Relay working principle and characteristics - Solutions - Huaqiang Electronic Network

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A relay is an electronic control device that switches the output circuit on or off when the input signal (such as voltage, current, temperature, etc.) reaches a set threshold. It can be categorized into electrical relays (e.g., current, voltage, frequency) and non-electrical relays (e.g., temperature, pressure, speed). Relays are known for their fast response, stable performance, long lifespan, and compact size. They are widely used in power protection systems, automation, motion control, remote operations, measurement, and communication devices.
A relay functions as a control system with an input loop and an output loop. It is commonly used in automatic control circuits, where it uses a small current to manage a larger one, acting like an "automatic switch." This allows it to perform tasks such as automatic regulation, safety protection, and circuit switching within the system.
Electromagnetic relays operate using components such as an iron core, coil, armature, and contact reeds. When a voltage is applied to the coil, current flows, generating electromagnetic force. This pulls the armature towards the iron core, causing the moving contact to connect with the normally open contact. When the coil is de-energized, the spring returns the armature to its original position, closing the normally closed contact. This mechanism enables the relay to switch circuits on and off. The terms "normally open" and "normally closed" refer to the state of the contacts when the coil is not energized.
Thermal reed relays are advanced temperature-sensitive switches that use magnetic materials to detect and regulate temperature. They consist of a temperature-sensitive magnetic ring, a permanent magnet, a reed switch, and other components. Unlike traditional relays, they do not rely on coil excitation. Instead, the permanent magnet generates the necessary magnetic force to trigger the switch, which is controlled by the temperature-dependent properties of the sensitive magnetic ring.
Solid-state relays (SSRs) are semiconductor-based devices with four terminals—two for input and two for output. They provide electrical isolation between the input and output circuits. SSRs can be classified based on the type of load (AC or DC), switch configuration (normally open or normally closed), and isolation method (hybrid, transformer, or optically isolated). Optically isolated SSRs are the most common due to their high reliability and safety. These devices are ideal for applications requiring fast switching, minimal wear, and noise-free operation.