How to choose a solid state relay: SSR vs electromechanical relay, heat sink sizing, zero-crossing vs random-fire switching, AC/DC load ratings, and when an SSR is the right call.
A solid state relay (SSR) switches a load with no moving contacts: a semiconductor does the job instead of a coil and armature. For panel builders, OEMs and importers buying control components from Asia, that single difference changes the failure mode, the noise, the lifetime and the heatsink you have to bolt on. This guide covers when an SSR actually beats an electromechanical relay (EMR), the load ratings and heat-sinking you must get right, and where an EMR is still the cheaper, safer call.
Inside an SSR a small opto-isolated input drives a power switching device - typically a TRIAC for AC loads or MOSFETs/IGBTs for DC. There is no arc, no bounce and nothing to wear out from switching. The control side is usually 3-32 V DC or 90-280 V AC; the output side carries the load. Because the switching is optical and solid-state, the relay can operate tens of millions of times where an EMR would have welded or pitted its contacts long before.
Use an SSR wherever switching is frequent, silent operation matters, or the load is sensitive to contact bounce and EMI - ovens, plastic welders, lighting dimmers, lab equipment.
| Parameter | SSR | EMR |
|---|---|---|
| On-state drop | 1-1.6 V across the device, dissipated as heat | roughly 0 V, negligible heat |
| Off-state leakage | mA-level leakage current | essentially zero |
| Inrush handling | limited by the junction; needs margin | very high make capacity |
| Overload breaking | cannot clear a fault | some types can |
| Cost | higher per pole | lower |
The two that bite most designers are heat and leakage. The voltage dropped across the SSR is dissipated as heat in the device, so every amp of load current needs a heatsink. And the small off-state leakage means a load is never quite fully dead - fine for heaters, a problem for a load that must be absolutely de-energised.
Power dissipated P is roughly load current x on-state voltage drop. For a 10 A load on a 1.2 V-drop SSR that is about 12 W of heat to remove. Pick a heatsink whose thermal resistance keeps the SSR junction below its rated maximum (often 100-125 C) at your ambient. A rough guide:
| Load current | Typical heatsink need |
|---|---|
| up to 5 A | small finned sink or DIN-rail mount |
| 5-15 A | medium extruded sink; forced air if enclosed |
| 15-40 A | large sink plus fan or panel mounting |
If the SSR lives in a sealed panel, derate hard or add ventilation - trapped heat is the number-one cause of field failures.
Zero-crossing SSRs switch on at the next AC zero crossing, suppressing EMI and inrush - the default for heaters and resistive loads. Random-fire (phase-control) SSRs switch immediately, needed for fast PID loops and dimming. Use the wrong type and a heater controller floods the panel with EMI, or a phase-control loop never fires cleanly.
AC SSRs (TRIAC-based) block both half-cycles when off; DC SSRs use MOSFETs and must be chosen for the correct polarity and voltage. Never fit an AC SSR on a DC load - it will stay partly on and burn out. Always specify the load voltage and current, including the cold inrush of a heater or lamp (often 10x running current).
ELEHPD supplies DIN-rail solid state relays in AC and DC versions, with zero-crossing and random-fire options, from 5 A to 100 A, built to IEC practice and ready to sit alongside contactors, thermal relays and power supplies in the same panel.
An SSR has no moving contacts, so its life is set by junction temperature rather than switching cycles; in frequent-switching duty it routinely outlasts an EMR by orders of magnitude. An EMR wins only when switching is rare and cost is the priority.
Any SSR carrying real load current dissipates heat equal to current x on-state drop (typically 1-1.6 V). Below a few amps a DIN-rail mount may suffice, but from 5 A up you need a proper heatsink sized to keep the junction under its limit. Skipping it is the most common cause of failure.
No. AC SSRs use TRIACs that block both polarities at zero crossing; DC SSRs use MOSFETs and need correct polarity. Fitting an AC SSR on a DC load leaves it partly conducting and destroys it. Always specify the load type.
Send the ratings, dimensions and quantities you are working to and we will confirm the closest match from the ELEHPD catalogue, the compliance documents available for it and the lead time. OEM branding, private model numbering and buyer-specific cartons are available on qualified volumes.