A relay is a device that allows for a smaller electronic switch to control a larger mechanical switch opening and closing a contact terminal with electrical isolation between the two. They’re found everywhere, from refrigerators to elevators to amplifiers to smart meters. Since most relays involve an ac load, arcing and extra power loss at switching can be a concern if the relay opens and closes the contacts while the ac signal is around its peak. This can be prevented by switching only while the ac signal crosses zero volts, or at zero-crossing.
What’s a Relay?
A relay consists of two isolated circuits: a controlling "primary" circuit and the controlled "secondary" circuit. The primary circuit commonly involves a transistor that controls an electromagnetic coil to pull or push a mechanical armature on the secondary circuit with its electromagnetic field. It’s often powered by a smaller dc voltage supply. The secondary circuit has the contact terminal that’s opened and closed by the armature. It often involves an ac load, such as a fan, light, amplifier circuit, or a smart meter.