SolidStateRelais

Solid-state relay (SSR) is a device that switches electrical loads using semiconductor devices rather than mechanical contacts. It provides electrical isolation between the control circuit and the load and offers fast, silent switching suitable for automation and power-control applications. A typical SSR contains an input optocoupler, which isolates the control input from the output, and a high-power output stage formed by a semiconductor switch such as a triac or SCR for AC loads, or back-to-back MOSFETs for DC loads.

AC SSRs commonly employ zero-cross detection to switch on near mains zero crossings, reducing electrical noise and inrush effects. DC SSRs use MOSFET outputs that can switch quickly and, depending on design, can turn on and off with little or no appreciable leakage in the output path. There are also latching SSRs, which stay in the last state after a pulse.

Key characteristics include the absence of moving parts, high reliability, and long life; fast switching with no arcing; and good electrical isolation between input and output. However, they exhibit off-state leakage current and an on-state voltage drop, which can require heat dissipation measures. Relative to mechanical relays, SSRs can be more expensive, and their behavior with highly inductive or capacitive loads must be considered when selecting devices.

Typical applications include motor and heater control, lighting, and power-supply regulation in industrial and consumer electronics. SSRs are well suited for resistive loads and many inductive loads where fast cycling, silent operation, and longevity are advantageous.