Oscilloscope Current Probe

Discussion in 'General Electronics Chat' started by Joster, Oct 5, 2014.

  1. Joster

    Thread Starter Member

    Jun 12, 2013
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    Hi All,

    I am testing a pulse charge battery charger. It has a cap bank that is charged up then discharge quickly into the battery. I need to get a current probe for my scope to be able to see the transient current pulses. All the probes I see on ebay (the good ones at least) are pretty expensive. Is there anyway to do this on the cheap or should I just start saving?

    Thanks,

    Joster
     
  2. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
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    BR-549 likes this.
  3. crutschow

    Expert

    Mar 14, 2008
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    Why not just put a small resistor in series with the charger common terminal and measure the voltage across it?
     
  4. Joster

    Thread Starter Member

    Jun 12, 2013
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  5. Lestraveled

    Well-Known Member

    May 19, 2014
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    Because in a high energy pulse environment, common mode and parasitic noise can easily overwhelm a reading using a resistor. I learned the hard way that a Pearson current transformer or a non-contact current probe is the only way to get accurate pulse current measurements.


    Joster, start saving.
     
  6. Joster

    Thread Starter Member

    Jun 12, 2013
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    ok so ur recommendation is that the diy version will not cut it and I should start saving then?..I have a good connection with good electronics surplus dealer who is hopefully going to have 1 or two.
     
  7. Lestraveled

    Well-Known Member

    May 19, 2014
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    Here are some examples:

    9 us pulse width, 6 amps

    - The top trace is the voltage across a .01 ohm non-inductive resistor.
    - Bottom trace is the output from a Pearson current sensor (model 4100, 1Volt/amp)
    [​IMG]


    Below
    - Same signal, different scale on the top trace.
    [​IMG]

    If you did not know what to look for in the top trace, by having seen the bottom trace, you would have little chance of getting any useful measurements.

    The Pearson 4100 current transformer that I used.
    [​IMG]
     
  8. Lestraveled

    Well-Known Member

    May 19, 2014
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    I tried and failed several times to build my own current transformer (similar design as the above white paper). Pulse widths over a few hundred microseconds, no problem. Under 100 microseconds, forget about it.

    There is a reason that the new list price for a Pearson CT is around $1000. I bought my 4100 used on E-Bay for a little over $100. It is worth every penny.
     
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