Hi current impedance measurement

Discussion in 'General Electronics Chat' started by Col, May 22, 2012.

  1. Col

    Thread Starter Member

    Apr 19, 2012
    44
    2
    I have a simple LC notch filter for a power lines communication application.

    I want to test my filters impedance change while carrying a high current, upto about 15A. In my application this current will be at 50Hz, but I'm happy enough to test at DC. My notch frequency is around 10MHz

    I have an impedance analyzer (Agilent 4294A). However this can only provide a bias current of 100mA. MY 15A will be provided by a DC power supply (at a pretty low voltage) but I am very nervous about my impedance analyzer. Can someone suggest a set up to measure my filter impedance while carrying 15A without breaking my analyzer.

    Simply, I consider DC blocking caps... but would they have to be huge and would they ruin my filter impeance measurement??
     
  2. crutschow

    Expert

    Mar 14, 2008
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    3,237
    You might check on the analyzers input DC voltage rating.

    But if you need a cap, if the notch frequency you want to measure is at 10MHz, then the blocking capacitor doesn't need to be that large. For example, a 0.1uF cap (film or ceramic type) has an impedance of 0.16 ohm at 10MHz which should have little effect on your measurement.
     
  3. Col

    Thread Starter Member

    Apr 19, 2012
    44
    2
    My filter has an extreamly low ESR so my externally applied DC voltage will be very low... 0-1,5V... The analyzer is good for up to 40V. Its the current I'm worried about. What if the cap blows, or shorts and my 15A ends up flowing into the ground of my anaylzer?

    Will an isolation capacitor not explode if it sees a step change of 0A to 15A

    Also my ESR is in the region of 0,1 Ohm (at 10MHz), so this 0,1uF cap will significantly change my filter.... however maybe it could work, since I am only interested in seeing if my resonance frequency changes under high current conditions
     
    Last edited: May 23, 2012
  4. crutschow

    Expert

    Mar 14, 2008
    13,028
    3,237
    If the analyzer is good for 40V then I don't see a problem. The analyzer has a high impedance so no significant current can flow through the analyzer.

    Note that you will need to add a high current inductor between the power supply and the filter, otherwise the analyzer will be measuring the impedance of the power supply, not the filter. The inductor value needs to be much higher than any inductance in the filter.

    The isolation capacitor sees the voltage change, not the current change.

    If you are interested in only a change in the resonant frequency, then the value of any isolation capacitor won't affect that.
     
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