12/24V Incandescent Light Bulb Dimmer to drive MR-16 LED Lamp

Discussion in 'The Projects Forum' started by monkey19939, Jul 17, 2007.

  1. monkey19939

    Thread Starter New Member

    Jul 10, 2007
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    I need to drive an MR-16 LED lamp using a 12/24V Incandescent Light Bulb Dimmer.

    Problem: that dimmer is expecting some very low resistance loads and MR-16 LED Lamps are designed with IC LED drivers (very high resistance).

    If I connect a 10-ohm 10-Watt Resistor in parallel with this MR-16 LED Lamp then it works. But wasting lots o energy on that Resistor.

    Solution: a trick circuit (low resistance and low current) to turn ON Dimmer so MR-16 LED Lamp can function normally.

    Thank you for helping out
     
  2. SgtWookie

    Expert

    Jul 17, 2007
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    Use a low-wattage incandescent bulb across the output of the dimmer.

    Then, connect up your LED's, and determine the setting where current through the LED's is about 5% less than the manufacturer's recommended max current.

    Then measure the voltage across the bulb.

    See if you can find another low-wattage bulb that's rated higher in voltage than what you measured. The closer the v rating of the bulb to what you measured, the brighter the bulb will be and the shorter it's life will be, but it will be more efficient due to the low current wasted. If you, say, doubled the voltage rating of the bulb, it would waste some current, but the bulb would glow only dimly, and would have a much longer-than-normal service life.

    At room temperature, the filaments of incandescent bulbs have very low resistance. However, as voltage is applied and the filament gets hot, it's resistance greatly increases. You can figure out the resistance of a bulb by measuring the current through, and voltage across it, then applying Ohm's Law.

    Apparently, the circuitry inside your dimmer requires a "jump-start" of low resistance - but once it has current flowing through it, it'll continue to function.
     
  3. monkey19939

    Thread Starter New Member

    Jul 10, 2007
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    Hi SgtWookie:

    You are right the dimmer is expecting a low resistance to function normally. So I need something low in resistance and low in current too, to get dimmer to work and not to waste too much turning dimmer ON.


    TKS
    Monkey19939
     
  4. SgtWookie

    Expert

    Jul 17, 2007
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    Ahhh, did you read the note that I wrote to you in it's entirety?

    Because what I already wrote describes what you are apparently still looking for.

    Or is there something that you don't understand?
     
  5. SgtWookie

    Expert

    Jul 17, 2007
    22,182
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    Something else you could try is to use a fairly large electrolytic capacitor across the output of the dimmer, say 220uF or so, that has a rating of at least 25v, in series with your lightbulb. When the dimmer is first turned on, the capacitor will have no charge on it, so it will pass current on through the bulb; to your dimmer it will look almost like a dead short until the bulb's filament begins to get hot. As more current flows through the bulb, the voltage potential increases across the capacitor until it is charged to nearly the same as the output of the dimmer.

    Meanwhile, the MR-16 should have lit up, and will be consuming current, which may be sufficient for the dimmer to remain on. If it doesn't - well, then you need more of a load.
     
  6. monkey19939

    Thread Starter New Member

    Jul 10, 2007
    3
    0
    Dear SgtWookie:

    Thank you for your reply, yes, a resistance of 50-ohm or less will turn ON the dimmer but that 50-ohm gets hot :eek:(

    Attached is a fakeload concept to share with you, not tested yet. Its a 20mA current limiting ckt, please comment :eek:)

    TKS
    Monkey19939
     
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