always connect to this, it is the moving part of the switch.
NC = Normally Closed, COM is connected to this when the relay coil is off.
NO = Normally Open, COM is connected to this when the relay coil is on.

Connect to COM and NO if you want the switched circuit to be on when the relay coil is on.
Connect to COM and NC if you want the switched circuit to be on when the relay coil is off.

Advantages of relays:

• Relays can switch AC and DC, transistors can only switch DC.
• Relays can switch higher voltages than standard transistors.
• Relays are often a better choice for switching large currents (> 5A).

Disadvantages of relays:

• Relays are bulkier than transistors for switching small currents.
• Relays cannot switch rapidly (except reed relays), transistors can switch many times per second.
• Relays use more power due to the current flowing through their coil.