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Markings on Big shunt - what is R?

Discussion in 'Electronic Design' started by Hawker, May 31, 2007.

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  1. Hawker

    Hawker Guest

    I have two large current shunts a client gave me.
    One is marked 25 amps 50mV SE company
    The other is 20amps 50mV.

    Am I to take it that they read 50mV at rated current?
    Or in other words the 25A one is .002 ohms and 20amp one is .0025 ohms?
    My VOM isn't very accurate down there.
  2. Steve

    Steve Guest

    As far as I know, you are correct. Every shunt I've used is marked
    for full scale current, and output voltage at that current.
  3. Guest

    If you want to measure that sort of resistance, you need a four-
    terminal (Kelvin) measuring system.

    Upmarket multimeters offer this facility - the Thurlby-Thandar 1906
    (Farnell order code 724-026) offers four terminal resistance
    measurement, but since the resoultion only goes down to 1 milliohm, it
    wouldn't do you much good.

    Thurlby-Thandar do offer a micro- and milli-ohmeter - the BS407
    (Farnell order code 381-2364) which is somewhat more expensive, but
    can resolve resistances up to 1.999 milliohm to one micro-ohm, and
    19.99 milliohm to 10 uohm.

    Top of the line Hewlett-Packard (now Agilent) and Datron multimeters
    do better than the TTI 1906, but cost quite a bit more.
  4. But for the OP to simply check his understanding of the markings, he
    just needs to stick a few amps through it and measure the voltage
    developed across the terminals of the shunt. I usually use a power
    supply with an adjustable current limit.
  5. Exactly. Most cheap DMMs have 100uV or better resolution at full
    accuracy, so at 3A you have 33 uohms resolution.

    Be sure to apply the 3A to the outer (usually larger) terminals of the
    shunt and read the voltage from the inner (usually smaller ) set of
  6. Guest

    He'd better measure the voltage twice, reversing the direction of the
    current between readings - low level voltage measurements are
    bedevilled by thermocouple voltages developed in the junctions between
    dissimilar metals, and measuring with AC or at least reversing DC is
    the standard way of getting rid of these offsets (or at least of
    getting some idea how bad they are).
  7. Would you not need quite a big temperature differential (between the
    ends of the shunt), for that to be significant?
  8. Guest

    The last time I was using a really good DVM to measure low voltages, I
    found draft shields were absolutely essential to keep the voltage
    stable. Most measuring set-ups have different metals all over the
    place, and base metal thermocouples give you about 50uV/C.

    You have to be very careful if you want to get anything useful out of
    the uV resolution of a good DVM, particularly when measuring
    resistances - which automatically involves dissipating some heat.

    The fact that the manufacturers data sheets derate resistors linearly
    against ambient temperature doesn't means that the temperature rise of
    a resistor is a linear function of temperature - a low dissipations
    the Maclaurin number is below 500 and you don't get any significant
    convective cooling at all, so the the resistor is a lot warmer at low
    power dissipations than you'd expect from linear extrapolation.
  9. Several degrees C (more than 5 is my guess) at least just to tick the
    LSD. Forget about it. You'd see it anyway when the power supply is
    turned off because of the large thermal mass.

    Best regards,
    Spehro Pefhany
  10. John Larkin

    John Larkin Guest

    Right. And manganin has a low thermoelectric potential relative to
    copper, just a few uV per K.

  11. So use a really bad DVM :)
  12. The pairs for base metal thermocouples are chosen to give a relatively
    high voltage for a given temperature difference (among other things).

    Typical connection material pairs are something like 5:1~10:1 better,
    and you'll very seldom see anything other than a "0.0" mV if you go
    around probing random bits of metal that have just been handled by
    30°C fingers. A DC shunt will be made symmetrical in part so the
    substantial self-heating at rated current won't cause thermocouple
    voltages to affect the reading. Immediately when the current is shut
    off, the meter should go to 0. If it doesn't, then the reading can be
    corrected by that factor, but it will not be a problem with such a
    setup and a 100uV resolution meter.

