Debug SONOFF AC relay with a thermal camera


James Lewis shares his experience in debugging SONOFF AC relay with a thermal camera:

The clever solution seemed to be clever, at least for a few minutes. Suddenly the light turned off. I thought maybe there was a timeout for the manual button. Annoying, but workable. The lamp remained off for about another 2 minutes when I started to smell that unmistakeable burning plastic odor. Touching the case of the SONOFF identified the culprit immediately.
Great. So I have an AC mains switch that isn’t working, but I do not want to go poking my multimeter into it. What do I do?
Turns out, that SONOFF module was defective. I wanted to debug it, but I did not want to measure anything while connected to AC. Here’s how I used a thermal camera to debug my SONOFF.

See the full post on Bald Engineer blog.

Join the Conversation


  1. 33ohms is just 0.1A at 3.3V. That does not sound like a short or a failed-short scenario. The 0.1A ballpark kinda reminds me of the relay coil… Anyway, this thing just gives me the jeebies. Open it up and screw in the wires? I would definitely prefer a socket, thank you very much but please no playing with wire terminals in this kind of toys.

    So they made a big show of adding slots and extra solder for the mains traces. But what about the power supply traces that is still at mains voltage? Does the power supply circuit look like its well-isolated from the low power circuitry? Okay, I would be too much of a coward to play with this toy.

  2. If the 1117 is dead, it could have been hit by a transient. Maybe the power supply or the relay coil. If the relay coil has a bad diode or badly-soldered diode across it, then it would be a nice transient generator.

Leave a comment

Your email address will not be published. Required fields are marked *

Notify me of followup comments via e-mail. You can also subscribe without commenting.