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Electronics Engineering QUESTION #11724
Question 1
A solid-state relay (SSR) using a TRIAC output fails to switch OFF when the control signal is removed. The most likely cause is:
  • Insufficient trigger current to the optocoupler LED
  • Snubber capacitor is too large
  • TRIAC is latched on due to load current exceeding the minimum holding current without zero-crossing✔️
  • The heat sink temperature is below 25°C
Correct Answer Explanation
A TRIAC-based SSR turns OFF only at a zero-crossing of the AC load current. If the load current is inductive and lags the voltage, or if the TRIAC's holding current is not dropped below its minimum during a zero-crossing, the device can remain latched. This is also caused by a shorted TRIAC output — verify with an AC ammeter and oscilloscope.