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lighting circuit help

Discussion in 'Electronic Basics' started by DH, Feb 21, 2005.

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  1. DH

    DH Guest

    I was wondering if any here could help with a bit of info. I've
    been asked to help wire up a kinda chandelier "fallen on the floor"
    lighting display. Basically it's going to be a mess of bulbs, fixtures
    and structure (copper tube, glass, etc...) on the floor. The "look"
    has not yet been completed by the designer and I'm just trying to be
    ready for whatever is thrown at me. It will mostly be made of standard
    25 - 40w bulbs that I will wire up to a variac (silent running is key)
    to control brightness. The designer also has her eye on some lower
    voltage bulbs she would like to mix in the display. Is it ok to add in
    parallel to the 120v lights a series of (lets say) ten 12v bulbs?
    Would the 12v bulbs on the neutral side be dimmer than those on the
    hot side? As this display will be frequently turned off and on (a few
    times a day for a few months) I would like to add an inrush current
    limiter to extend bulb life but am unsure of the placement. Right at
    the variac on the hot or neutral side? Or multiple locations? The
    display will use upward of 1000w and possibly more. Go big or go home!

  2. You means series, I think.

    I'm not sure what would happen (I've never tried it). I know that bulbs
    are very low resistance until they heat up. This causes a surge of
    current which quickly heats the bulb. Once it's at temperature, the
    resistance is such that it limits current to whatever is required for
    the power rating. If your 12V bulb is in series with the big bulb, and
    subjected to 120V, it's possible the 12V bulb would be destroyed by the
    initial surge of current. Once the bulbs are at temperature, though,
    what'll happen is that the voltage across the 12V will be affected by
    the relative wattages of the bulbs. Consider two 100W bulbs, one 12V,
    one 120V. The resistances will be

    120^2/100 = 144 for the 120V bulb, and

    12^2/100 = 1.44 for the 12V bulb.

    Thus, the voltage across the 12V bulb would be

    120 * 1.44/(1.44 + 144) = 1.18V

    For a 100W 120V bulb and a 10W 12V bulb, we have

    120 * 14.4/(144+14.4) = 11V, which is closer.
    No. Elements in a resistive circuit are generally independent of where
    they are in the circuit. It's possible the non-linearities of the bulb
    heating will muck with this, but it'll only be a factor at startup.
    Thus, it may affect whether the 12V bulb will survive.

    As this display will be frequently turned off and on (a few
    Again, it shouldn't matter. These devices can be seen as a resistance
    which gradually diminshes with time. Initially, the resistance is much
    greater than the resistance of the bulbs, and gradually subsides. Thus
    the voltage divider which is created causes most of the voltage to be
    across the limiter. However, again, non-linear effects may be an issue.
    This device may also enable your 12V bulb thing.

    However, if you are using a variac, why not just use it to slow start
    the whole system? That would protect your 12V bulbs.


    Robert Monsen

    "Your Highness, I have no need of this hypothesis."
    - Pierre Laplace (1749-1827), to Napoleon,
    on why his works on celestial mechanics make no mention of God.
  3. Rich Grise

    Rich Grise Guest

    DO NOT put bulbs in series unless they came that way on an Xmas tree

    Get a low-voltage (i.e. 12V) lawn-light transformer, and drive the
    12V bulbs with it.

    Give each bulb its own two-wire feed - you can use surprisingly small
    and inconspicuous wire to power ONE BULB, whether it's a 120V 150W
    or a 12V garden-path light. Put a terminal block in the base, and
    connect each bulb to its own terminals, much like a breaker box.

    But if you want to use 12V bulbs, get a proper transformer.

    Good Luck!
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