Solid State Relay (SSR) Failure: Diagnosis And Heatsink Compatibility Guide
Diagnosing A Failed Solid State Relay
Solid state relays (SSRs) are reliable, wear-free switching devices with no moving parts — but they're not indestructible, and heat is by far the most common cause of failure. Here's how to diagnose an SSR that's stopped working correctly, and how to match a replacement or heatsink to the relay you have.
Common Symptoms And What They Usually Mean
Load stays on permanently, even with the control signal removed. This is the classic symptom of a solid state relay that has failed from thermal stress — the output stage has failed in a way that leaves the load circuit continuously closed. If you see this, isolate the load electrically (don't rely on the controller) before doing anything else, since the SSR itself is no longer switching it off.
Load doesn't switch on at all. Before assuming the SSR itself has failed, confirm the control signal is actually present and within the SSR's rated input range — Novus's DC-controlled SSRs need 3-32VDC on the control side. A control signal outside this range, wired backwards, or a controller output that isn't actually driving (worth checking the controller's own output configuration) will look identical to a failed SSR from the load side.
Intermittent or erratic switching. Can point to a marginal connection on either the control or load side, a control signal close to the SSR's minimum trigger voltage, or — particularly if it's a new development on a relay that's run fine for a while — the early stages of thermal degradation.
Why SSRs Fail — And Why Heat Sinking Matters
Unlike a mechanical relay, an SSR doesn't wear out from switching cycles — its output stage dissipates heat every time it's conducting, and if that heat isn't removed fast enough, the internal temperature climbs until the device fails. This makes heat dissipation the single biggest factor in SSR lifespan, and it's why current rating alone doesn't tell the whole story: the same SSR run continuously near its full rated current, in a warm enclosure, without adequate heat sinking, will run hotter and fail sooner than one with a properly sized heatsink and some safety margin on current.
As a rule of thumb: the higher the current, the more critical proper heat sinking becomes. At 10A it's less critical but still worth doing for continuous duty; at 25A a heatsink is recommended for any continuous high-current load; at 40A and above it's strongly recommended in every case.
EMAS Heatsink Compatibility
EMAS's SSS-series heatsinks are a universal fit for standard "hockey puck" style SSRs — the form factor used across both the EMAS and Novus ranges we stock — and are sized by current rating rather than brand:
- SSS1X — sized for 10-12A relays, matching the EMAS SSRP1DA012 and the Novus 10A SSR.
- SSS2X — sized for 25A relays, matching the EMAS SSRP1DA025 and the Novus 25A SSR.
- SSS4X — sized for 40A relays, matching the EMAS SSRP1DA040 and the Novus 40A SSR.
All three are rated for a -25°C to +70°C operating range and mount to either DIN rail or a flat chassis. Fan-assisted versions are available across the range for applications running close to continuous full load, or in a warm enclosure, where the extra cooling margin is worth having.
Frequently Asked Questions
My SSR failed and it's stuck on — is that dangerous?
Treat it as a live circuit until you've isolated the load some other way (upstream isolator or breaker) — don't rely on the controller, or the failed SSR itself, to switch it off.
Do I need to replace the heatsink when I replace the SSR, or can I reuse it?
If the heatsink is the correct size for the new relay's current rating and shows no damage, it can normally be reused — just check it's rated for at least the same current as the replacement SSR.
Can I use a Novus SSR with an EMAS heatsink, or vice versa?
Yes — both ranges use the same hockey-puck form factor, so the heatsinks are sized by current rating rather than brand and will fit either.
Is a heatsink always required?
For light, intermittent loads at low current it's less critical, but for any continuous-duty application — especially at 25A and above — a heatsink is recommended to avoid premature failure from heat build-up.
Need More Help?
If you're not sure which SSR or heatsink size matches your application, please contact us with the current rating of your load and we'll help you pick the right combination.