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Using a Wall Dimmer on LED Lightrope

Discussion in 'Electronic Design' started by D from BC, Apr 1, 2007.

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  1. D from BC

    D from BC Guest

    I'm dimming a 9ft piece of manufactured LED lightrope with an ordinary
    household wall dimmer.

    The nice thing about using a wall dimmer is that it doesn't get hot,
    it's cheap, easy to get and no lightrope mods are needed.
    The lightrope has an internal diode and internal resistors to run off

    With the dimmer full on, I don't notice any flicker.
    However, flicker seems more noticeable (but still subtle) when the
    lightrope is dimmed.
    I suspect some combo of LED curve + dimmer waveform + eye persistence
    time to account for the flicker.

    So.... how to get rid of the flicker and still use an ordinary wall

    Could I put the dimmer in series with a bridge rectifier? That'll put
    the flicker at 120Hz..

    Maybe design a more appropriate dimmer?

    D from BC
  2. Lionel

    Lionel Guest

    That's the best way to go. My suggestion would be to run it from DC,
    pulse-width modulated via a high current MOSFET or IGFET switch at
    (for example) 1KHz. That should be reasonably cheap, & won't flicker
  3. D from BC

    D from BC Guest

    Mmm.... 170Vpk pwm on 40ft of lightrope (~300mA)...Isn't that make a
    big EMI transmitter?
    D from BC
  4. Lionel

    Lionel Guest

    It certainly can be, yes. The trick is to put a cap after the switch
    output to remove the harmonics.
  5. Guest

    Going back to the original plan of a bridge rectifier - it may not
    work because then the average power through the light-rope is doubled
    (full wave not half wave), and it may overheat. You could compensate
    for that by setting an upper limit on the dimmer, or rewiring the
    series/parallelness of the lights in the tube.

    Another alternative is to put a large capacitor in parallel with the
    lightrope, as while the lightrope will still get dimmer through the
    "off" parts of the cycle, it won't be fully off and that looks better
    to the eye.
  6. D from BC

    D from BC Guest

    Ahh..good point about the average power... I didn't think about
    that..The lightrope has probably been designed to be max brightness
    (with long life) with half wave.. (The lightrope has that internal
    diode + LED's are diodes.)
    I'll compensate with full wave..Thanks.

    Seems a bit scary to put a filter cap in parallel with the lightrope.
    Let's say power it's switched on right at the sine peak
    (~170V)...That's a big inrush current spike.
    Wouldn't that blow the dimmer(triac) + bridge rectifier combo?
    D from BC
  7. jasen

    jasen Guest

    put a bridge rectifier and an inductor (fluorescent lamp ballast? or three?)
    after the dimmer.

    X LOAD
    is it not currently at 120Hz ?
    probably the best way.
  8. GregS

    GregS Guest

    Or use a transformer with a lower output voltage and a DC filter.

  9. D from BC

    D from BC Guest

    IIRC a (triac based) wall dimmer doesn't rectify the line voltage.
    Maybe call it 4 quadrant phase dependent switching.

    Why the inline inductor after the dimmer? To reduce EMI?
    The triac suddenly conducts at phase>90deg (and >270) across the
    inductor and the light rope.
    Wouldn't the inductor then return with a big nasty spike which will
    stress the dimmer and the lightrope??
    D from BC
  10. D from BC

    D from BC Guest

    Nooooo! :p
    Not variable, bulky, can get hot, expensive compared to wall dimmer,
    needs to ordered and needs a box.
    However, a linear supply will have no flicker.
    D from BC
  11. GregS

    GregS Guest

  12. jasen

    jasen Guest

    an attempt to reduce flicker. the load is basically a fixed voltage drop
    No, inductors don't send spikes to triacs - triacs turn off when the current
    stops. You do get a step however if the current isn't in phase with the

    The bridge recitifier howver should stop the current through the triac at 0V,

    Hmm maybe it needs a 1 ohm series resistor just to make sure, because the
    bridge will be forwards biased by ithe inductor abd so looking like no
    resistance to the dimmer as voltage crosses 0.

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