A relay (English: relay) is an electrical control device. It is an apparatus designed to induce a predetermined step change in a controlled variable within an electrical output circuit whenever a change in the input quantity (excitation signal) meets specific requirements. It establishes an interactive relationship between a control system (also known as the input loop) and a controlled system (also known as the output loop). Typically employed in automated control circuits, it effectively functions as an "automatic switch" that utilizes a small current to control the operation of a larger current. Consequently, within a circuit, it serves functions such as automatic regulation, safety protection, and circuit switching.
Fundamentally, a relay is an electrical control device capable of inducing a predetermined step change in a controlled variable within an electrical output circuit, provided that a specified input quantity is applied and sustained for a sufficient duration. Conversely, when the input quantity drops to a certain level-and remains at that level for a sufficient period-the device reverts to its initial state.
