A relay is an electromagnet with movable contacts that make or break connections with fixed ones (depending upon the construction). The current flowing through the relay coil creates a magnetic field that activates the movable contacts. When the current stops flowing through the coil, the movable contacts are returned to their unenergized state by an external force, usually a spring or a permanent magnet. The force applied may be sufficient to break circuits through one set of contacts, or it may close them through a marginal gap.
This type of relay is used when the control circuit needs to be able to reliably return to the original state after a power interruption. When the contacts are energized with a current pulse of the proper duration and polarity, the relay will persist in the new position until the next current pulse toggles it to the other state. This feature allows the relay to be interrupted several times without damaging the circuits that it is controlling.
Often it is not possible to use a relay with the required operating voltage, current, or time-on delay because of factors such as cost and size. In these cases it is useful to design a special circuit that will reduce the coil current and time-on delay, or even eliminate them entirely. This is known as a snubber. The snubber consists of a capacitor and resistor in series. Suitably rated components are available in pre-packaged forms for this purpose.
The snubber circuit can also be useful in reducing the surge currents that tend to form around the relay output contacts when they first open and then close. This surge can damage the contacts and the circuit tracks on a printed circuit board. By absorbing the initial surge current with a snubber circuit, the surge can be safely dissipated and the contacts will function as intended.
Another consideration is mechanical vibration and shock. Relays that are exposed to a significant mechanical shock, such as those on avionics equipment or automotive vehicles, need to be designed with sufficient strength and resilience to withstand the expected level of shock. Some relays can resist mechanical shock loads much better than others, depending on their construction and the size of their moving parts.
If a relay with two opposing coils is to be used, the coils are usually connected by an over-center spring or permanent magnet. A pulse of positive and negative voltage to the coils is needed to actuate the armature, and then the force of the magnetic field holds the movable contacts either closed or open (depending on the relay construction). When the current to the coils is switched off, the contacts are released by a mechanical force to their original position. This type of relay is commonly found in telecommunication equipment, security alarm systems, office equipment, and aircraft cockpit controls. It is also used in many industrial applications. The terminals of this relay match up well with the ISO body style and relay diagram in a vehicle.
