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Accurately Measuring Precision Resistors

Discussion in 'Electronic Components' started by Watson A.Name - \Watt Sun, the Dark Remover\, Apr 12, 2005.

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  1. I acquired a couple dozen Micro-Ohm 400 ohm, 0.1% resistors, and some
    other values. They've probably been around a decade or more, and/or may
    have been culled. In any case, I would like to check them to make sure
    they're within tolerance. I thought about doing this a couple ways:

    Measuere them with a Fluke 4.5 digit meter, which hasn't been cal'd in
    decades, but it could give me an idea of how close they are to a common
    value, even if that value isn't exactly 400.

    Measure them with a Leeds Northrup Wheatstone bridge. It's been around
    decades and hasn't been cal'd in a long time. But it's a bridge, so if
    there is any long-term drift, the resistors should drift the same way,
    assuming the resistors are all the same. My guess is they're wirewound,
    which is fairly stable. But there's grunge on the switch contacts, etc.

    Make a bridge out of four of the 400 ohm resistors. Again, use this to
    compare them to one another, to see if they are all about the same
    resistance. If I apply a few dozen volts to the bridge, I should be
    able to measure a few millivolts, as long as I don't heat up the
    resistors too much and cause them to drift.

    One other thought. I have a precision power supply with terminals on
    the back to allow me to hook up a resistance in series with the 1.000 mA
    constant current source. If it is 400 ohms, then I should measure 0.400
    VDc output.

    I realize that 0.1% is one part in a thousand, so that's 400 + or - 0.4
    ohms. That's about the residual meter lead resistance I see in my
    meters. I don't have a standard resistance, traceable to the NIST or
    whatever. And I don't have a friend who's working at a cal lab. So I'm
    trying to make do with what limited resources I have to get the most
    accurate measurement. Any helpful advice would be appreciated. Thanks.

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  2. Phil Allison

    Phil Allison Guest

    "Watson A.Name -

    ** Convert resistance to frequency - since that is a dead easy to measure

    A 555 oscillator ( using a polystyrene timing cap) running at 10kHz to 20
    kHz will allow you to compare the 400 ohm resistors with each other and a
    new 400 ohm 0.1% precision resistor for calibration.

    ............ Phil
  3. John Fields

    John Fields Guest

  4. Basically, without accurate voltage *and* current measuring
    facilities, he's screwed. OTOH, if all four resistors measure exactly
    the *same* (a much easier job than an absolute measurement), then it
    boosts confidence that they are all very close to the nominal 400 ohms
    absolute value.
  5. I read in alt.binaries.schematics.electronic that John Fields

    AARGH! You've criticised Allison! You will regret it. Actually, I think
    he's got a good solution, and if the project is serious, the cost of one
    reference resistor is justified.
  6. John Fields

    John Fields Guest

  7. The Phantom

    The Phantom Guest

    Post your snail mail address, and I'll send you several .01% resistors, checked with a
    6 1/2 digit DVM.
  8. Well, converting to a freq is about the same as what the DMM does but
    the DMM uses 1/F or time. And I don't have an accurate freq meter, I
    have a DMM with a freq range but it's nowhere as accurate as a frequency
  9. Well, I checked the two dozen on the Leeds & Northrup wheatstone bridge.
    And contrary to what you just said, above, the wheatstone bridge does a
    *comparison* of the unknown to an accurate known value, so it doesn't
    really care what voltage or current you use. I used a 30VDC PS for the
    batt. Most were within a few dvisions on the meter (I think five
    divisions is an ohm at that range). I had two 'outliers' that were
    greater, and one that was less than that. But the rest were fairly
    close. I used heavy short leads to minimize their resistance and their
    effect on the readings.
  10. So what's it cost for a resistor that's more accurate than the 0.1%
    resistors I have? In this same batch, I have a few resistors that are
    0.05 percent. ;-) But they're as old as the others.

    Sometimes I think I'm trying to talk myself into buying a more accurate
    bench DMM. I have an old 4-1/2 digit, how many X's in X-1/2 digits do I
    need to measure the 0.1% resistors accurately? How about a used HP or
    FLuke DMM from Ebay?
  11. Phil Allison

    Phil Allison Guest

    "Watson A.Name -
    "Phil Allison"

    ** A 4.5 digit multimeter meter permits 0.1 ohm resolution with a 400 ohm

    My suggestion offers about 0.02 ohms resolution.

