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small, very accurate shunt resistors?

Discussion in 'Electronic Design' started by Michael Noone, Jul 24, 2005.

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  1. Hi - I'm looking for some small (physically), very low resistance, very
    accurate resistors to use as shunts for current sensing. Ideally I'd like
    something like a .05 ohm resistor in a 0805 package with maybe .1% or
    better accuracy. I've been able to find plenty of larger resistors that
    match these specs, and among 0805s I've been able to find ones of the right
    resistance but that are less accurate (lots of 1% parts), and I've found
    many that are the right accuracy but not the right resistance (normally 10-
    100 ohms is the smallest I can find among the .1 and .05% parts).

    Any ideas?

    Thanks!

    -MJ Noone
     
  2. An 0.1% 50 milliohm resistor must be accurate to within 0.05 milliohms. If
    you really need that kind of accuracy, then I think that trace resistance
    will be a catastrophic problem. Is your circuit designed for a Kelvin
    (four-wire) connection? I have very little knowledge in this area, but I am
    sure that it would be necessary and I doubt that it would be sufficient.

    Also, you implied in a previous posting that you were trying to use a
    smaller current sense resistor so that your circuit would produce 0 to 2.048
    volts instead of 0 to 4.096 volts. Clearly there is more to your circuit
    than just the resistor; perhaps if you posted that then someone with
    experience in this area could help you.

    Jonathan
     
  3. Keep in mind that when you get to such low value resistors, you need 4
    connections to them to get an accurate voltage proportional to the
    current through them. The best you may be able to do is to have the
    current carrying traces going into two of the outside corners of the
    resistor and the voltage connections coming out from the opposite
    inside corners of the pads. Very nearly as good is current into one
    side of the resistor, voltage from the other side. The idea is to
    include as little trace and pad drop as possible in the measurement.
     
  4. John Larkin

    John Larkin Guest

    Try Isotek for their 4-wire parts; but your specs are tough.

    John
     
  5. Guest

    Guest Guest

    You need a 4-terminal shunt. I happen to have one 0.1 ohm shunt that was
    used for current measurement. The accuracy is .02%. I also have a matched
    set of 0.1, 1.0, 10, 100.1 and 1010 ohms. They were designed to be loaded
    by a 100K input impedance. Let me know by e-mail if this is what you want.

    Norm Strong
     
  6. Ol' Duffer

    Ol' Duffer Guest

    Shunt resistors 0.1 Ohm and lower are mostly 4-terminal even in
    through hole mount, and anything below 10 Ohms is rare in SMD.
    The parts you think you want may be custom (interpret as $$$$).
    You could make your own from manganin sheet cut a few percent
    low calibrated with software or dremel...
     
  7. Uwe Bonnes

    Uwe Bonnes Guest

    www.isabellenhuette.de
     
  8. And the problem with a paper clip is....?


    --
    Many thanks,

    Don Lancaster
    Synergetics 3860 West First Street Box 809 Thatcher, AZ 85552
    voice: (928)428-4073 email:

    Please visit my GURU's LAIR web site at http://www.tinaja.com
     
  9. I can't tell from the description if these are anything near an 0805,
    but Digikey does sell some 0.5%, 50 milliohm 4 terminal current sense
    resistors. Part # RW1S0CKR005D
     
  10. John Larkin

    John Larkin Guest


    TC.

    John
     
  11. http://www.vishay.com/docs/63023/vcs2516.pdf

    These are about 3 times the size you asked for and I don't know who
    stocks them but they look like fine devices.

    By the way, have you figured out how you are going to amplify and
    digitize the signals from your shunts in the presence of trace drops,
    while keeping the accuracy any where near 0.1%? I have managed 2% or
    3% without heroic efforts, but not much better.
     
  12. If you use Zeranin paperclips rather than the plated steel ones from
    Wal*mart it would be better, but you still would have the tolerance
    issue (unless you can handle the calibration issues). ;-)

    Tempco of steel is about +0.5%/K, worse even than Cu, so the desired
    0.1% accuracy is lost with tenths of a degree change in temperature.

    Can you use a less-than-spec accurate (but stable) Kelvin-connection
    shunts that other have mentioned and calibrate? As others have
    mentioned, you really don't want hardly any of the +0.39%/K copper
    traces in the measurement path.

    Do you need a shunt made with four terminals? If the copper is to
    contribute, say, 0.5%, then you need an equivalent length of about
    3mils of 12 mil wide 1-oz traces on each end. Maybe this is achievable
    by running traces off of either side of an SMT pad for a four-wire
    connection, but it sounds a bit dodgy. I've done something similar for
    an instrument that measured standard CT output...

    Bare (low tempco alloy) shunts are made in high volume for multimeter
    use (both a jumper style for low accuracy meters, and IIRC, some
    4-terminal types) but generally not nearly that accurate.

    Then there are possibilities like this:
    http://www.precisionresistor.com/LVS2.htm
    http://www.precisionresistor.com/LVS3.htm

    For an LVS3 (0.5" x 0.25" SMT) 0.05 ohm 0.1% with 15ppm/K tempco they
    want $8.30 each, which is a bit pricey, but maybe tolerable for you.
    They're obviously a lot bigger than 0805, but it meets all your other
    specs (and appear to be in stock).

    Here are some in 2516 (0.25" x 0.16") which are offered in your specs
    as well: http://gensemi.com/docs/63023/vcs2516.pdf (watch the tempco
    and maximum current rating)

    And some in 2512 (0.25" x 0.125") which also seem to be offered in
    your specs: http://www.powertron.de/POWERTRON_RESISTORS_GROUP_SMD.PDF
    (UHS 4-2512)


    Best regards,
    Spehro Pefhany
     
  13. Robert Baer

    Robert Baer Guest

    A major problem with all of the SMD resistors, is that there are *NO*
    kelvin contacts!
    So, assume you have a 50 milliohm SMD resistor that you determined
    was within 0.1% of that value.
    Now solder it to your PCB, and measure the resistance on the traces
    close to the resistor.
    Hmmm...seems to me that you will not be any where close to 50
    milliohms + 500 MICRO-OHMS!!!
    The damn solder joints will have far larger resistance, and one might
    ask if you remembered to attempt kelvin connections???
    And..where does one measure these low-R SM units? at the top, close
    to the "J" bend? half way down from the top? at the bottom of the "J"
    bend where it would connect to the PCB trace?
    Furthermore, what about thermo-electric effects?
    I presume one could go on about the problems...
     
  14. Robert Baer

    Robert Baer Guest

    Maybe the paper clip is still attached at the top of his Word
    document, and will not motorcycle away...
     
  15. Robert Baer

    Robert Baer Guest

    I stand corrected about Kelvin SMD resistors.
    I see that Vishay complains about tin/lead oxidation and about poor
    reliability of conductive epoxy.
    BUT in the same breath they seem to recommend !gold! which has a
    history of seperating from solder at the interface...
     
  16. The Phantom

    The Phantom Guest

    Go have a look at:

    http://www.koaspeer.com/koa/master.asp?URL=/koa/resistors.asp

    or

    http://www.isotekcorp.com/productDisplay.asp?CatID=1&SubCatID=

    That ought to get you started.
     
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