Solidstatereläer

Solidstatereläer, or solid-state relays (SSRs), are electronic switches that replace mechanical relays with semiconductor devices. They provide galvanic isolation between control and load circuits and switch without moving parts, offering silent operation and high switching frequency.

An SSR typically comprises an input control circuit, often an LED-driven optocoupler, and an output switching element such as a triac/thyristor pair for AC loads or MOSFETs/IGBTs for DC loads. The optocoupler isolates the control side from the load side. Zero-cross switching is common in many AC SSRs to minimize EMI and surge, though non-zero-cross types exist for fast or non-sinusoidal signals.

Control signals are usually DC (for example 3–32 V) or a 4–20 mA current loop. The output is a normally open or normally closed electronic switch, chosen by design and vendor.

Advantages include no contact wear, long service life, silent operation, fast switching, and compact form factor with good isolation. Disadvantages include off-state leakage current, heat generation in the conduction path, and limited suitability for very low-power or high-frequency DC switching in some configurations. Some SSRs are designed for AC loads only; DC SSRs use MOSFET- or transistor-based outputs.

Applications include industrial automation, heating control, motor starters, power supplies, and lab equipment. When selecting an SSR, engineers consider load type (AC vs DC), required isolation, allowed leakage, thermal management, and whether zero-cross switching is appropriate for the application.