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Calibration resistors

Discussion in 'Electronic Repair' started by N Cook, May 18, 2007.

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  1. N Cook

    N Cook Guest

    What structure/type of resistors would be best for purest R resistors to
    make up and be measured with an accurate calibrated measurement bridge.
    Normal off the shelf resistors that is, secondary standard level of accuracy
    is not required.
    Required a few, odd values like 374 ohm +/- 2 ohm , butactual values known
    accurately to +/- 0.01%, in the range 20 ohm to 100K
     
  2. You can buy 0.01% resistors, but they're not cheap!

    Alternatively, there used to be a guy on eBay that sold resistor
    kits-- He'd measure some wirewound resistors with a GOOD ohmmeter, and
    mark them with their measured values, to 0.01% or so IIRC.
     
  3. bz

    bz Guest

    Over what frequency range will the bridge be used? Over what range of
    resistances?

    If you are only working with audio frequencies, you can use carbon
    resistors, wire wound resistors and metal film resistors.

    If you are working at higher frequencies, in the RF region, you must make
    sure your resistors are non inductive. Even some metal film resistors have
    spiral patterns cut in the metal film, making the resistor into an RF choke
    at some frequencies.

    For ultra low resistance readings, 4 wire kelvin connections are necessary
    to reduce the error due to the resistance of the leads.

    What is the temperature range under which you will be doing calibrations?
    All resistive devices have some kind of temperature coefficient of
    resistance.

    What is your application?




    --
    bz 73 de N5BZ k

    please pardon my infinite ignorance, the set-of-things-I-do-not-know is an
    infinite set.

    remove ch100-5 to avoid spam trap
     
  4. N Cook

    N Cook Guest

    only 100Hz to 10K and 15 to 25 degree C
     
  5. bz

    bz Guest

    Go to radio shack and buy some resistors with the necessary precision. For
    your application, the construction will not matter.


    If you need odd values, make them from series/parallel combinations of
    standard values.







    --
    bz 73 de N5BZ k

    please pardon my infinite ignorance, the set-of-things-I-do-not-know is an
    infinite set.

    remove ch100-5 to avoid spam trap
     
  6. Jim Yanik

    Jim Yanik Guest

    I'd want to know what "good ohmmeter" he was using before I spent any money
    on his "precision" resistors.
     
  7. Guest

    The worst R type for drift is carbon, so forget about those. Picking
    cheap Rs using a meter isnt really effective: you may know its value
    when measured, but theyre sold as rough tolerance precisely because
    they cant be relied on to stay that value. Temperature and time cause
    wander.

    As already mentioned, metal film are routinely spiral cut, so are no
    use for hf, but should be good for 10kHz, and are relatively stable.

    The main thing is to pick an R with a very small tolerance - not only
    does this tell you what value you're getting, but also that the R
    should remain within that small tolerance.


    NT
     
  8. bz

    bz Guest

    wrote in
    Resistors used for calibration purposes should NOT be used for ANY other
    purposes.

    NEVER subject them to any current other than that of your ohm meter.

    In such a case, long term chemical stability would be your main concern.

    Even most carbon resistors would probably be ok provided you store them
    under normal 'comfort level' conditions (0-80 percent RH, 65-85 degrees F).

    Ceramic encapsulated metal film or wire resistors should be more resistant
    to environmental factors than composite body carbon resistors.

    When used within certain environmental and power limits that may or may not
    be apparent to the user.

    HIGH REL or MIL SPEC resistors usually have tighter limits and longer
    lifetimes under load than 'ordinary resistors'.





    --
    bz 73 de N5BZ k

    please pardon my infinite ignorance, the set-of-things-I-do-not-know is an
    infinite set.

    remove ch100-5 to avoid spam trap
     
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