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0.01 Ohm resistor

Discussion in 'Electronic Design' started by kell, Jan 24, 2006.

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  1. kell

    kell Guest

    I'm looking for a way to make a .01 Ohm resistor for a circuit I'm
    building. The current in the resistor will max out at 10 amps, but
    there will be times when it runs 10 amps continuously. This resistor
    will be potted in silicone and I can't really put a heatsink on it,
    though I can put the resistor in a spot where it's in contact with the
    circuit enclosure.

    I took out my trusty milliohmmeter and measured the resistance of .030"
    steel baling wire, but the resistance of that kind of steel seems so
    climb significantly with heating. I can't have this resistor varying
    by much more than 1 or 2%.
     
  2. John Fields

    John Fields Guest

     
  3. How about using a length of copper wire? #20 can handle 11 amps, and
    one foot of it is 0.011 ohms according to one wire table. If you run
    it back and forth, not too tightly bundled it will dissipate the 1 watt
    with little temperature rise.
     
  4. Geez, does it really need to be that accurate? If this is for your current
    limiter, I wonder whether it really matters if you limit at 9A or at 11A.
    After all, the downstream stuff won't be precise enough to know the
    difference.

    But anyway, you can buy .01 ohm resistors. You can also buy 0.1 ohm
    resistors and parallel 10 of them, which will help spread the heat load.
    I've seen that done in switching power supplies.

    A bit of climb-with-heating could be a good thing, for a current sense
    resistor; it would tend to decrease the current limit when overheated.
     
  5. Noway2

    Noway2 Guest

    You can buy two and for terminal current sense resistors. Search
    digikey for those terms. Based on the fact that you say you can't
    tollerate a variance of much more than a couple of percent, this is
    probably the way to go. It sounds like you are interested in using
    this to make current measurements, which is what I am doing in my
    project and I used a four terminal resistor for this purpose. I am
    able to read the current to an accuracy of better than 1% with it and
    have pushed over 10 amps through it. I believe I may have used a .005
    ohm because it was readilly available and the .01 ohm was out of stock.
     
  6. Yukio YANO

    Yukio YANO Guest

    How much of a problem is heat dissipation at 100 mWatts !!! IxR= 10
    Amp x 0.010 Ohms = 0.1 Watts !!! The real trick is how do you measure a
    0.010 Ohm resistor, now that is the real Problem !

    Yukio YANO
     
  7. Phil Hobbs

    Phil Hobbs Guest

    You don't want to use any pure metal for shunt resistors--their
    resistances rise very rapidly with temperature (something like 6000
    ppm/K for copper, IIRC). Nichrome works, or constantan thermocouple
    wire (which is easily solderable).

    Cheers,

    Phil Hobbs
     
  8. Ian Bell

    Ian Bell Guest

    No, it's I^2*R which is 1 watt.

    Ian
     
  9. Phil Hobbs

    Phil Hobbs Guest

    I misspoke. 4000.

    Cheers,

    Phil Hobbs
     
  10. kell

    kell Guest

    1 watt, as Ian pointed out.
    Measuring .01 or even .001 ohms isn't that hard. All you need is an
    accurate current source and a volt meter. I made the current source
    with a LM317 and a length of resistance wire. Calibrating such a
    current source to 1% accuracy or better is fairly easy to do just by
    locating the right point to tap the nichrome wire. I wound the length
    of nichrome around a straw and secured it with heat shrink tubing.

    Anyway I thought I could fabricate a low value sense resistor, but it
    looks like buying one is the better solution. Thanks all for the
    advice.
     
  11. Rich Grise

    Rich Grise Guest

    Go to the hobby shop (model trains, RC airplanes, etc) and get some brass
    shim stock. Cut a strip of the appropriate size - maybe a little
    oversized, so that you can trim it with a Dremel or so.

    Figuring out the dimensions of the strip is, of course, left as an
    exercise for the reader. ;-)

    Or, bite the bullet and get a proper current shunt. ;-)

    Good Luck!
    Rich
     
  12. Rich Grise

    Rich Grise Guest

    What problem? All you need is a known current source and a millivoltmeter.

    Done! ;-)

    Cheers!
    Rich
     
  13. John Fields

    John Fields Guest

    ---
    "The appropriate size"???
    ---
    ---
    STM like the writer can't offer a lot of help, but wants everyone to
    know that he's still around and pretending to be a teacher.
    ---
    ---
    Actually, it's not a shunt, it's a resistance in series with the
    load.

    The "shunt" name for it came about originally because it was a
    resistor which lived in parallel with the ammeter which was inserted
    in series with the circuit, and diverted a known amount of current
    around the meter, the remainder being used to make the meter needle
    deflect enough to indicate the current "shunted" acoss/around the
    meter.
     
  14. Mook Johnson

    Mook Johnson Guest

  15. Phil Allison

    Phil Allison Guest

    "Ancient_Hacker"
    ** It is often important that current shunt have low *self inductance* so
    its value remains the same to some high frequency.

    A 1 foot length of #20 looped wire has enough inductance to make it no
    longer an accurate shunt above about 1 kHz.



    ......... Phil
     
  16. Based on a diameter of .032 inches and the formula on this page:
    http://www.ee.scu.edu/eefac/healy/indwire.html
    I calculate an inductance per foot of about 360nH.

    That produces a 45 degree frequency of about 4.5kHz. I wonder how
    that would change if you zig zagged the wire into a 1 inch square.

    Much lower inductance, I am quite sure.
     
  17. Phil Allison

    Phil Allison Guest

    "John Popelish"
    ** I used the rule of 25 nH per inch - close enough.

    * What a strange criterion.

    By the time the phase angle is 45 degrees, the impedance value has risen by
    a whopping 41 %.

    An increase of about 1% happens at a mere 450 Hz.



    ** Much easier to use a short length of a high resistance, low tempco
    material.

    As used to make WW resistors.

    Get L down to a maybe 20nH then.

    Moves the 1% error frequency up to 9kHz.


    ......... Phil
     
  18. Jasen Betts

    Jasen Betts Guest

    Hmm 1 Watt...
    Nichrome wire keeps a failry constant resistance, it's not the easiest stuff
    to solder though and you'll need to parallel several pieces of
    nichrome wire to get the current capacity you want unless you can get some
    thicker than usual nichrome somewhere.

    Bye.
    Jasen
     
  19. The Phantom

    The Phantom Guest

    According to Grover and Rosa of the NBS, such a non-inductive (so-called)
    zig-zag arrangement would give about 103 nH. If instead of making it into
    a 1" square, you made it 1" by .384", and kept the strands right next to
    each other, the inductance would be 39 nH.

    Just folding the 1 foot of wire into a hairpin, with the two strands as
    close together as possible would give about 57 nH.
     
  20. Guest

    The real problem with measuring low value resistances is contact
    resistance - soldered contacts do have very low and stable resistances,
    but a lump of solder on the resistor side of the joint can affect the
    resistance of your resistor!

    The solution is to take four terminal (Kelvin) measurements.

    The voltages that you measure tend to be fairly low, so an AC excited
    bridge can get you better accuracy than DC - reversing DC can be almost
    as good. If you are really pusshing for high accuracy (as national
    standards laboratories do) you use an AC excited transformer-based
    (Blumlein) bridge, where the transformers are precision ratio
    transformers, with ratio's stable to about one part in ten million. See

    http://www.npl.co.uk/electromagnetic/publications/guides/ac_bridges.html

    Lots of good advice. My copy has seen very little use, but when I've
    needed it, it has been invaluable.
     
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