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Dimmer won't work when connected to power inverter

Discussion in 'General Electronics' started by Chuck, Oct 13, 2007.

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  1. Chuck

    Chuck Guest

    Dimmer won't work when connected to power inverter

    I successfully tested an 18ft long section of rope lights (Hunter brand) and a 3-stage touch dimmer in my house before taking out to install it in my RV. After plugging it into my 12VDC to 110VAC power inverter it just has one brightness level which is maximum brightness. Once its on, I can't turn it off again without unplugging it. I tried using a standard incandescent rotary dimmer with the exact same results. Why is this happening?

    Why does it work properly on 110V 60Hz household current but not with the power inverter? Thanks.
     
  2. Dimmer won't work when connected to power inverter
    I successfully tested an 18ft long section of rope lights (Hunter brand) and
    a 3-stage touch dimmer in my house before taking out to install it in my RV.
    After plugging it into my 12VDC to 110VAC power inverter it just has one
    brightness level which is maximum brightness. Once its on, I can't turn it
    off again without unplugging it. I tried using a standard incandescent
    rotary dimmer with the exact same results. Why is this happening?

    Why does it work properly on 110V 60Hz household current but not with the
    power inverter? Thanks.
     
  3. Chuck

    Chuck Guest

    OK... I will try various dimmers and try adding caps across the output to
    see if I can get it to dim. Thanks.
     
  4. Mmm... not sure if adding caps will work. The type of dimmer that is most
    prevalent in the US uses a TRIAC
    (http://www.epanorama.net/documents/lights/lightdimmer.html).

    These things simply trigger on the voltage level as the sine wave swings up
    and down. This controls the "on" time for each cycle, and hence the
    brightness of the (incandescent) lamp. If you feed in a square wave, there
    is no variable voltage threshold to trigger on, because the square wave
    turns on and off sharply. The dimmer will not dim.

    You need a dimmer that is based on timing instead of threshold, but they are
    not as common. The article I linked to should explain just about everything
    you need to know about dimmers.
     
  5. Eric Sears

    Eric Sears Guest

    I suspect that "Chronic P" is correct - but its a combination of TWO
    factors - 1) the mod square wave of the inverter and
    2) the way in which that particular dimmer works.
    Not all dimmers are the same. Some use the slope of the sine wave, but
    others use the charging of a capacitor to trigger them.

    I use a mod square inverter ( probably like yours, only 230v), and it
    works perfectly with a number of dimmers I have tried - including with
    both incandescent lights and battery chargers (for varying the
    charge). In fact I charge my electric car (a converted Daihatsu Mira)
    with a battery charger that runs on my mod sq inverter. Charging is
    controlled with a dimmer, and gives smooth charging from zero to 8
    amps at 60 volts - through a transformer.
    Suggestion - try a cheaper dimmer! Most ordinary light dimmers,
    designed for mounting in a wall switch are still the cap type here in
    NZ - but they are probably electrically "noiser" than the more
    expensive ones.

    Eric Sears.
     
  6. Chuck

    Chuck Guest

    Thanks for the link. I have found a few "pure" sine wave inverters but the
    cost is considerably higher.
     
  7. Eric Sears

    Eric Sears Guest

    NOTE! I did not say anything about adding a cap to get it to dim. I
    said they use a cap - its in the trigger circuit. You can't just add a
    cap across the input or output.
    Yes, they all use triacs - but there are various ways of firing or
    triggering it.
    If you use the straight sine wave, then when the voltage of the sine
    wave reaches a certain level (set by the "dimmer" pot), the triac
    turns on - and won't turn off again until the half-cycle finishes.

    However, if you incorporate a cap with the dimmer pot, you have a time
    constant, so that the cap begins charging, and reaches a point when it
    triggers the triac "on". For this charging to occur, it doesn't matter
    whether it is a square wave or sine wave (just a bit different
    adjustment of the pot). The lower the resistive value that the pot is
    set to, the quicker the cap charges on each half cycle, and the sooner
    the triac turns on.

    They are actually very simple devices and if you google "light dimmer
    circuit" who will find many on the internet. Basically they have just
    4 components - triac, pot, cap and a diac (some don't need the
    latter).

    Hope this helps.

    Eric Sears.
    Note - the problem with those that do NOT use a cap is that they must
    trigger "on" during the first half of the "half-cycle". Since they
    cannot turn off again until the zero-crossing point (for ordinary
    triacs), full dimming is not possible. However, they are some triacs
    which act more like a (bi-directional) transistor, and turn off with a
    lower threshold of input - these will give full dimming.
     
  8. Yeah, that's the problem with the sinewave inverters. A compromise is a
    stepwise inverter, which might give you a number of discrete levels of
    dimming.
     
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