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All relays operate using the same basic principle. Our example will use a commonly used 4 -pin relay. Relays have two circuits: A control circuit (shown in GREEN) and a load circuit
(shown in RED). The control circuit has a small control coil while the load circuit has a
switch. The coil controls the operation of the switch.




















Current flowing through the control circuit coil (pins 1 and 3) creates a small magnetic field which causes the switch to close, pins 2 and 4. The switch, which is part of the load circuit, is used to control an electrical circuit that may connect to it. Current now flows through pins 2 and 4 shown in RED, when the relay in energized.
























When current stops flowing through the control circuit, pins 1 and 3, the relay becomes de energized. Without the magnetic field, the switch opens and current is prevented from flowing through pins 2 and 4. The relay is now OFF.

























When no voltage is applied to pin 1, there is no current flow through the coil. No current means no magnetic field is developed, and the switch is open. When voltage is supplied to pin 1, current flow though the coil creates the magnetic field needed to close the switch allowing continuity between pins 2 and 4.























Other relay variations include three and five pin relays. A 3-PIN relay instead of two B+ input sources, this relay has one B+ input at pin 1. Current splits inside the relay, supplying power to both the control and load circuits. A 5-PIN relay has a single control circuit, but two separate current paths for the switch: One when the relay is de-energized (OFF - no current through the control coil) and the other the energized (ON - current is flowing  through the control coil). When the 5-PIN relay is de-energized (OFF), pins 4 and 5 have continuity. When the relay is energized (ON), pins 3 and 5 have continuity.