Actions
Forum Thread Details
  • Replies 27 replies
  • Subscribers 526 subscribers
  • Views 3745 views
  • Users 0 members are here
Related

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

scottiebabe

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

image

image

image

image

The schematic is the same as drawn by this author:

image

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

Searching for an X25783 datasheet led me to this,

image

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

A more common brand that may be similar could be the 

image

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...

Parents
No Data
Reply Children
  • 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.