What's inside a 20+ year old dimmer switch?

Not much. The dimmer function failed and only the switch function remained functional.

The schematic is the same as drawn by this author:

The Quadrac (an internally triggered triac, a triac + diac) failed short between all three terminals.

Searching for an X25783 datasheet led me to this,

https://images.100y.com.tw/pdf_file/31-Teccor-Quadrac.pdf

A more common brand that may be similar could be the

https://www.littelfuse.com/media?resourcetype=datasheets&itemid=b4817091-0710-4ac7-a446-a7ffcca0a634&filename=littelfuse_thyristor_q6008lth1led_datasheet.pdf

The potentiometer used as a rheostat has a switch stacked on top.

Not much inside...

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dougw over 1 year ago

Thanks, I always wondered what was going on in these circuits, but never explored. I have one where the rheostat has dead spots.
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scottiebabe over 1 year ago

Update! There was a party inside.

The pot terminals flashed over to the case at some point...
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dougw over 1 year ago in reply to scottiebabe

Amongst the mayhem lies a nice fail-safe design. Nobody got electrocuted and the house didn't burn down.
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scottiebabe over 1 year ago in reply to dougw

Thankfully its all good news stories. If any fault current went into the earth ground of the mounting box a GFCI breaker would trip.

I have never tried or played with Arc Fault breakers (AFCI). I may have to buy one to tear apart...

https://en.wikipedia.org/wiki/Arc-fault_circuit_interrupter
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robogary over 1 year ago

The nice thing is they only stay short briefly if there is a bolted fault :-)
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dang74 over 1 year ago

I had no idea there was any silicon in a dimmer. I had assumed it was only a potentiometer forming a voltage divider with the light bulb. I suppose though that an approach like that would generate a lot of heat inside the dimmer.
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scottiebabe over 1 year ago

The modern replacement is using a PCB for component connections. No more point to point wiring...

https://www.leviton.com/en/products/rdl06-10z

They also reference patent https://patents.google.com/patent/US8664886B2/en

Alluding to the idea that the phase firing angle is microprocessor controlled. There is a little MCU everywhere you look.
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DAB over 1 year ago

You confirmed what I had thought was in a dimmer.

The SCR was the easiest way to get reliable control of an AC wave.
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scottiebabe over 1 year ago in reply to $parentForumReply.Author.DisplayName

Not sure when it happened or how. There appears to be about a 1.5 mm air gap between the tabs and case.
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electronicbiker over 1 year ago in reply to scottiebabe

Assuming a 240v AC supply, 1.5mm isn't much of a gap. It can be bridged by a drop of water, the conductivity of which would depend on where it came from. That distance can also be bridged if the surrounding air or material (plaster, concrete, etc.) was damp. I've seen earthed conduit boxes, with switches fitted and wired, where plastering to the wall above them had been carried out. Some of the wet plaster had got inside the conduit boxes and was still slightly damp when I investigated. The damp plaster caused fuses to blow, and the circuit failed a Megger test in a big way. Thinking about triacs, a blob of wet plaster connecting any of the three terminals to the conduit box (and preferably one of the other two) could cause severe damage to most of the components within milliseconds.
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