What is the difference between a solid state relay (SSR) and a contactor?
Compare semiconductor SSRs and electromechanical contactors across switching frequency, PID temperature control, acoustic noise and thermal dissipation.
What is the main difference between a solid state relay (SSR) and a contactor? An SSR switches loads electronically using semiconductors (thyristors/triacs) with no moving parts, zero acoustic noise and millisecond response for high-frequency PID temperature control. An electromechanical contactor uses a coil-actuated physical contact system suited for motor starting (AC-3) and galvanic isolation.
SSR vs Contactor comparison table
| Criterion | Solid State Relay (SSR) | Electromechanical Contactor |
|---|---|---|
| Switching Mechanism | Semiconductor (no moving parts, arc-free) | Coil-driven silver-alloy mechanical contacts |
| Switching Frequency | Very high (ideal for fast PID SSR output pulses) | Moderate (frequent cycling wears mechanical contacts) |
| Acoustic Noise | Silent (0 dB) | Audible click on pull-in and drop-out |
| Heat Dissipation | ~1.2–1.5 W per amp per phase; requires heatsink above ~5–10 A | Very low contact resistance; no external heatsink required |
| Primary Application | Resistance heaters, extruders, ovens, injection molding PID loops | Squirrel-cage motors (AC-3), pumps, fans and power isolation |
| Category Link | Solid State Relays (SSR) | Contactors |
Frequently asked questions
Why do SSRs require a heatsink?
Semiconductor junctions drop approximately 1 to 1.5 V in the ON state, generating ~1.2 W of heat per ampere. Without a properly sized aluminum heatsink and thermal interface pad, the internal junction exceeds its thermal limit.