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UPS gives funny voltages - why?

Discussion in 'Electronic Basics' started by larry moe 'n curly, May 27, 2005.

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  1. I have a Conext (by APC) model CNB-325 battery backup power supply (not
    a real UPS) that shows funny voltages when the power cord is

    Backup supply plugged into AC outlet;

    hot-neutral: 120 VAC
    hot-ground: 120 VAC
    neutral-ground: 0.7 VAC

    Backup supply unplugged from AC outlet, computer used as load:

    hot-neutral: 115 VAC
    hot-ground: 58 VAC
    neutral-ground: 58 VAC

    Backup supply unplugged form AC outlet, no load:

    hot-neutral: 115 VAC
    hot-ground: 19 VAC
    neutral-ground: 60 VAC

    A different model Conext, model CNB-300, behaved similarly, only the
    no-load voltage on battery power was 90 VAC neutral-ground and 40 VAC

    I didn't get such wierd voltages on battery operation with a much older
    APC BK-300 (much larger battery, all-metal case); hot-neutral and
    hot-ground both measured 118 VAC, neutral-ground was slightly under 1

    I measured with both a Fluke 73 digital meter (true RMP) and an analog
    meter with 10K/volt sensitivity. The analog meter gave very different
    voltages with each backup supply running off battery, but I assume it
    was caused by the AC not being a real sine wave.
  2. Suggests the output is connected directly to the mains.
    This is not unusual; the output is symmetrical about earth for safety
    (as the biggest shock you can get is 58 volts). Most isolation
    transformers are wired this way.
    It sounds like the lack of load is doing strange things to the
    regulation, and probably the waveform. Many supplies that are derived
    electronically exhibit this effect - and remember that uneven
    waveforms will play havoc with a meter because it won't know which bit
    it's supposed to be measuring.

    Look at it on a scope and see what's actually coming out.



    ...."Dial only the last eight digits for calls within the London area"
  3. colin

    colin Guest

    the chances are when its not conected to an ac suply the output is floating
    with respect to ground, so any voltages relative to gnd are due to leakage,
    with a load conected there is more balanced leakage.

    Colin =^.^=
  4. Jim Adney

    Jim Adney Guest

    Backup is line powered and is just passing thru the relationship that
    your AC line has.
    As someone else said, ground is floating and the load leakages are
    pretty nicely balanced, and lower impedance than your Fluke.
    Here the output is completely floating, but you can see that the
    leakages in the backup are not particularly balanced. The 10 MOhm
    impedance of the Fluke is significant here; it has pulled each one
    down a bit so that the last 2 numbers no longer add up to the first.
    If you had a second Fluke so that you could actually make those 2
    measurements simultaneously, you would get numbers that add up right.

  5. Franc Zabkar

    Franc Zabkar Guest

    The PC PSU has RF suppression (?) capacitors between A-E and N-E. If
    the earth pin is disconnected, then its potential floats to a point
    midway between A and N.

    - Franc Zabkar
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