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
www.howstuffworks.com) 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 he

http://i598.photobucket.com/albums/t...ies/dimmer.jpg

Any help would be much appreciated!

Thanks

Luke

Luke wrote:
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
www.howstuffworks.com) 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.


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.
..

SNIP TALE OF KNACKERED DIMMER

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.
.- Hide quoted text -

- Show quoted text -

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

On Wed, 5 Aug 2009 04:12:25 -0700 (PDT), Luke wrote:
SNIP TALE OF KNACKERED DIMMER

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.
.- Hide quoted text -

- Show quoted text -

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?

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.

Luke wrote:
SNIP TALE OF KNACKERED DIMMER
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.
.- Hide quoted text -

- Show quoted text -

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
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

On Aug 5, 1:14*pm, Bob Minchin wrote:
Luke wrote:
SNIP TALE OF KNACKERED DIMMER
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.
.- Hide quoted text -

- Show quoted text -

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

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- Hide quoted text -

- Show quoted text -

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

In article
,
Luke wrote:
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
www.howstuffworks.com) 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.

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

--
*Support bacteria - they're the only culture some people have *

Dave Plowman London SW
To e-mail, change noise into sound.

In article ,
"Dave Plowman (News)" writes:

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

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

--
Andrew Gabriel
[email address is not usable -- followup in the newsgroup]

"Andrew Gabriel" wrote in message
.. .
In article ,
"Dave Plowman (News)" writes:

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

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

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

In article ,
Andrew Gabriel wrote:
Usually the triac has failed short circuit. Replace it and the diac with
ones from the cheapest dimmer you can find.

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

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.

--
*Why are a wise man and a wise guy opposites?

Dave Plowman London SW
To e-mail, change noise into sound.

Dave Plowman (News) wrote:

[...] - but it's a long time since I've bothered fixing them.

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

http://www.maxwell.myzen.co.uk/uk.d-i-y/Dimmer.jpg

--
Andy

In article ,
Andy Wade wrote:
[...] - but it's a long time since I've bothered fixing them.

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

http://www.maxwell.myzen.co.uk/uk.d-i-y/Dimmer.jpg

Ah - a challenge. ;-)

--
*You are validating my inherent mistrust of strangers

Dave Plowman London SW
To e-mail, change noise into sound.