Relays are used wherever it is necessary to control a high power or high voltage circuit with a low power circuit, especially when galvanic isolation is desirable. The first application of relays was in long telegraph lines, whereas the weak signal received at an intermediate station could control a contact, regenerating the signal for further transmission. High-voltage or high-current devices can be controlled with small, low voltage wiring and pilots switches. Operators can be isolated from the high voltage circuit. Low power devices such as microprocessors can drive relays to control electrical loads beyond their direct drive capability. In an automobile, a starter relay allows the high current of the cranking motor to be controlled with small wiring and contacts in the ignition key.
relays are used wherever it is necessary to control a high power or high voltage circuit with a low power circuit especially when galvanic isolation is desirable the first application of relays was in long telegraph lines whereas the weak signal received at an intermediate station could control a contact regenerating the signal for further transmission high-voltage or high-current devices can be controlled with small low voltage wiring and pilots switches operators can be isolated from the high voltage circuit low power devices such as microprocessors can drive relays to control electrical loads beyond their direct drive capability in an automobile a starter relay allows the high current of the cranking motor to be controlled with small wiring and contacts in the ignition key