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

scottiebabe

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

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The schematic is the same as drawn by this author:

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

Searching for an X25783 datasheet led me to this,

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

A more common brand that may be similar could be the 

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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...

  • in reply to scottiebabe

    Hi scottiebabe, sorry about the misunderstanding - I thought you were Scottish and lived in Scotland!

    So, ok, the 1.5mm gap should be fine at 110v. But I've just thought of something else...  Over a long period of time, soldered connections (especially when close together on a PCB) can exhibit a strange phenomenon caused by 'creepage'. This is where a soldered connection through holes in a PCB (amongst other places) will produce radial hair-like dendrites growing in length across the surface of the PCB. If they meet dendrites from another soldered connection then there may be trouble! I have seen solder connections where dendrites have grown all around them, looking like multipointed stars with very sharp points. The sharp edges around the ends of the through-pins used for connecting the two sides of a double-sided PCB are quite good at dendrite-growing, and an electromagnetic field helps to 'steer' them. Such as the ones produced by inductors.

    This is not a very common failure mode, but it can happen. If anything much more than a small current passes along a dendrite it immediately blows just like a fuse, thus destroying the evidence. One place where they might have formed is between two of the rivets holding the terminals in contact with the track on the pot.

    A lot depends on the age of components and PCB material. Conformal coatings on PCB's will usually prevent creepage effects.

  • in reply to scottiebabe

    I do like that oscilloscope! Might even get one myself to replace the ages-old Tektronix battery portable. Does it have BNC sockets for X, Y, and external timebase inputs?

  • in reply to electronicbiker

    It's my understanding that 20+ year old electronics would have lead/tin solder, which generally would not form tin whiskers (unlike lead-free solder). REALLY old solder (think 1920s) did have whiskers, either because there wasn't enough lead or too much.

  • in reply to scottiebabe

    Lots of LED lights don't work well with dimmers either.

  • in reply to scottiebabe

    My homeowner insurance company is involved with a company that monitors your electrical system to watch for various faults. They claim to find all manner of physical issues from open neutral wires to arcing wires. Obviously their intention is to minimize electrical fires, which I think is a very good idea. I wish I'd had this service twenty years ago!

    For the curious, It's State Farm and Whisper Labs ("Ting"). So far it's only warned me that I've had power interruptions, which I find more useful than the old 'quarter on the ice cube tray' method. 

  • in reply to electronicbiker

    It has BNC inputs for channel 1 and 2. Not for external timebase.