Glossary · Electrical engineering and control panels
Solid-state relay
Also known as: SSR, Semiconductor relay
German: Halbleiterrelais
In electrical engineering, a solid-state relay (SSR) is a switching device that uses semiconductor components, such as thyristors, triacs or MOSFETs, instead of mechanical contacts to switch a load circuit, usually with optical isolation between control and load side.
- Control panels
In one sentence
A solid-state relay switches loads with semiconductors instead of contacts, without wear and silently, usually with optical isolation.
Example
Heating elements of a plastic extruder are switched by solid-state relays that the temperature controller pulses several times per second.
How it applies
- Engineering: SSRs suit high switching frequencies and silent operation. They produce heat (heat sinks needed), have leakage current in the off state and usually fail short-circuited. Zero-crossing types reduce interference for resistive loads.
- Safety: Because an SSR has leakage current and can fail closed, an SSR in the off state does not isolate the load. It can't replace a disconnecting device or be used for Energy isolation.
- Documentation: Document type, load rating and heat sink requirements. Maintenance manuals should warn that terminals on the load side can carry voltage even when the SSR is off.
Solid-state relay vs. electromechanical relay
An electromechanical Relay provides a physical air gap when open and wears mechanically. An SSR has no wear and switches fast, but offers no air gap and produces heat.