SSR (Solid-State Relay)
Switches a load electronically with no moving parts — fast, silent, ideal for frequent switching like heaters.
What it is
An SSR switches a load using semiconductors instead of mechanical contacts. With no moving parts, it can switch rapidly and silently for millions of operations — perfect for temperature control.
How it works
A small control signal triggers an internal optocoupler, which switches a power semiconductor (a triac or thyristor) to pass the load current. Many SSRs switch at the AC zero-crossing to reduce electrical noise.
SSRs fail differently to mechanical relays: they commonly fail short-circuit (stuck on), so the load stays powered even with the control off. They also have a small leakage current when 'off'.
Where it's used
Heating control, where a controller pulses the SSR on and off to hold temperature. They need good heatsinking — they run hot and fail if under-cooled or under-rated.
Safety first
A failed SSR can stay on with no control — don't rely on the controller to make it safe. Isolate upstream. Runaway heating is a fire risk.
Isolate, lock out / tag out, and prove dead before working unless a live test is specifically required, authorised, and carried out under proper supervision. Always follow local regulations, your site procedures, and the equipment manufacturer's documentation.
Related faults
Solid-state relay (SSR) stuck on — heat won't switch off
A heater (or other SSR-driven load) stays on even when the controller commands it off. Temperature overshoots, or the load runs continuously regardless of the control signal.
SSR not switching on (load won't energise)
A solid-state relay won't turn its load on even when commanded — the heater/load stays off because the SSR isn't conducting.
Heater not heating at all
A heater (element, band, or bank) produces no heat — temperature won't rise, the process stays cold, despite the control calling for heat.
Related definitions
Relay
A small electrically-operated switch — like a miniature contactor — used to switch or route control signals.
Contactor
An electrically-operated switch that uses a coil to make or break a load circuit, usually three-phase power.
Heating element
A resistive conductor that turns electrical energy into heat — the business end of most electric heating.
Thermostat
Switches a heating or cooling load on and off to hold a temperature at a setpoint.