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Lamp Dimmer

Discussion in 'Electronic Repair' started by Bob Simon, Mar 19, 2007.

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  1. Bob Simon

    Bob Simon Guest

    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?
     
  2. Not easily. What happens if you bypass the dimmer?
     
  3. SparkyGuy

    SparkyGuy Guest

    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.)...
     
  4. Probably, but assumptions waste more repair time than anything else.
     
  5. Bob Simon

    Bob Simon Guest

    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?
     
  6. Bob Simon

    Bob Simon Guest

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

    Meat Plow Guest

    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.
     
  8. Bob Simon

    Bob Simon Guest

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

    James Sweet Guest


    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.
     
  10. Bob Simon

    Bob Simon Guest

    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?
     
  11. 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?
     
  12. James Sweet

    James Sweet Guest


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

    Bob Simon Guest

    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?
     
  14. James Sweet

    James Sweet Guest



    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.
     
  15. Guest

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


    NT
     
  16. Bob Simon

    Bob Simon Guest

    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?
     
  17. James Sweet

    James Sweet Guest

    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.
     
  18. Guest

    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
     
  19. James Sweet

    James Sweet Guest


    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.
     
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