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Dumb question ...

Discussion in 'General Electronics' started by Jeff Strickland, May 30, 2007.

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  1. .... mostly because I'm dumb.

    What's the difference in a volt-amp and a "regular" amp?

    I have a transformer that makes 24vAC, and is rated at 40 VA, but the amp
    rating is only 1.67.

    I have a lamp (fiber optic system) that takes 24v and 250W, which works out
    to 10.4A. The VA rating does not give me the current I need, and when I plug
    the light in, the voltage drops because the draw exceeds the power by a wide

  2. Rich Webb

    Rich Webb Guest is a starting point. To avoid
    recapitulating what's explained better and in more detail elsewhere,
    volt-amps and WATTS (not "'regular' amps") are the related

  3. Okay, so to distill it down, 40VA is not near enough to drive a 250w light?

    Actually, this is a bit moot because another multi meter in my fleet has
    determined that the lamp is open. I can't explain why I couldn't figure this
    out before I bought the new power supply ...
  4. Daniel

    Daniel Guest

    Gee, that Wikipedia explanation is a bit hard to understand, so here is
    my attempt.

    In D.C. circuits, the voltage peaks occur at the same time as the
    current peaks, so the power in the circuit is found by multiplying the
    voltage applied, i.e. if applying 6 volts causes 2.5 Amps to flow, the
    power dissipated by the circuit is (6 x 2.5) 15 watts.

    However, in A.C. circuits, the effects of the components in the circuits
    can cause the peak current to not flow at the same time as the peak
    voltage occurs. So you may apply 6 volts a.c. to a circuit, and at some
    times 2.5 Amps a.c. may flow at some time, but because the 6 volts and
    2.5 amps do not occur at the same time, it is, technically, incorrect to
    to same the power in the circuit is 15 watts, it is correct to say that
    the circuit draws 15 VA.

  5. Jasen

    Jasen Guest

    It's similar to the difference between a foot-pound and a regular pound.

    it's mreasuring a different quantity.

  6. George

    George Guest

    ....In other words, VA is the power rating, similar to saying 40 Watts. So a
    40VA transformer cannot drive a 250W lamp.
  7. Daniel

    Daniel Guest

    Watts is Power measure in DC circuits where as VA is power measurement
    (sort of) in AC circuits because of possible phase shifts between the
    applied voltage and the Current.

    If, and that's a very big IF, the circuit is PURELY resistive, the the
    AC Power will be the same as the VA rating.

    40 VA transformer can, usually, very easily drive a 250 W lamp, but the
    lamp will, most likely, be fairly dim.


  8. Daniel

    Daniel Guest

    Good explanation, Jasen. Like it.

  9. George

    George Guest

    Sounds to me that the lamp is going to try to get its 10 Amps and will blow
    up the transformer.

  10. Daniel

    Daniel Guest


    The lamp will try to draw its 10.4 amps, but the supply cannot deliver
    that much, so the voltage out of the supply will drop.

    The 40 VA does NOT mean that it can supply 24 V at 1 A. It can provide
    40 VA, which could be 0.5 A at 80 V, 2 A at 20 volts, whatever the
    output of the Supply is set to provide, so it may be that you are
    connecting your 250 V lamp to a supply that only gives 40 volts, so the
    lamp will only be one sixth as bright a it could be, if it were
    connected to the right supply, something that can deliver 250 V at upto
    1 amp.

  11. neon


    Oct 21, 2006
    40va means that @40v you expect to get 1amp with 20 v you can expect 2 amps that is the rating 40v x 1a =40 watts 20v x 2a=40w nothing has changed. to get 250 w this device will not do it it is only 40watts.get the idea
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