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Measure average current

Discussion in 'Electronic Design' started by Hammy, Oct 3, 2010.

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  1. Hammy

    Hammy Guest

    I want to be able to measure a pulse unipolar with a DC offset
    (programmable) from 10mV to 250mV; maximum amplitude of 500mv square
    wave Variable duty cycle 10 to 50%.The frquency of the pulse is
    seletable from 10kHz to 100kHz. The measurement is a square wave from a
    resistor current shunt.

    The idea I have is use an rms to DC converter IC and feed the DC
    output to a PIC 10 bit ADC clocked at 20MHz and do the math on the
    pic to get the average current then calculate the average power. I
    was wondering is this the best way to go about this or is their a
    better way? Looking at rms to dc IC's with 100 kHz + BW aren't
    exactly cheap.
  2. Sylvia Else

    Sylvia Else Guest

    RC filter and measure the resulting voltage?

  3. Hammy

    Hammy Guest

    I could cheat the PIC will be setting the pulse frequency and duty and
    magnitude and range so I know what the value would be assuming
    everything works as it should.;-)So far it does but it would be nice to
    know for sure by being able to take an actual measurment.

    I just thought it would be a nice feature to add to my programable
    load. That way I could display total power and avg current.But its
    looking like it might be to complex and to expensive to do so I might
    be having to settle with just the ability for DC mesurments.

    I dont have FPGA hardware or experience using them,that is likely the
    way it should be done though.
    The fastest rise times are about 100nS or so. Thanks its something I'll
    look into.
  4. Hammy

    Hammy Guest

    Thanks Jan you gave me a couple of ideas I can look into further.

    The pulse amplitude is 500mV max. I'm using a 16F887 for know; it
    has 2 comparators and a 10 bit ADC.
  5. Jamie

    Jamie Guest

    I don't know why you can't just directly have the ADC input read it
    as is and do the math in code ?

    The ADC most likely won't produce the exact square wave result but
    simply round it a bit how ever, all you're looking for is a change in
    amplitude from high to low/ low to high and just measure the Pk-PK
    values in code..

    I did this a few years ago experimenting with a AM+FM type carrier
    protocol to pass information over short wave using a simple PIC with a
    ADC on board. The problem I ran into was some of the receivers in use
    didn't have a slow enough AGC if I extended amplitude change for too
    long. So a sync signal had to be applied with full amplitude with
    short frames.. Much like Analog Video..

  6. Hammy

    Hammy Guest

    Its for a programable load I built. I want to add the ability to
    measure the current pulse. The pulse is square and can between 10mV to
    500mV. The maximum frequency is 100kHz, minimum duty cycle is 10%. So
    a 1uS pulse width is the worst case.

    By takeing the average current I can calculate the power dissapeted in
    the pass FET's.
  7. Hammy

    Hammy Guest
    It will be a square wave on a DC offset that can vary from 10 to 60%
    Vmax where Vmax is 500mV. The voltage signal is from a current
    shunt.I want the average so power dissapation can be calculated and
    I've done some more research and the typical way to do it seems to
    be sample over a period and take the average of the ADC. But hey if
    you know of a simplier way to do it I'm all ears.

    The way I was thinking of doing it was start sampling at the start
    of one period and stop at the end "do the math" to figure the

    Its for an adjustable load I built for testing power supplies just
    another little feature I thought I would add. Its simple to
    calculate the power for DC I just sense the input voltage (scaled)
    using the PICS ADC and sense the current and voila I can display
    current voltage and power. I'm also displaying heatsink or case temp
    of the FETS. I want to do the same thing when I test PSU's with a
    load step.

    I'm still looking into it.;-)
  8. Hammy

    Hammy Guest

    It is mosfets that I'm mesuring the power in.
    Huh did you read my whole post? I'm sensing the input voltage its
    for a programable electronic load 300W countnous to 600W peak.
    The load is 6 mosfets. The voltage across the FETs is the input
    voltage minus the shunt voltage. So P=(Vin - Vshunt)* Iavg.
    Yep your right that is an option I never thought of.
  9. Hammy

    Hammy Guest

    Actually P= Vin * I avg. I would want to display the power the PSU
    under test is delivering.

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