Solid State Relay (SSR) Explained
A Solid State Relay (SSR) is a semiconductor switching device that replaces an electromechanical relay or contactor for moderate-current AC or DC loads. SSRs use thyristors, triacs, or MOSFETs to switch loads with no moving parts — meaning unlimited cycle life, silent operation, and microsecond switching speed. The trade-off: SSRs dissipate heat continuously while switched on, requiring proper heatsinking.
SSR vs electromechanical relay
| Aspect | SSR | Electromechanical relay / contactor |
|---|---|---|
| Cycle life | Unlimited (no moving parts) | 100k - 10M operations typical |
| Switching speed | Microseconds | 5-25 ms |
| Noise (audible) | Silent | Audible click |
| Voltage drop / heat | ~1.5 V drop = continuous heat (~1.5W per amp) | ~0V drop, near-zero heat |
| Heatsink required? | Yes for >5A | No |
| Failure mode | Often shorts (load stays on!) | Usually opens (load goes off) |
| Cost (10A AC) | $15-$40 | $10-$30 |
| Best for | High-cycle (heaters, lighting flicker, valve cycling) | Low-cycle, safety circuits, large currents |
Zero-crossing vs random-firing AC SSRs
Zero-crossing SSR only turns on when the AC waveform crosses zero volts. This eliminates inrush current spikes and EMI, ideal for resistive loads (heaters, incandescent lamps).
Random-firing SSR turns on instantly regardless of waveform position. Required for phase-control applications (dimming, motor speed) and inductive loads where you need fast response.
Heat dissipation
An SSR drops about 1.5 V across the output when switched on, dissipating ~1.5 W per amp. A 10A load = 15W of heat that must go somewhere. Without a heatsink, the SSR overheats and fails (often shorted, a dangerous failure mode).
Sizing rule: heatsink rated for at least 1.5W × max load current, with 30°C/W or better thermal resistance for free-air; better with forced air. Always derate SSR current rating to 50% for sustained loads — a 25A SSR running 25A continuously is at thermal limit.