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halogen dimmer

Discussion in 'Electronic Repair' started by Sammy, Nov 11, 2006.

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  1. Sammy

    Sammy Guest

    I have a 12v halogen dimmer light - a 'mother and child'
    uplighter (240v). The diac is faulty. I don't need to have
    the dimmer function working.
    It would be ok if I could just have it at full brightness
    all the time. Could I substitute 2 diodes in parallel, one
    in either direction, to achieve full
    brightness?. Any particular diodes?. == Sammy ==
  2. Not quite sure what you mean? Simply bypassing the dimmer and running the
    bulb straight off the transformer should be ok. Or if you know it's the
    DIAC, replace it.
  3. I think he means replace the diac with the two diodes. Sure, that would
    work but as noted above, just bypassing the whole thing should will work also.

    But why does he think the diac is faulty?

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  4. There's a tendency to assume it's the bit you don't understand or can't
  5. default

    default Guest

    The obvious answer would be to just get rid of the dimmer entirely.

    One assumes this: 'mother and child' uplighter (240v). Is something
    that already has a transformer stepping the mains down to 12 and not
    just some scheme of firing the triac to maintain 12 volts???????

    If this is one of the schemes that uses the firing angle to keep the
    voltage from the 240 mains at 12 or lower - you don't want to mess
    with it. Two diodes will replace the diac - but diacs usually fire at
    ~30 volts and putting diodes in will have them fire at .6 volts that
    would really screw up the thing if there's no transformer . . .
  6. Morse

    Morse Guest

    I think it's highly unlikely that would work- a diac isn't simply two
    standard diodes bunged back to back, it is IIRC more like two 30v zeners
    back to back.

  7. Zeners aren't right either.

    Why not replace the diac?
  8. Morse

    Morse Guest

    Diac? Are you sure it's a diac you mean? Can you describe the circuit and
    the component in question, and how you tested it? The answer is no, if the
    diac is faulty it should be replaced with a diac, it's not as simple as two
    standard diodes back to back.

    I've repaired stacks of dimmers and rarely had to change a diac. They tend
    to fail only when the triac they drive shorts out, which often results in
    other burnt out components.

    Where is the 12V derived from? Does it come from an isolating 240V-12V
    transformer then go through the dimmer? If so then *theoretically* you'd
    just wire the 12V straight to the bulb bypassing the dimmer circuit, no
    faffing with diodes is necessary. If the dimmer is on the mains side then
    bypass the dimmer and take mains straight to the transformer primary.

    However, without knowing exactly what you have I couldn't recommend the
    above procedures as they may raise important safety issues.

    If the 12V is derived by dropping 240V to 12V by a triac chopper (not likely
    legal in the EU) then it's strongly recommended you get it professionally
    repaired or bin it.

    I've seen halogen lights with 'electronic transformers', which are
    switchmode power supplies designed to be dimmed. I wouldn't recommend
    delving inside these as they provide isolation from the mains as well as
    240v-12v conversion and can't just be bypassed.

  9. Morse

    Morse Guest

    I didn't say they were, I mentioned them as a closer approximation
    electrically than standard diodes.
    Why not indeed? I certainly didn't imply any different course of action.

  10. Morse

    Morse Guest

    Thinking about it a bit more- the closest approximation I can come up with
    would be four 30v zeners in a serial-parallel combo. Two zeners in series
    with anodes together in parallel with two zeners in series with cathodes
    together. This would drop 30.6 ish volts in either direction if I'm not

    Still, a proper diac is the right way to go, messing with Frankenstein jobs
    is pointless other than to satisfy one's curiousity ;-)

  11. I've never known one blow on its own - it's usually the triac failing that
    takes it.
  12. IIRC, a diac is like a triac with no gate - when it fires the dropped
    voltage is a couple of volts only. This is unlike a zener.
  13. James Sweet

    James Sweet Guest

    Just two zeners in series with the anodes together would do this.
  14. Morse

    Morse Guest

    Or cathodes together, either way.
    You're right :) Too late at night and too much beer for thinking!

  15. Morse

    Morse Guest

    Ah right, that buggers up that theory then!

  16. Pretty sure you need a real diac. For higher voltages a neon lamp can do it
  17. James Sweet

    James Sweet Guest

    Yeah a diac is the way to go for what the OP is looking for, I was just
    responding to another part of the thread.
  18. [two zeners as replacement for a diac]
    You're right: but would there be a difference in function between a
    triac triggered (at 30 V) by a diac and a triac triggered (at 30 V) by
    two back-to-back zeners? Wouldn't both of them conduct current until its
    next zero crossing?


  19. Sammy

    Sammy Guest

    Thanks to all of you who replied.
    The diac is split and measures 0.640 ohms in either
    direction. marking on it: BTB04 600SAP.
    Another similar diac on the board, for the 300w
    halogen circuit, is high resistance both ways.
    I'll find a replacement diac for the faulty one rather than
    experiment with diodes.
    There is so little space for diodes anyway.
    A low value fusible resistor in the 12v circuit is o/c.
  20. The point of the diac is to dump enough current into the gate to turn on the
    triac. A zener doesn't do that - it just passes the current once the voltage
    is exceeded.
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