    If you want to calibrate a really high current shunt (not just check
    it) at a current orders of magnitude less than the rated current, then
    such things would come into play (and perhaps you'd be using a meter
    with 100nV resolution rather than 100uV), but the OP just wants to
    assure himself that it's actually 50mV at rated current as marked.

    We've supplied many, many, high current (up to 15,000A) DC measurement
    systems using such shunts, BTW. They're usually between 50mV and 150mV
    output at rated current.

    Best regards,
    Spehro Pefhany
  13. If you're down in low uV DC territory or below, for sure. But we can
    get 30 micro-ohm resolution out of a 100uV resolution measurement @3A,
    which requires no special care. That's a resolution of ~1.5% of the
    expected value, which is fine for the intended purpose.

    Do you have a reference on techniques for nanovolt DC measurements?
    I'd be interested in that.
    Heat in itself is not a problem. Nor even are thermal gradients. It
    has to be an asymmetrical thermal gradient with dissimilar metals.
    Maclaurin number? Do you mean the Reynolds number? or maybe the
    Nusselt number?

    Keep in mind that shunts typically dissipate a fair bit of heat at
    full rated current. The OP's wee 25A one will dissipate in excess of
    1W in normal use. Larger ones are in the hundreds of watts. The lack
    of significant dissipation might affect the reading a bit.

    Best regards,
    Spehro Pefhany
  14. Guest

    No. My impression is that any such reference would start off by
    recommending that you immersed the active part of the experiment in
    liquid helium and go on from there. Microvolt DC measurements are
    already tricky enough.

    The English national standards laboratory at Teddington does offer "A
    guide to measuring resistance and impedance below 1MHz" ISBN 0 9044557

    I've got a copy, but can't recommend it - it doesn't say anything
    silly, but it doesn't help you understand what is going on.
    You can't dissipate heat without creating a thermal gradient. Ad hoc
    connections are always asymmetric.
    Oops. Raleigh number - the Reynolds number applies to flow, the
    Raleigh number applies to convection. Both show up in my Ph.D. thesis.
    Why I keep on thinking the Rayeigh number is called the Maclaurin
    number I'll never know. Check out

    if you want a bit more detail.
    Heat dissipation is proportional to the square of the current - 3A is
    going to generate about 1.44% of the heat dissipated at 25A. 3mV isn't
    too hard to measure, unless you expect the voltage to be accurate to a
    couple of uV, but reversing the current is a useful check.
  15. Guest

    Never had any manganin voltage probes for any voltmeter I ever used.
    Most of them looked like chromium-plated steel.
  16. The shunt is probably manganin. The wires you put under the screws are
    probably plated copper, (AWG22 or something like that) so copper for
    T/C purposes, so the number is applicable.

    BTW, the probes (if they are the probe type, and cheap) are most
    likely nickel-plated brass.

    Best regards,
    Spehro Pefhany
  17. joseph2k

    joseph2k Guest

    Go Sphero, i breifly googled and did not find any appropriate Mclaurin
    number application, Reynolds number, as i had learned in school, is pretty
    strictly related to aerodynamic drag of various shapes, but Nusselt number
    appears to be applicable. Well placed question, i learned something as a
  18. Steve

    Steve Guest

    Probably not the most accurate thing, but may be helpful for smaller
    shunt measurements.
  19. Thanks. We use the Reynolds number (and the Nusselt number) in mold
    cooling calculations- you want a Reynolds number high enough to assure
    turbulent flow and therefore efficient heat transfer from the metal to
    the cooling medium. A small number like 1,000 means you have laminar
    flow for sure, whereas a larger number such as 10,000 or larger means
    you have turbulent flow in the cooling passages. In between is
    transitional. I wish the powers that be had deemed a basic education
    in fluid mechanics to be of more importance to us EEs.

    Best regards,
    Spehro Pefhany
  20. John Larkin

    John Larkin Guest

    We have a similar 1-amp home-made box. We used a good voltage
    reference, a 4-lead Vishay power resistor as the shunt, and a 10-turn
    pot to trim current. Once it warms up it is short-term stable to a few
    PPM. It has a load-short switch to keep it warm. We'll typically drive
    a precision 1-ohm oil-filled resistor in series with an unknown,
    measure the drops, and compute the unknown.

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