Bad Triac or something else?

[Steven]

CTB 16PC v1, channel 10 stopped working.

The 220Ω resistor that connects the optoisolator to AC shows heat damage, and is now an open circuit.

https://photos.google.com/share/AF1QipNXsojh675QWAEm3iDBIVzlUl31LDyadKwa7RszSvuND17zE86bZCzMNVboUk_7Zg?pli=1&key=TklQb2x0U2t6UTNOTG1JOFQ5U19xNWZnUGdYU29n

The corresponding triac measures the same as the other triacs so I suspect it is still okay. Could it just have been a bad resistor? I'm going to replace it to see if that fixes it.

[Steven]

Update: I replaced the resistor, and the channel appeared to work. However, the resistor started smoking when the channel was turned on.

Clearly the triac needs to be replaced.

[bdwillie]

Usually a resistor does not burn like that on it's own, I bet it's the triac!

 

[Orville]

Only times I've ever seen a resistor burned like that was due to overvoltage, short to ground or a stray lightning strike managed to damage the controllers circuitry that may lead to the resistor, it may or may not be the Triac, but that would be my next step, replacing the Triac and resistor. 

But if it happens again after doing that, I'd start next with ground tracing, if that checks out.  Then there is possibly a defective integrated circuit chip somewhere allowing to much current flow into channel 10. 

Might need to have to go back to LOR so their techs can find the problem and fix it.

Just my thoughts on this.

[Steven]

Sorry I forgot to report my results. It was a bad triac.

A triac triggers by applying a current (not a voltage, because unlike a FET, a triac is a current controlled device) through the gate and MT2. The triac will then turn on, effectively shorting MT1 and MT2. In a typical triac circuit, this will remove the gate current, as it is supplied by an opto-isolater from the voltage present at MT1.

However, if the triac is damaged in such a way that it doesn't trigger while still passing current through the gate, then the current through the gate will rise to an excessive value (e.g. 120v / 220Ω =~ 500mA, which is too much for a 1/8W resistor). Since the light was actually being controlled before the resistor blew, the failure was probably only in one polarity.

The result was the resistor acted as a fuse, protecting the opto-isolater. Replacing both the resistor and the triac fixed the channel.

[Orville]

Glad to hear it was the resistor, triac combo that was the issue and fixed by replacing both.  Easier fix than having to send it in for repairs.