The small signal that moves a separate circuit
Inside a relay, current in a coil creates a magnetic field that pulls a movable metal piece, called an armature. The armature presses contacts together; when the coil is de-energized, a spring usually returns it to its resting position. That simple motion lets a modest control circuit operate equipment whose power supply and wiring are separate from its own.
This separation is useful when a sensor, timer, or computer cannot safely or directly handle the voltage or current required by a motor, heater, or lamp. The control side only has to operate the coil, while the contacts are chosen for the load. In many designs, the two circuits are electrically isolated: their conductors do not need to share a direct connection.
Contacts have limits
A relay is not a universal switch. Its contacts must be rated for the load’s voltage and current, and switching can wear them. When contacts open, an electrical arc may briefly bridge the gap, especially with demanding loads. Motors and other inductive devices can also produce voltage spikes as their current changes, so a circuit may need extra protective components.
Relays remain useful because their action is visible and physical: energize the coil, hear or feel a click, and a separate path changes state. That makes one compact component a bridge between delicate decision-making electronics and equipment that needs substantial power.
