Relays

A relay is n electromagnetically-operated remote-control switch in which a small current operates a switch which controls a much larger current. examples of relays in a vehicle are the horn relay, starter relay, headlight relay and so on. the relay is usually mounted in a location that forms a short path between the battery and the operating unit. A relay consists of a soft iron core wound with many turns of thin insulated wire. the coil is insulated from the core and one end of the coil is connected to the battery and the other is earthed through, for example, the horn button. There is a soft iron frame around the outside and the core has a springloaded armature across the top. One contact is carried on the movable armature which is connected to the frame and then underneath the battery terminal. the other contact is mounted on a fixed support which is connected to the "horn" terminal. when the horn button is pressed, current is fed to the fine winding around the central soft-iron core to the earthed base of the relay and so back to the earthed terminal of the battery. the armature is attracted towards the core and closes the contacts. therefore when the contacts close, the horn is connected directly to the battery. An advantage of the relay system is that the horn button wire has to carry only sufficient current to energize the relay winding (usually around 0.5-1amp), and therefore the horn button contacts remain in good condition almost indefinitely. another advantage is that the voltage drop in the main horn circuit is reduced because of the shorter length of cable required.
Relays usually have a wiring diagram printed on their cover to show how the relay can be used. Most relays usually have pin numbers on them to designate what they are usually connected to

86- positive side of control circuit

85- negative side of control circuit

30- battery supply for switched circuit

87a- normally closed switch circuit

87- other switch circuit

During one of our practical lessons we had to wire up a circuit and test our relay, we wired up a parallel 3-bulb circuit, the use of the relay was to see if the switching circuit of the relay would turn on all three light bulbs. After wiring up the circuit we had to measure available voltage at the following terminal points when the circuit is off and then when the circuit is on. 

CIRCUIT OFF

86- 12.5V

85- 12.5V

30- 12.5V

87a- 12.5V

87- 0.0V

CIRCUIT ON

86- 12.4V

85- 0.01V

30- 12.4V

87a- 0.0V

87- 12.4V

There are three terminals where the most voltage change is seen, these are terminals 85, 87 and 87a. This is because when the circuit is off it is a closed circuit so voltage flows through it, because no components are using an current, so when you turn the circuit on there is current being used by the components so there is voltage changes in the relay terminals.