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Shunt Resistors?

Discussion in 'Electronic Design' started by @donis, Apr 18, 2006.

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  1. @donis

    @donis Guest

    Hi guys,

    Im working on a 30A/120V power meter and would like to use a shunt
    resistor? what value do you guys recommend for my application? Any
    manufacturer's in particular?

    Thanks In Advance.
     
  2. John Fields

    John Fields Guest

     
  3. @donis

    @donis Guest

    Certainly John, sorry about that...

    Im working on a power meter residential 3-wire 120VAC. 30 AMPS MAX
    current. I want to use a shunt resistor as my current sensing
    transducer but I'm not sure what criteria should I use in selecting the
    shunt. What manufacturer or what material type should be the best for
    handling a current of 30A.
     
  4. John Fields

    John Fields Guest

     
  5. Robert Baer

    Robert Baer Guest

    If you need isolation, you would be better served using a current
    transformer.
     
  6. Guest

    The easiest shunt to get hold of would be the Hobut ammeter shunts
    stocked by Farnell/Newark. The smallest on offer are two 60A shunts
    with 60mV and 75mV voltage drops at rated current. Both cost close to
    $50.

    Farnell also stock a fair range of low resistance resistors which are
    quite a lot cheaper - the Tyco parts mostly specify temperature
    coefficients of resistance, which puts them ahead of the Vishay parts
    that Farnel stocks, but the specifications range from +/-250ppm/C down
    to +/-100pp/C for the thick film parts down to one 75ppm/c part.

    None of them are remotely competitive wuth manganin.
     
  7. John  Larkin

    John Larkin Guest


    Need isolation? Cheap single-element meters don't, but 2-element US
    120/240 volt meters do.

    If you don't need isolation, or can get it somehow, punching or
    chemically milling shunts out of sheet manganin is the way to go. You
    can even use wire manganin, soldered to a pc board, at 30 amps. IRC
    sells some nice croquet-hoop manganin shunts. Beware magnetic
    pickup... zero is a surprisingly small number.

    I did a meter a few years ago, designed to be manufactured in India,
    where it's 240 volt, single-phase for residences. The parts cost was
    $9 or something like that. We used a simple manganin shunt in the low
    leg and a MC68HC05 uP with the built-in 8-bit mux'd ADC and a lot of
    number crunching. It had a bunch of anti-tamper provisions, since
    stealing power seems to be a national pastime in India. Worked great
    but didn't sell.

    John
     
  8. Ross Herbert

    Ross Herbert Guest


    Why not use an off the shelf clamp type sensor/transducer?
    eg. http://www.industrysearch.com.au/products/viewrecord.asp?id=4815

    All the hard work is already done for you.
     
  9. Guest

    AFAICR Farnell also stock IRC's OAR resistors down to 5 mOhms at 5W,
    and around 20ppm/K for under $2; parallel two if needed for less
    thermal effect at 30A.
     
  10. Guest


    Thanks Tony. I'd missed the "Low Ohmic Value" section in the Farnell
    "resistor, potentiometer and thermistor" section. The OAR and OARS
    parts from TT Electronics/Welwyn are indeed very attractive and cheap,
    and I should have found them myself.

    20ppm/C is definitely close to what you can get out of manganin,
    although manganin's resistance doesn't vary linearly with temperature
    around room temperature - see the data sheet (in English!) at

    http://www.isabellenhuette.de/pdf/widerd/MANGANIN-ISABELLENHUETTE-R.pdf

    The curve in Graph 2 at the bottom of the first page is informative.

    Rayner and Kibble, in "Coaxial AC Bridges" (ISBN 0-85274-389-0
    http://www.npl.co.uk/electromagnetic/publications/guides/ac_bridges.html)
    prefer Evanohm
    (http://www.goodfellow.com/csp/active/static/A/Evanohm_-_Precision_Resistance_Alloy.HTML).
     
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