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Wrong transformer or resistance??

Discussion in 'Electronic Basics' started by jan, May 20, 2004.

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  1. jan

    jan Guest

    Hi All,

    You've helped me out before and am hoping you can do it again.

    I bought three battery operated light fixtures that you use in tight
    places with no electricity [like a small closet]. Each takes four
    batteries, so 1.5volts in series=6 v DC.

    They work great and I wanted to use all three under a cabinet and,
    instead of batteries, I would use a transformer. I found an 8volt
    transformer rated at 325ma and wired the lights in parallel. With one
    light on, all is fine. Two or three lights on and they get dimmer and
    dimmer. Finally, two of the bulbs burned out, one after another.

    Should I get different bulbs [screw base], a different xformer or need
    to wire them differently??

    Thanks in advance.

    Jan
     
  2. grahamk

    grahamk Guest

    You need a 6 volt transformer. 8 volts is too high.
    To find the current required for each lamp, divide its wattage by 12.
    Multiply this by 3 (for 3 lamps).
    The transformer must supply this current at a minimum, preferably a bit more.
     
  3. grahamk

    grahamk Guest


    SORRY!! I mean divide it by 6.
     
  4. CFoley1064

    CFoley1064 Guest

    Subject: Wrong transformer or resistance??
    Assuming your transformer can handle the current of all three bulbs in
    parallel, here's a way to drop the voltage -- it aint pretty, but it'll do the
    job (view in fixed font or M$ Notepad):

    6VAC From 8VAC

    FU1 D D D
    ____ .-->|-->|-->|--.
    o-|_--_|--. ,--o o----o------o------.
    )|( '--|<--|<--|<--' |
    120 VAC )|( D D D | | |
    o---------' '--. .-. .-. .-.
    | ( X ) ( X ) ( X )
    | '-' '-' '-'
    | | | |
    | |
    '-------------------o------o------'

    No matter which way the current is going, there'll be about 2V dropped across
    the three diodes before any significant current flows. That will give you an
    effective 6VAC.

    Now either set of three diodes is only conducting half the time, so you can
    play with the current rating of the diodes a little. If your total current is
    less than 1.3 amps or so, you can go with 1N4001 diodes. If it's less than 4
    amps, you can use 1N5401 diodes for D.

    It's probably more sensible to just get a 6VAC transformer, but if not, I hope
    this helped.

    Good luck
    Chris
     
  5. jan

    jan Guest

    Hi Chris,

    Since the xformer is too low on current, would it be better to get a
    higher current 6vdc xformer? Acutally, I don't understand why I
    should go to AC.

    Jan
     
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