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Simple Triac dimmer circuit question

Discussion in 'Electronic Projects Design/Ideas/Reviews' started by earckens, Aug 31, 2017.

  1. earckens

    earckens Member

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    Hi, I have a selfmade (30 years ago) dimmer circuit which uses the schematic attached, except for these values in the schematic:
    C1, C2 and C3: 100nF
    R1: 12k
    R2: 470R
    R3: 100k pot

    I recently opened the box it had been sitting in for decades and for safety resaons I had to remove a 100nF cap which had been soldered rather unsafely in parallel to C3. Result is now that the dimmer does not go to 0V anymore, at the minimum it lights any load still to about 20%.

    For practical reasons (board layout, tight space etc..) I would rather replace a resistor instead of having to solder back a cap parallel to C3 or rather then to replace C3 altogether.
    Is a solution with a resistor replacement possibel?

    Thanks,
    Erik

    Edit: the reason my values are different from the schematic simply is that my actual circuit was assembled 30 years ago, and this schematic is just plucked from internet, although it is the same layout for my circuit).

    Edit 2: or would the use of polypropylene 400V capacitors be safe and right?
     

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    Last edited: Aug 31, 2017
  2. schmitt trigger

    schmitt trigger Active Member

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    For the simple reason that the RC time constant is now shorter and now it won't span the full zero-crossing to zero-crossing period (10 msecs for you).
     
  3. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    Increase the value of the pot, it's too low to reach the higher time constant required.
     
  4. dave

    Dave New Member

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  5. earckens

    earckens Member

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    Thanks!

    Parallel to the polypropylene 0.1uF 400V cap (C3) was soldered a 0.1uF 250V metalfilm cap (photo); I am surprised it never blew all these years :eek:

    Erik
     

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  6. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    No reason for it to blow, as it's not subjected to anywhere near it's 250V rating.
     
  7. earckens

    earckens Member

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    Unless in Europe where we have 220V rms
     
  8. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    Nope, not even in the UK where we have 240V mains :D

    There's no mains voltage across it.
     
  9. earckens

    earckens Member

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    220xsqrt(2)=311V ptp

    Sorry, forgot to say, C1 is the same 250V cap

    Edit: stupid of me, I had not seen C3 is not under mains. I almost thought UK rules were different :facepalm:
     
    Last edited: Aug 31, 2017
  10. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    C1 is a different matter - but neither C2 or C3 have mains voltage across them.

    As for your problem, couldn't you just replace C3 with a 0.22uF?.
     
  11. earckens

    earckens Member

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    Indeed, I had meanwhile been looking at polypropylene capacitors and their size must have shrunk the past 30 years :cool:, I ordered some different values from Aliexpress; it will certainly not be a dimension issue in my cramped student-era box.
     
  12. Dick Cappels

    Dick Cappels Member

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    The Maximum voltage across the capacitor is just a little more than the voltage rating of the diac.
     

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