SSR not switching on (load won't energise)
A solid-state relay won't turn its load on even when commanded — the heater/load stays off because the SSR isn't conducting.
Safety first
SSRs switch live power. Prove dead before working on the load side. Note SSRs can have small leakage even when off — don't treat the load as dead based on the control signal alone.
Isolate, lock out / tag out, and prove dead before working unless a live test is specifically required, authorised, and carried out under proper supervision. Always follow local regulations, your site procedures, and the equipment manufacturer's documentation.
Full detail — causes, the why, and common mistakes.
Likely causes
Ranked from most to least likely.
- 1
No control signal to the SSR
Most likelyThe control/trigger voltage isn't reaching the SSR input, so it never turns on.
- 2
Control signal below threshold
#2The input voltage is present but too low to reliably trigger the SSR.
- 3
SSR failed open
#3The SSR has failed in the non-conducting state and won't pass power.
- 4
No load-side supply
#4The power feeding the SSR's load side is missing, so nothing can flow even if it switches.
- 5
Wrong SSR type (DC vs AC control / zero-cross)
Least likelyA mismatch between the control signal type and the SSR input prevents triggering.
Reports are saved on this device to reflect what you actually find.
Testing sequence
Work through one test at a time. Expected reading and what each result means.
Measure the control/trigger voltage at the SSR input when 'on' is commanded.
Control voltage present and within the SSR's input range.
Control signal good — check the load-side supply and the SSR output.
No/low control signal — trace the controller output and wiring.
View all expected readings at once
Fault-finding flowchart
The same logic as a decision tree.
- 1start
SSR won't switch on
→ step 2 - 2decision
Is a valid control signal present at the SSR input?
Yes→ step 3No→ step 4 - 3decision
Is the load-side supply present?
Yes→ step 5No→ step 6 - 4result
No/low control signal — trace the controller output and wiring.
- 5decision
Does the control type match the SSR input?
Yes→ step 7No→ step 8 - 6result
No load-side supply — restore it.
- 7result
SSR failed open — replace it.
- 8result
Type mismatch — fit the correct SSR.
Common mistakes apprentices make
- Not checking the control signal actually reaches the SSR input.
- A control voltage too low to trigger the SSR reliably.
- Forgetting the SSR needs its load-side supply too.
- Mismatching DC/AC control type to the SSR input.
When to stop & escalate
Repeated SSR failures point to heatsinking or rating problems to correct. Confirm the correct SSR type/rating for the control and load when replacing.
If you're past your competence, authorisation, or the safe limits of the job — stop and hand it on. There's no fault worth getting hurt over.
Related faults
Solid-state relay (SSR) stuck on — heat won't switch off
A heater (or other SSR-driven load) stays on even when the controller commands it off. Temperature overshoots, or the load runs continuously regardless of the control signal.
Heater not heating at all
A heater (element, band, or bank) produces no heat — temperature won't rise, the process stays cold, despite the control calling for heat.
PLC output LED is on but the device doesn't work
The PLC output indicator says the output is energised, but the connected device (valve, contactor, lamp, motor starter) does nothing. The program thinks everything is fine.
Learn the theory
How the gear and circuits behind this fault actually work.