SSR (Solid-State Relay)

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.

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'.

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.

SSRs fail short far more often than open — the load stays on with the control signal dead. On a heater that's a runaway: controller says off, element keeps cooking, and the high-limit (if fitted) becomes the only thing in charge. Fail-open happens too — no output ever — usually after running under-cooled or overloaded.

Half-failures are common: leakage through the 'off' SSR keeps indicators glowing and small loads dribbling along, and a marginal heatsink gives random shutdowns that recover once everything cools.

Test it in circuit, under power, with the load connected — live testing, so licensed work and due care. Control off: full supply volts measured across the SSR's output terminals means it's blocking properly; near zero volts across it with control off means it has failed short. Control on: the drop across it should collapse to a volt or two.

An ohmmeter or diode test across the output usually reads open even on a good SSR — the semiconductor needs real voltage and load current before it conducts. Don't condemn one on a bench continuity test; test live or substitute. Check the control side too: input volts in the rated range, status LED on.

Never treat an SSR as isolation — it leaks when off and its favourite failure is stuck on. Isolate upstream before touching the load, and on heating give the circuit something electromechanical (contactor or high-limit) that can genuinely break it.

Match the type to the load: an AC-output SSR on DC never turns off, and zero-cross types can misbehave on transformers and other inductive loads. And the heatsink isn't optional — most field failures are simply cooked SSRs.