Dimmer Switch repair

[Luke]

Hi all, My sister has a rather nice floor mounted uplighter which incorporates a dimmer switch which has knacked. The symptoms are that the lamp simply did not light no matter what the position on the dimmer.

I took the lamp apart and disconnected the controller. I found that the fuse had blown, so I replaced it with another, and found that the lamp then worked on full brightness regardless of the dimmer position.

Reading around the subject I found out how a dimmer worked (from

and suspect that the firing capacitor has failed causing a short circuit between Live and the bulb, explaining why the lamp is going full blast.

I therefore reckon I just need to replace this capacitor, but I have been trying to find a suitable replacement but I don't know much about them and the choice is mind-boggling! Hopefully someone here will be able to point me in the right direction. The key information (I hope) is:

Capacitance: 0.1 uf (I think this is micro-farads from my A-Level physics days)

275v~

-40+100C

The size of the capacitor is about 17mmW * 10mmH * 4mmD

I have put a photo here:

help would be much appreciated!

Thanks

Luke

[The Natural Philosopher]

No. The triac has popped, because the bulb blew, and fried it.

Happens often and its always the case.

Usually not worth fixing, but if its part of a lamp, then yes, solder in replacement triac - bigger is better and Robert is a relative. Or pirate a triac from a cheap dimmer that DOES work. .

[Luke]

Thanks, (and I see your reasoning) but I'm pretty sure the lamp

*didn't* blow (I will check with my sister). Thing is, I have put my multimeter across the firing capacitor, and it shows a dead short - surely this is wrong as capacitors tested in this way should show infinite resistance shouldn't they?

Is there any easy way to test the triac?

Thanks again

Luke

[pete]

Well, it could've been a mains spike, or just a shoddy component who's time had come. You may be able to replace the whole dimmer PCB with one from a (new) gutted dimmer. When my Maplin "temperature controlled" soldering iron died, I took it apart and discovered it was just a bog-standard dimmer cct inside. The board from a cheapo dimmer switch worked fine - though I had to enlarge the hole as the pot. had a different sized shaft.

[Bob Minchin]

It is only valid to measure resistance across a component if one end is disconnected first otherwise you are possibly measuring the effects of other components.

If the firing capacitor is short circuit and the Triac still functioning then the lamp would be OFF all the time. I strongly suspect the triac is dead and maybe, Just maybe, the capacitor is too but this would be very unusual.

Bob

[Luke]

Thanks Bob, OK I take everyone's comments on board regarding the Triac and have bought an IDENTICAL replacement from Farnell for the princely sum of

67p. I will replace it and hopefully this will sort the problem out.

I drew a circuit diagram, which I will post a link to later when I have copied it up, that seems to suggest that if the firing capacitor goes short circuit then the lamp *would* be ON all the time, the flow of the circuit going LIVE > CAPACITOR > FUSE > LAMP > NEUTRAL. Obviously I have missed out the branch to the Triac here.

I still can't help suspecting the capacitor!

Luke

[Dave Plowman (News)]

Usually the triac has failed short circuit. Replace it and the diac with ones from the cheapest dimmer you can find.

[Andrew Gabriel]

I've never come across a case where the diac failed.

[brass monkey]

The ones i've played with all had quadracs in 'em Q4004LT

[Dave Plowman (News)]

I have - but it's a long time since I've bothered fixing them. And if robbing one of the triac you might as well change the diac too.

[Andy Wade]

The last faulty dimmer I dealt with was a little beyond repair. Fortunately it had been installed in a metal back-box:

[Dave Plowman (News)]