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newbie question on probing transformerless ac power supplies with an isolation transformer

Discussion in 'Electronic Repair' started by costab06, Jul 9, 2005.

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  1. costab06

    costab06 Guest

    Hi,

    This is a general question about the use of isolation transformers for
    probing ac powered circuits. The circuit in question is from the
    following application note:

    http://ww1.microchip.com/downloads/en/AppNotes/00954A.pdf

    which describes several designs for transformerless power supplies for
    PIC microcontrollers.

    I built the capacitive power supply in FIGURE 1, and my DMM (battery
    powered, ungrounded) reads 4.8 vdc at vout as expected. Now I need to
    see the waveform. I've been trying to use an isolation transformer
    to be able to see the output wave using my scope, but I can't
    eliminate an ac potential between the ground for the transformerless
    power supply circuit and the ground for the oscilloscope probe. I
    tried the following:

    1) The scope and the circuit under test are each on their own isolation
    transformer. Neither isolation transformer is grounded at the wall,
    and neither the scope nor the circuit are grounded to the isolation
    transformers. All three wires are isolated for the scope and for the
    circuit under test (the line and neutral are isolated via the
    transformer, and the grounds are disconnected and floating). I checked
    the potential between the grounds to be sure I wasn't going to burn
    anything before clipping the probe ground to the circuit. Measuring
    the voltage between the probe ground clip and the ground for the
    transformerless power supply circuit shows 20 vac between the two.

    2) If I tie the grounds for the scope and circuit under test to the
    neutrals on the secondaries of their respective isolation transformers
    (recreating a neutral to ground bond at the secondary at each isolation
    transformer), the voltage jumps up to 48 vac between the probe ground
    and the ground on the circuit under test.

    3) If I put both the scope and the circuit under test on the same
    isolation transformer, with the grounds floating as in number 1 above,
    the voltage between the probe ground and the circuit under test ground
    is then 52 vac

    4) If I put both the scope and the circuit under test on the same
    isolation transformer with the ground bonded to the neutral of the
    secondary of the transformer (as in number 2 above), then the voltage
    between the probe ground and the circuit ground is 115vac.

    In the application note are pictures of startup waveforms for this
    project. I would like to recreate those, but I cannot seem to
    eliminate the potential between the probe ground clip and the circuit
    under test ground, so I haven't actually hooked up the scope....

    Any help would be greatly appreciated.
    Thanks,

    Brian
     
  2. In practice, in a repair shop, the device to be tested is isolated,
    and the testing device is simply plugged into the AC receptacle.

    Tom
     
  3. NSM

    NSM Guest

    Ground the scope normally.

    Isolate the capacitive power supply.

    Connect a ground wire from the scope to the capacitive power supply - I
    assume you can find the correct point to ground.

    N
     
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