Lamp Dimmer

My wife's reading lamp failed. I replaced the 100W 120V halogen bulb,
which did no good so I pulled out the dimmer circuit. I know nothing
about these but after googling for light dimmer I found this:
http://www.epanorama.net/documents/lights/lightdimmer.html#120v

The "typical 120V AC dimmer circuit" uses almost the same components
that are on my dimmer:
two diodes
cap
resistor
pot
diac
triac

The diodes test good one way so I wonder if either the BT137F triac or
the DB3 diac are the problem. Can these be tested with a VOM?

 

Not easily. What happens if you bypass the dimmer?

 

The diodes test good one way so I wonder if either the BT137F triac or

With that low part count, replace both. Sometimes "shotgun" troubleshooting
pays off compared to time spent testing.

After confirming that all the other components test good (cap, *switch*,
etc.)...

 

Probably, but assumptions waste more repair time than anything else.

 

You mean send 120V directly to the bulb? I just tested resistance on
the leads that go to the socket and found 10.3 ohms. Doesn't this
prove the same thing: continuity to the bulb and a non-open filament?

 

Thank you. That sounds like a good idea -- especially since I've
already removed the diac from the board.

 

http://en.wikipedia.org/wiki/Triac
http://en.wikipedia.org/wiki/DIAC

look at the diagrams and you might be able to answer this one yourself.

 

I tried it anyway and it lit up brightly. At least my wife can use
the lamp until I get the dimmer fixed. Thanks.

 

Usually triacs fail shorted, you'll want to rule out the easy stuff
first. Switch, variable resistor, etc. If the lamp doesn't light at all
my first suspect aside those already mentioned is the diac.

 

I went to the local parts store and they did not have either the triac
or the diac. They did have a TNE5646 "Triac with Internal Trrigger
Diac", which I purchased. This is also a 600V device and it handles
10A.

Can I simply install this in place of the BT137F and jumper between
the two holes on the circuit board for the missing diac?

 

Pretty much. If you have a large library close by try to find a copy of a GE
SCR Handbook or similar. This will explain a lot of the way these work.

Will the new device fit OK?

 

You should be able to, give it a shot, worst case it won't work.

 

It didn't. The pot/switch also had problems. I removed it from the
circuit board, squirted in some tuner cleaner, and worked the shaft.
Now the pot works good mechanically and electrically but the switch is
still unreliable.

I hate to give up when I'm so close. Is there any hope of repairing
the switch if I open up the pot? What about finding a replacement
that will fit into the board?

The only markings on the pot are T S on the bottom of the casing.
The circut board it came from is labeled Golo. Does anyone recognize
what either of these are?

 

If you open the switch you might be able to repair it. Worst case you
can replace the pot with an ordinary one of the same resistance and then
put an inline switch on the cord.

 

switches are easy to repair, they usually just need a scraping of the
contact points.


NT

 

Thanks to all who helped me on this project. The lamp is now back in
service. The dimmer works perfectly. I wasn't able to fix the switch
because I couldn't figure out how to replace the spring contact that
came out when I opened the bottom of the potentiometer case, so I
jumpered accross the contacts and plugged the line cord into a
switched power strip.

When I turn the dimmer to the off position (max resistance), the bulb
turns off completely. Does the lamp still use power in this position?

If it's not too much trouble, I'd like to fix this "right". Is there
any reasonable chance that I could find a 300K ohm pot with integrated
switch that would fit directly onto the circuit board? If not, where
is a good place to buy a 100W 120V dimmer module?

 

Yes, it may still draw some power, but very little. You could check it
with a multimeter if you wanted, just use the AC amps function and wire
the meter in series.

I've seen the dimmer modules various places online, personally I'd just
install one of those inline switches on the cord near the lamp.

 

yes, and quite a bit too. Dimmers are often described as devices for
turning down the efficiency of light bulbs. No light output comes from
somewhere roughly in the region of 50% power consumption.

yes certainly. 300k is an unusual value though, are you sure thats
right?

NT

 

Unless the triac is not triggering at all. I'll have to experiment with
some dimmers and see what happens. Easy enough to measure and find out.