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
     
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  2. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
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  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
    95
    1

    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.
     
  9. daniyalmb

    New Member

    Sep 8, 2017
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    Use a shunt resistor in circuit but it will disturb your circuit, However if you're looking for a cheap current probe thn go for fluke 80i or intellisens i2. Fluke is an established brand and clampable, intellisens is good for research and cheap but not clampable which is a con for me. On the other hand you can get some cheap 100$ Chinese probes on amazon too but they often come with some problems.
     
  10. Janis59

    Member

    Aug 21, 2017
    81
    14
    Hmmm, when the low frequency, there any resistor or coil will be good enough. But when one have a deal with high frequency, at least I was incapable to solve this thing well. I tried a noninductive resistors, I tried a capacitative dividers, I tried an impedance compensated adjustable circuits, but anyway I got that I utilize the many-fold more power out of my `black-box` than it consumes from PS, where to DC ampermeter I can believe absolutely. Thus, I decide any current probe for RF may be considered only to show the +/- 1000% accuracy, and this is the best case scenario.
     
  11. DickCappels

    Moderator

    Aug 21, 2008
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  12. crutschow

    Expert

    Mar 14, 2008
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    Why does the top trace look like it's AC coupled?
     
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