A relay is a device that can be wired to switch between different circuits. They are very useful in many situations, and can be programmed to automatically turn on or off when a certain event occurs. This can include anything from the temperature of a sensor to a button being pressed.
Relays can be either square or round in shape, and come with two or three sets of contacts depending on the type you buy. The contacts on a relay can be designated as normally open (N.O.), normally closed (N.C.), or common (C).
Before electricity is applied to the coil, each of these switch positions is marked with a symbol or is labeled as “normally open”. For example, a rectangular symbol with two parallel lines is labeled as normally open.
When a small current flows through the coil, it creates an electromagnetic field that attracts one of the poles and pulls it to a particular position on the armature. The resulting movement of the poles mechanically turns the relay switches on or off.
This is where the term ‘electrical switch’ comes from. A relay is a mechanical device that acts as an electrical switch when energized with a signal or control voltage. It is very similar to a traditional switch in that the switch is connected to a power source, and the switch can be moved from an open position (ON) to a closed position (OFF).
However, unlike a normal switch, there is no physical movement or ‘flip’ between the ON and OFF positions of the relay. Instead, the magnetic field of the relay coil ‘pulls’ a relay pole to a particular position on the armature.
There are three contacts on the relay – the central pole, the N/O contact, and the N/C contact. The N/O contact is usually connected to a ground, and the N/C contact is usually connected to a positive voltage source.
A relay is also usually protected by a flywheel diode that clamps the reverse voltage across the coil to about 0.7V, dissipating any stored energy. This helps protect the switching transistor that is used in a relay.
Test the Relay Coil
A good way to test a relay coil is to use a multi-meter set up in continuity mode, and place a meter probe on each of the coil terminals. If the meter beeps or shows a digital reading, the coil is working. If it does not, then the coil is probably open or damaged, and a new one should be ordered.
If the contacts on a relay coil get damaged by arcing, they will eventually “weld” together and cause the volt drop across the contacts to increase significantly. This could be unacceptably high for a load circuit, especially if it’s operating at 12 or 24 volts.
So it’s worth knowing how the arcing process works in advance of trying to fix any problems that you might find with your relay coil. If you do experience a problem, be sure to check the contact resistance of your relay coil before removing it. If the relay contacts have too much contact resistance, it can be a sign that they’re getting damaged by arcing and need replacing.