Measuring I with oscope in a HV circuit?

Discussion in 'General Electronics Chat' started by electronice123, Aug 3, 2014.

  1. electronice123

    Thread Starter Senior Member

    Oct 10, 2008
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    I want to use my scope to measure current in a series circuit, the only problem is the circuit operates at higher voltage than the scope can handle.

    I was thinking of soldering (3) 1Ω resistors together and placing them in series in the circuit.

    Each resistor will have 20mV max across it (Ipk is less than 20mA). My thinking is that I will measure only across the center resistor, that way I have 20mV on either side instead of -500V or +500V.

    Is this an effective way to measure current in my circuit and protect the scope from the HV?
     
  2. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
    2,503
    380
    hi,
    You will still have HV present on the 3, 1R resistors relative to common/ground.

    Post a diagram showing the resistors etc.
    E
     
  3. to3metalcan

    Member

    Jul 20, 2014
    228
    23
    I think the OP is planning to connect his probe to one end of the resistor and his scope's ground to the other. You actually don't even need three resistors...one in series with the rest of the circuit would be fine. I do this to check bias on vacuum tubes in amplifiers, with B+ voltages above 500V, which would nuke my meter. The resistor has to go in series between the circuit and ground...this is the ONLY safe place to put it.
     
    Last edited: Aug 3, 2014
  4. alfacliff

    Well-Known Member

    Dec 13, 2013
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    there are clamp on probes for scopes to measure current. I use one from tektronix at work. not the kind you use to measure the current in a 60 hz circuit, these are made for higher and lower frequencies. check on some of the test equipment sites.
     
  5. Lestraveled

    Well-Known Member

    May 19, 2014
    1,957
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    What kind of bandwidth do you need?

    There are also hall effect current sensors. These are great devices in that they measure down to DC with complete isolation.
     
  6. electronice123

    Thread Starter Senior Member

    Oct 10, 2008
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    Yes I thought I could just place the three resistors in series with the rest of the circuit, then place the probe leads across the center resistor. Frequency is 5kHz.

    Buying a current probe is out of the question for me as they are way to expensive.
     
  7. MrChips

    Moderator

    Oct 2, 2009
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    You cannot simply connect the probe leads of a grounded scope across any resistor.
    You have a few options:

    1) Put the current sense resistor on the ground side of the circuit.
    2) Use a floating or battery operated scope such as a Fluke Scopemeter.
    3) Power the scope from an isolation transformer.
     
  8. electronice123

    Thread Starter Senior Member

    Oct 10, 2008
    302
    0
    Ok, here's a diagram of what I was thinking.

    The circuit is pulsed DC at 500V, the HV side is isolated with no ground.

    I guess a diagram is worth a thousand words, sorry I didn't include it earlier.
     
  9. MrChips

    Moderator

    Oct 2, 2009
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    Why do you need three resistors when one will do?
    There is no protection provided by the extra two resistors.
    You need to connect the resistor to GROUND.
     
  10. electronice123

    Thread Starter Senior Member

    Oct 10, 2008
    302
    0
    Ok, so you mean something like this?

    The only thing I don't understand about that is then the resistor is in parallel with the circuit, so how can you measure the current?
     
  11. MrChips

    Moderator

    Oct 2, 2009
    12,442
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    No.

    Put R3 in series with your circuit as you did before. Eliminate R2 and R4 like you did.
    Ground one end of R3.
     
  12. electronice123

    Thread Starter Senior Member

    Oct 10, 2008
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    Ok, I get it now.

    Thank you very much for the help!
     
  13. ericgibbs

    AAC Fanatic!

    Jan 29, 2010
    2,503
    380
    hi,
    Your circuit looks OK.
    Ensure that the 1R wattage rating can carry the load current, also many low value resistors are not tight tolerance resistors.
    E
     
  14. alfacliff

    Well-Known Member

    Dec 13, 2013
    2,449
    428
    you measure the voltage drop across the resistor.
     
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