faulty 12vac dimmer/switch

Discussion in 'General Electronics Chat' started by atlantis43, Apr 30, 2014.

  1. atlantis43

    Thread Starter New Member

    Apr 30, 2014
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    I'm having problem with a 12vac halogen lamp rotary dimmer/switch, which does not turn "off", with light returning to partially "on" when switch in "off" position.
    (Sometimes bulb actually turns off, but if I return to lamp later I find that bulb is again lit!)
    Sorry that I can't take a clear picture of this mini-switch, but I've attached the best I can do of the interior of the switch, and I hope that you'll excuse the verbiage----a picture is worth 1000 words :).


    The original switch has 4 terminals, 2 on top, and 2 on bottom , but only 2 wires (which are connected to the bottom of the unit) which connect to another box of electronic circuitry (which connects to 120vac, and also to the bulb itself). There also is a "jumper" wire connected from one of top terminals to one of the bottom terminals.

    I purchased a replacement dimmer unit, but didn't realize that it was for a 120vac bulb, and was therefore unsuitable. It had a similar looking switch/rheostat (with marking B500k), which was solder-mounted to a circuit board by its "top" two and "bottom two terminals.

    When I disassembled the two switches, they both had identical contact assemblies (see picture), but the actuator facets of the two switches are not identical, so I can't just switch partial assemblies from one to the other.

    I'll try to help describe what is shown in the attached picture, which is:
    1) a small fixed contact shaped like a small spoon, to which one terminal is connected. This is the smaller shiny object in the picture
    2) a larger half-circular metal contact, with a small pin at one end and a contact knob at the other end. This is the larger shiny object clearly evident in the picture.
    3) a piece of springy metal (seen end-on in the picture, so it is barely visible) which is mounted inside of the above two structures, and I guess serves as the on-off switch when moved by the actuator ridges on the knob.


    My Question is:
    Is the above-mentioned "jumper" wire on my original switch the thing that converts a potentiometer into a switch-potentiometer? If I were to install such a jumper on the replacement potentiometer, will it have the same effect?
    (I'm assuming that my original unit was also a B500K pot).

    I hope that I've given a clear enough understanding of the problem that someone might be able to provide an answer.
     
  2. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,553
    2,375
    In many cases the 120v or similar dimmer type switches are not voltage sensitive, I have used high voltage type on 15vac.
    The circuit is basically the same regardless, the components have to be higher rated of course for 120v etc.
    I would reverse engineer it, there is usually only 5 components at the most.
    All the ones I have seen, the switch removes the input power to the circuit, so it should be completely off.
    I have used them also to make an 'improved' version as they are often the most basic of circuits, no diac or UJT etc.
    There are numerous circuits out there via Google.
    Max.
     
    Last edited: Apr 30, 2014
  3. atlantis43

    Thread Starter New Member

    Apr 30, 2014
    15
    0
    Max,
    Not quite sure I understand. The available dimmers of the type that I need to fit the lamp have 120vac output, which I'd think would blow my 12vac halogen bulb, would it not?

    Actually, there are errors that I just discovered in the wiring I described for my original switch. I'm going to re-post with a corrected version of the wiring, so that perhaps I can "reverse-engineer" only the new switch.
     
  4. djsfantasi

    AAC Fanatic!

    Apr 11, 2010
    2,810
    834
    I think Max is suggesting that either a) you feed the dimmer with 12VAC - not line power, or b) you study the circuit t learn how they work.
     
  5. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,553
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    Yes, I was making the point that many of these are not voltage sensitive, IOW they will work on a lower AC voltage.
    I suggest you put a detailed photo or a reverse engineered drawing up.
    BTW, it is better and correct to put up one post rather than another identical to this! ;)
    Max.
     
  6. atlantis43

    Thread Starter New Member

    Apr 30, 2014
    15
    0
    I understand what you are saying, but isn't the bulb voltage-sensitive? This replacement unit that I purchased was made for 120v incandescent bulbs, which I realized upon receipt. On contacting the distributor, they said it was not suitable for my needs. The original unit was made in China, and nothing is marked; but the box that the switch is connected to is principally a transformer coil.
    Perhaps my question should be:

    I'm assuming that a pot/switch is a combination of a pot and a SPST switch.
    Could anyone show me the basic wiring diagram for such a combination switch, so that I can understand how the connections on the replacement switch from the 120v unit might be connected to the transformer box by only 2 wires?
     
  7. ian field

    Distinguished Member

    Oct 27, 2012
    4,415
    784
    The typical line powered dimmer usually has a diac in the gate circuit of the triac - most diacs have a breakover voltage somewhere in the region of 30 -34V. This could make things difficult if the lamp has an iron-cored transformer with a 12V secondary.

    It might be easier to pin down if the OP posted a photo or two of the lamp innards.
     
  8. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,553
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    I have rarely seen a diac in the ones I have stripped, but I suppose there could always be an exception.
    Sounds like the OP has a transformer based system for his lamp, the question is, is what side the controller is on?
    The few component there are it should be easy to repair the existing one?
    It shouldn't take much to draw up what is there now, or post a photo/picture.
    Max.
     
  9. atlantis43

    Thread Starter New Member

    Apr 30, 2014
    15
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    Ok, what you mean finally got thru my thick head, but I disposed of all of the new dimmer except for the pot switch.
    I managed to take some clearer pictures of the works in the lamp, and perhaps this will help toward what I was hoping to do. There is one pic of the electronics, and 2 views of the pot-switch.
     
  10. atlantis43

    Thread Starter New Member

    Apr 30, 2014
    15
    0
    Oh! I finally realized that the part of the pot that I thought was the pot was actually the switch, and it is just in series with the pot.
    Problem solved, though I don't know how to indicate that fact on this forum.
    Yhanks to all for your endurance.
     
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