A relay is an electrical device that?

A relay is a device that takes a smaller source of electrical power and using it to manually control a larger source of electrical power. Typically, a low voltage or low current source, which is more easily manageable with control circuits, sends a signal to energize an electromagnetic coil. When the coil is activated, the magnetic field moves a plunger type device which has contacts that serve as a switch for the larger voltage or current device. In essence, the contacts complete the path for the circuit and allows a load to begin to operate using the higher voltage or current that is supplied through the contacts' connections. A good example is a house air conditioning system's condenser unit (the part which contains the compressor and sits outside of the house). The condenser requires 240 Volts Alternating Current (AC). But the thermostat inside the house uses a 24 Volt power source. When it's time to cool the house down, the thermostat on the wall sends 24 Volt signal to the relay in the condenser unit. The 24 volts causes the coil to energize and makes an electrical connection to the 240volts required to power the compressor. The compressor now runs and your house begans to cool off. Once the set temperature is reached, the thermostat discontinues the 24 volt control signal and the 240 Volts is disconnected from the compressor by the relay, thus shutting off the condesor unit. In cars and vehicles, a relay performs the same function, except that the voltage is not 240 Volts AC. It is 12 volts Direct Current (12VDC). It is used in the air conditioning system in much the same way, but is also used to control the blinking of turn signals, the horn, and in some cases headlights. The voltage remains at 12VDC, but large current flow is controlled by small current flow. For example, 12VDC at 10milliamps through the relay could contol the headlights which may require 12VDC at 5 Amps to shine brightly enough to see at night.