    ** Boo - hoo !!

    High time you got yourself one or simply built one.

    ............ Phil
  12. *Most* 4-1/2 digit DVMs are pretty linear and repeatable, so all you
    need is a reference resistor in the same range. You can buy or borrow
    one, or compare the L&N bridge vs. the DVM and if they happen to
    agree, call it a day. Chances are if you can't tell the difference, it
    doesn't much matter to you, right?

    Best regards,
    Spehro Pefhany
  13. Which remindw me, I have some precision resistors made by Cinema
    Engineering out in the garage. I think some of them are that low a
    tolerance, but they're big ol' bobbins wound with lots of very fine
    wire. I'll have to dig around and check them out.
  14. I can compare them to each other, and to the known resistances of the
    bridge and the DVM's (uncal'd) accuracy. But my concern is that the
    measurements I'm making are comparing or measuring resistances that are
    close relative to each other, but may be off from the absolute value.
    If the DMM and the bridge are both off in the same direction by about
    the same amount, then I would be fooling myself if I accepted them as
    absolutely accurate because they happen to closely agree. No? (BTW,
    the L&N bridge has a cal sticker that says it's due for recal in Nov.
    1969. :p )

    I measured some very old Cinema Engineering wirewound resistors with the
    bridge. The label says 3390 but I got 3404 ohms. That's 0.4% high, but
    since it's only a 1% tolerance, it's still within. Another one is
    labeled 100 ohms 0.1% and I set the bridge for 100.0 and I can't see the
    needle deflect, so I'm getting the feedback that the bridge agrees
    closely with the marked resistance.

    Most of these resistors are bobbins manually wound with a measured
    amount of resistance wire, so I feel that they are still accurate even
    if they're very old. But the two dozen Micro-Ohms look like chokes
    about the size of 1W resistors, plastic cylinders with one end shiny
    where they poured the potting epoxy in. I found this info on them.
    They're part number 135, subminiatures, but they're so old that the
    markings don't obey their current system. Can you imagine winding these
    with 0.6 mil wire? That's a quarter the thickness of a human hair!
  15. John Fields

    John Fields Guest

    4-1/2 digits = 1.XXXX, where the X's can go to 9, so you've got a
    meter which can read to one part in 20,000. That's +/- 0.005%.

    There'll be a spec on the number of counts that it can be in error,
    and if you look on Fluke's web site you can get that spec.

    Why don't you just spend a few bucks to get the thing calibrated and
    be done with it?
  16. I'm building a measure bridge, and I'd love two of these. How much do
    you want? I've turned "on" this address, so you can mail me at
  17. Tim Shoppa

    Tim Shoppa Guest

    But my concern is that the
    An older 400 ohm 0.1% axial resistor is almost certainly a wire-wound
    unit. Long-term stability is very good. If operated over the power
    limit or in extreme humidity/temp there may be some irreversible shifts
    in resistance, but if never used then you probably don't have to worry.

  18. J. Yazel

    J. Yazel Guest

    Thanks. You really made my day.

  19. Something doesn't seem quite right about that +/- figure. When I
    meaasured the 2 dozen, I had to use the 2k range. Most of them measured
    400.3 give or take a few tenths of an ohm, except for 3 or 4 outliers
    which were the same as the ones the bridge IDed (I marked them with some
    white paint). On the 2k range, the Fluke can only see a tenth of an
    ohm, so it seems to be able to do no better than .01% not including the
    +/- counts.

    I still have to deal with the R in the leads (I shorted the 12 gauge
    wires together and it measured zero). Mine's an 8600 and doesn't have
    4-wire R measurement (I used short lengths of 12 AWG in banana plugs to
    make the measurement). Here's a 5-1/2 on Ebay for probably not much
    over a hundred, and it has the 4-wire R measurement. I think the Fluke
    8600s like mine go for well under a hundred on Ebay, and the cost of
    calibrating them would be muich more than that, so it's kind of not
    worth it. It's probably wiser to buy a 5-1/2 or better meter on Ebay,
    just to get the 4-wire capability. Anyway, my Fluke measurements agree
    closely with the L&N wheatstone bridge measurements, which is
    reassuring. Yhanks.

  20. ???
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