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best way to measure current?

Discussion in 'Misc Electronics' started by Ken Williams, Jun 29, 2009.

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  1. Ken Williams

    Ken Williams Guest

    I have a coil. I want to accurately measure the amount of
    energy/current that it uses when pulsed with one single square wave. I
    know I can take a voltage and current measurement and multiply them
    together to get a wattage. is that the only way? is there such as
    thing as a wattmeter? that just simply tells me how much energy was
    just used? I guess its a stupid question.

    a side question - if I set my power supply to "constant current mode"
    (say at .3A) and at 12V the most my coil can draw is 3.6 Watts correct?
    so this would happen when I simply short the power supply with my coil.
     
  2. Don Stauffer

    Don Stauffer Guest

    Not a stupid question. In fact, it is a very involved problem in AC
    measurement. One needs to consider the phase of the current and
    voltage. Google "AC current" and phase. The current through, and
    voltage across an inductance (and a coil has a LOT of inductance) occur
    at different points in the cycle. So the determination of power in an
    AC cicuit containing a big inductor or capacitor is not a trivial problem.

    And pulsed power IS AC. There are simple power meters (wattmeters) and
    complex ones. The later, which do what you want, are not cheap.
     
  3. Charles

    Charles Guest

    A sqaure wave (I'll assume a constant voltage source) across a coil will
    produce an exponential current response with a time constant equal to L/R.

    The coil will store the energy and give it back and the resistance of the
    coil will convert some of it to heat.

    Your question can only be completely answered if you provide more
    information. Circuit analysis is often divided into transient and
    steady-state and average categories. It all depends ..............
    Watts are not always the same as volt-amperes. Assuming a DC supply, the
    power will be dissipated in the resistance of the coil.
     
  4. Resqueline

    Resqueline

    2,848
    2
    Jul 31, 2009
    You have to use an oscilloscope and a boosted pulse generator.
    Measure the voltage across a 0.01 Ohm series resistor for instance to read the current.
    You'll find that the current starts at zero and rises linearly with time, until the coil core saturates - at which point the current starts to rise more rapidly. The current will eventually level off at U/R (if you pulse that far) but the stored energy won't increase over the saturation level mentioned..
    It's easy to boost a pulse generator to 50A capability using a darlington or a MOSFET (and a big cap for current storage). A snubber circuit is required if using a MOSFET. You only need a 100mA 12V supply if you use low repetition rate (1-2Hz).
    The inductance of the coil would be: delta time * pulse voltage / delta current
     
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