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40v offset to ground?

Discussion in 'Electronic Basics' started by ScottM, Apr 21, 2006.

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  1. ScottM

    ScottM Guest

    I'm using a small, switching power supply for a project. (It's a Kaga
    sx15u-05s). When hooking it up, I found terminals for line and neutral,
    but no place for a ground wire. It ran fine without one, but after a
    bit I decided I'd be happier running grounded, and looked a bit more
    carefully.

    I found a floating ground (FG), but after reading up on this I decided
    it probably wasn't meant to be hooked to the ground prong of a plug.
    Then I found that the metal cage of the power supply was marked with a
    ground symbol, and had a screw hole to attach a wire to. Ah ha, I
    thought.

    But being nervous, before I hooked up the ground wire, I checked the AC
    voltage between the cage and the outlet's ground - and it came up about
    40vac.

    Is that normal? I'm not going to attach the outlet ground there until
    someone tells me that it is. :)
     
  2. Tom Biasi

    Tom Biasi Guest

    No its not normal.
    The chassis with the screw and marked with the ground symbol is where you
    are supposed to connect earth ground.
    You could be getting stray readings because of the high impedance of your
    meter or something could be wrong. If it were me I would hook the ground to
    the chassis and see.
    Regards,
    Tom
     
  3. Chris

    Chris Guest

    Hi, Scott. First, FG is usually interpreted to mean Field Ground.
    It's there for safety -- to ensure that, if something goes wrong, a
    fuse will blow instead of putting the chassis at line potential.
    Safety first -- use it.

    Your floating power supply will have a small leakage current between
    the line and the chassis, and also between the line and output. That's
    primarily capacitive, and is nothing to worry about for most
    applications. One exception is medical, where tenths of a mA of
    leakage current are critical, for instance with EKG readings -- you can
    get medical-rated power supplies where this problem is greatly reduced.
    The thing is, a DVM can have up to 10 megohms of impedance, so it's
    possibly too good at picking this up.

    Try placing a 47K resistor between the cage and GND, and then measure
    it with your DVM. I'll bet the voltage reading goes way down. If not,
    your power supply is defective.

    Good luck
    Chris
     
  4. ScottM

    ScottM Guest

    I'm using a crappy, old analog meter, and I doubt the impedance is
    megaohms. I'll try the 47k resistor and retest.

    I have two of these supplies, a -12s and a -05s. They both report this
    40v difference, so if it's a bug, it's an infestation. Anyone know if
    Kaga supplies are prone to this kind of thing?

    I'm still not clear on the difference between an FG and a Gnd, but if
    it's ok to tie the outlet's ground to FG, I'm happy to do that. There's
    no actual screw in the screw hole marked Gnd, but there is one in the
    FG position.
     
  5. Ralph Mowery

    Ralph Mowery Guest

    While not always correct, I have found that if you have the old 20,000
    ohm/volt meter or similar that if you change the range to another scale and
    the meter pointer (not the voltage) stayes in about the same position then
    the meter is picking up some high impedance leakage.

    Sometimes a piece of equipment will have the signal ground isolated from the
    actual chassie (frame) . That gives you two grounds. It is similar to the
    way the power company and your house is wired. You have a neutral wire
    going from the braker box and also a ground wire. They are seperate as far
    as the power transmission goes, but are connected at one point back at the
    braker box.
     
  6. Chris

    Chris Guest

    Hi, Scott. Real men use analog meters. ;-)

    Most all switches are made to have the cage connected to GND. You need
    to find the actual current, and to do that, you'll need a realistic
    value resistor. A 47K 1/4 watt resistor is about the smallest value
    that can handle line voltage for a few seconds (.3W), so I'll typically
    use one of those. Use your meter to read the voltage across the
    resistor, and you'll have current. You shouldn't be reading more than
    a couple of volts.

    Power supplies are supposed to be tested for hipot, so this shouldn't
    be a problem unless the power supply is defective. Do the test, and if
    it's OK, connect the line cord GND to the chassis (cage) of the
    switcher, like the manufacturer recommends.

    Good luck
    Chris
     
  7. I consider "FG" to mean "Frame Ground" - it is a terminal used to
    connect the frame of the device to ground.
     
  8. Pooh Bear

    Pooh Bear Guest

    I understand FG to mean 'frame ground' as in chassis i.e. the metalwork.
    Connecting this to the supply ground wire should not affect anything most
    likely.

    Graham
     
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