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Measuring Current

Discussion in 'General Electronics Discussion' started by rob_croxford, Mar 4, 2011.

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  1. rob_croxford


    Aug 3, 2010
    Hey folks,

    I have a requirement to measure current using a PIC. so it must be able to measure the current proportionally using a 0-5V reference.

    My first idea was to use a current transformer, lets say, (100:1) with a reference resistor of 5(ohms) therefore 5V is proprtionally equal to 100A (2.5V = 50A, 1.75V = 25A etc...)

    is this the best method??

    Cheers :)
  2. Resqueline


    Jul 31, 2009
    That'll give you a dissipation of 5W in the load resistor at max current.. And you'll need to rectify and RMS the AC there. Is the requirement AC current up to 100A?
  3. rob_croxford


    Aug 3, 2010
    For testing purpose i am going to use 1-100 amps as thats a relativly easy scale to work with. Once i have the basics working i will be able to adjust for other requirements.

    will i need some sort of isolation circuit?? another method maybe a small inductor?? I came accross this kind of method when measureing current in busbars. have you come accross this? The accuracy doesnt need to be that good as im going to use the current feedback to meassure power disipated at the load and will be using 230V as a fixed number (which i believe is 2% above and 6% bellow as a national standard?)

    Thanks for your help :)
  4. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    I would consider the following:

    1) measure the magnetic field strength with a hall effect sensor (this will work with DC unlike a current transformer, also isolation will not be a major issue)

    2) have a very small resistance (in the order of milliohms) and amplify this with a stable low-offset op-amp (possibly a chopper stabilised op-amp)

    If you choose option 2, I have a 200A shunt that has (from memory) 200mV across it at full scale. It has about the dimensions of a hacksaw blade, but only about 75mm in length. There are slots cut in it (perhaps with something like a hacksaw) that have clearly been used to calibrate it. Needless to say, you need a 4-wire connection to any device like this.

    Here is one that looks way better than mine :)

    if your op-amp allows inputs to zero volts, and you can insert this on the low side, then you may be able to get away without isolation.

    If you can't then perhaps use a small isolated dc-dc converter to power the thing and send the output via an optocoupler.
  5. rob_croxford


    Aug 3, 2010
    Hi Steve thanks for the info :)

    Hall effect sensor, could this be what i have encounted with a variety of busbars (used to measure current?)

    I Have been having a look (altho i am awear of the theory of "hall effect") I have not come into contact with these devices before. The ones i have been looking at require a magnet to toggle the Drain pin - is it safe to assume that not all of these devices require an external magnet to operate?

    Sor far my requirement is as follows: I have written some code to meassuer the current and power seen at a load. This is to display these values on an LCD screen. at the moment (lowest spec) it will be trialed on a 3KW Heater control board. My idea was that aswel as being able to meassure the current and power if it was possible to get a small voltage signal that is directly proportional to the current this could also be used for feedback.
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