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Isolation transformer?

Discussion in 'Electronic Basics' started by Hammy, Mar 14, 2008.

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  1. Hammy

    Hammy Guest

    I have a 450VA isolation transformer. The transformer is supplied from
    a three prong outlet. I've taken some measurements with my DVM they
    show a voltage between lines to ground on both outputs. The combined
    voltages between both lines to ground don't even add up to the line to
    line voltage.

    Heres a schematic of the measurements.

    I just got this transformer but I've already used it to test a few
    line powered circuits. These measurements must be wrong or I would
    have blown fuses when hooking up my scope to the circuits.

    I use the isolation transformer on the test circuits not on my scope.
    For example if I were to hook up a flyback to the isolation
    transformer and use my scope (un-isolated) to take primary side
    measurements according to those measurements I should be getting a
    short through my scope ground lead. This hasn't happened both the
    isolation transformer and the flyback are fused; neither blew.
    Can anyone explain this?
  2. If you want to do a simulation, add a small capacitor from the primary to
    the secondary of the transformer, and remember that your meters have about
    10 MegOhm resistance. The transformer has some capacitance from primary to
    secondary due to the overlapped windings and the insulation.

    A really good isolation transformer (such as medical grade) will have
    separate bobbins for primary and secondary, as well as electrostatic
    shielding connected to ground, which will greatly reduce this capacitive

    You can get an idea of this leakage current by putting a 1 k resistor
    across your voltmeters. You might read a couple of volts, which indicate a
    couple milliamps of leakage, which is nothing to be concerned about. The
    threshold of sensation is about 2-5 mA, and dangerous levels are generally
    20 mA or more. But you really should not be touching any part of the 120
    VAC secondary circuit.

  3. Hammy

    Hammy Guest

    Thanks. I was concerned about my scope when I seen those measurments.
    When you start paying for your own instruments you stop takeing things
    for granted.
    I'm using an inexpensive TEMA model. Safe to say not in the medical
    grade catagory.
  4. Jamie

    Jamie Guest

    Your seeing the effects of the over laid windings which creates a
    capacitance coupling.
    Try placing a load on the output and then test it."
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