The relay is a versatile electromechanical component that switches heavy currents in relatively small spaces and provides galvanic separation between control circuits and load circuits.
Relays are especially useful in applications where it is important to switch power reliably and where space is limited or a high degree of safety is required. They are available in many different sizes and types, with differing technologies and contact materials for various applications. Miniature Relays are compact versions of the basic electromechanical relay, and are often used for low current switching duties (such as actuating headlight main beam bulbs or indicating lamps).In its basic form the relay consists of an electromagnet coil and a metal contact connected to a frame or 'yoke' from which one end of the armature is hinged.
When the coil is supplied with voltage (the 'ON' state) a magnetic field is generated around the coil which pulls the armature down onto the contact to complete the 'high current' circuit. The yoke and armature are then held by a spring (the 'at rest' state). Once the contact is closed the coil can be de-energised and the yoke and armature return to their original positions.
This action can be repeated if the voltage to the coil is applied again. When the coil is de-energised the magnetic field collapses rapidly, generating a large voltage spike across the terminals connecting to the contacts (again, very low current). Resistors or diodes are sometimes fitted between the relay coil and the contacts to stop/reduce these voltage spikes from travelling back into the control circuit and damaging sensitive components.
Relays have been put to some quite clever logical functions in vehicle electrical systems and they can still be a useful, fun and cost effective way of performing a simple task such as latching a +12V output on and off from momentary inputs or flashing alternative left and right lights when the ignition is switched on (e.g. after an accident).Relays are also useful for reducing the size of control circuits.
In some applications it is possible to achieve the same effect using an electronic switch with a built-in microprocessor that performs the same logical operations as a relay (for example by linking up input signals from sensors or switches to trigger certain output actions from the ECU). However, for some after-market applications it can be much simpler to use a standard ISO mini or'standard' relay.