Measuring current with o'scope using shunt resistor?

  1. I am having a hard time understanding how to measure current safely in the following circuit.

    The circuit consists of an isolated step up transformer. 50V 1kHz pulsed dc on the primary and 500V 1kHz pulsed dc on the secondary @ 10mA max. I am wanting to measure current on the secondary side of the circuit.

    I am using a propscope (USB oscilloscope).

    Since the secondary circuit is isolated where would I place the 1 ohm resistor?
    Second, how can I measure the voltage across the resistor safely without damaging my scope, computer, or circuit?

    Thanks in advance
     
  2. jcsd
  3. Give us a schematic.
     
  4. Here's the schematic,

    I just want to make sure I protect my circuit, scope, and laptop.

    I'm not sure what the scope probe ground lead is grounded to since the computer operates on the battery when not plugged in. It's not like a typical mains grounded scope I presume...

    Also, since the circuit is isolated I don't know how to connect the ground lead without shorting the circuit?
     

    Attached Files:

    Last edited: Aug 4, 2014
  5. Born2bwire

    Born2bwire 1,776
    Science Advisor
    Gold Member

    If you are ambiguous about the ground, why not use a current sense transformer? Place the primary in series with the low side return and place a current sense resistor across the secondary. Only problem is that you have a small current and low frequency. Off the shelf current sense transformers are usually for high currents that you cannot measure with a resistor directly. But any suitable transformer should work.

    There are other solutions that provide isolation but again are mainly for high current sensing. You could just use an isolation amplifier. Your low frequency does away with most concerns but there is no need to be so fancy-schmancy. A simple isolation transformer is most likely all you need.
     
    Last edited: Aug 4, 2014
  6. jim hardy

    jim hardy 5,187
    Science Advisor
    Gold Member
    2014 Award

    Oscilloscopes generally tie the outside of their BNC input connector to chassis, which is earthed through the power cord's third prong.

    Your 1khz signal generator probably has a BNC connector too. See with your ohm-meter whether it's earthed to power cord "ground" prong. That'll likely be a convenient place for you to use as "ground" .

    Propscope's schematic here
    http://www.parallax.com/sites/default/files/downloads/32220-PropScope-Schematic-RevA.pdf

    shows it tied to pin 5 of a mini-B USB connector
    which is designated "ground" or "signal ground"
    so it probably ties to the power supply return inside your laptop.
    Whether that ties to earth depends on your laptop and its power supply.

    With your ohm-meter, read from pin 5 of the USB connector to 4 places:
    1. To outside metal shell of that connector
    2. To outside shell of other connectors,
    3. To case if it's metal not plastic
    4. To third prong of power cord

    That'll tell you whether connecting your scope 'ground' to something that's unearthed could raise your laptop to a dangerous level of voltage.

    What i would do is:
    First, make sure that transformer really is isolated like your schematic shows, use your ohm-meter;
    Then:
    tie the junction of your 1 ohm shunt resistor and the transformer winding to earth via third prong of power cord (green wire in US, i dont know what color in your country), or via signal generator's grounded(?hopefully it's grounded) BNC connector.
    and connect your 'scope common to that point.
    I made lots of o'scope current measurements that way back in the seventies, before usb's and laptops.

    Fundamental rule from the 1960's: NEVER tie a 'scope common to ANYPLACE besides earth ground.
    Still a good safety practice .
    A lot of 'scope probes go up in a shower of sparks when that advice is ignored.
     
    Last edited: Aug 4, 2014
  7. sophiecentaur

    sophiecentaur 13,916
    Science Advisor
    Gold Member

    As you are only dealing with 500V, why not use a pi network with two high value (say 1MOhm or more) resistor chains 10:1 ratio, from each end of the 1Ohm resistor and use the X-Y facility of the scope? You would need to be much more circumspect if it were several kV on your circuit but 500V is only over the level for worrying too much about.
    You would need to calibrate the measurement circuit against a known lower voltage first.
     
  8. berkeman

    Staff: Mentor

    What is the application? What is that 50kOhm load, physically?
     
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