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Wheatstone bridge problem

Discussion in 'Electronic Basics' started by Matty F, Jun 5, 2005.

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  1. Matty F

    Matty F Guest

    I've used a Wheatstone Bridge to measure the resistance of 4 large
    solenoids. The readings were: 0.3, 0.28, 0.27, 0.26 ohms.
    I measured all of them again and then they were all about 0.20 ohms.
    As you can see, the readings have slowly dropped from 0.3 to 0.2 ohms.
    Can anyone offer suggestions why the readings keep changing?
    I assume that the battery condition is independent of the measurements.

    Each time I did a measurement I reconnected the alligator clips to the
    terminals and wriggled them around to get a good contact, and checked
    the reading three times for each solenoid. Each of those three readings
    was the same. i.e. I've done a total of 24 readings.

    When I connect the two leads from the Bridge together I get a reading of
    0.05 ohms. There appears to be several hundred feet of 14 gauge wire on
    each solenoid. Each time I press the check button on the bridge the
    meter needle swings slightly to the left of its final position. Each
    solenoid weighs about 20kg. There is no way they can be changing
  2. Chris

    Chris Guest

    Hi, Matty. You haven't said specifically what instrument you're using,
    so I can't give specific advice. But I can say for sure that
    Wheatstone bridges which only have two terminals for Rx aren't made to
    measure low resistance. You might be getting self-heating in one of
    the bridge's internal resistors, or if there's an active null, battery
    voltage may be affecting that. These are the first causes that come to

    Why don't you try using a 10 volt or so regulated power supply capable
    of cranking an amp or so, a 10 ohm or so resistor, and a voltmeter and
    ammeter. Connect the resistor in series with the power supply to give
    you an approximately 1 amp source. Put the ammeter in series with the
    circuit to measure current. Then put the solenoid in the circuit, and
    measure voltage across the solenoid.
    ` 10 ohm _
    ` ___ / \
    ` .-|___|-----( A )-----.
    ` | \_/ |
    ` 10V | |
    ` --- .-.
    ` - Rx | |
    ` | | |
    ` | '-'
    ` | |
    ` '---------------------'
    created by Andy´s ASCII-Circuit v1.24.140803 Beta

    Even though this looks a little cheesy, it is in fact a true Kelvin
    connection, unlike your Wheatstone setup. This measurement will be as
    accurate as your ammeter. You don't need exactly 1 amp of current,
    because you can just do the math using Ohms Law to get the inferred
    resistance value:

    R = V / I

    Measure current each time you apply power -- it will change a little
    for every time you get a different Rx. Try to keep the application of
    power to the solenoid down to a few seconds or so if you can. That
    will reduce self heating, which is always a problem in measuring
    resistance of copper wire. Also look to ambient temperature, and prior
    heating of the solenoid coil from use. Make sure the coil is cool
    before you measure it.

    Good luck
  3. Chris

    Chris Guest

    I guess I should mention one or two other things here. The Kelvin
    measurement of resistance shown above didn't show the voltmeter:

    ` ___ / \
    ` .-|___|-( A )----.
    ` | 10 ohm \_/ |
    ` | o<----.
    ` | | |
    ` 10V | .-. / \
    ` --- Rx | | ( V )
    ` - | | \_/
    ` | '-' |
    ` | | |
    ` | o<----'
    ` | |
    ` '----------------'
    created by Andy´s ASCII-Circuit v1.24.140803 Beta

    Make sure you place the voltmeter leads right on the soleniod leads
    where you want to make the measurement. If you place the leads on the
    ammeter and the battery, you will be measuring the resistance of the
    leads between the ammeter and the solenoid, and between the solenoid
    and the battery. And also, you'll need a 10 ohm, _10 watt_ resistor.

    10V * 1A = 10 watts, P = V * I

    Second, a lot of people have those cheapie DVMs that only measure DC
    current to 200mA or so. If you can't find a DVM that can measure DC
    current of 1A, use a 100 ohm 1 watt resistor in place of the 10 ohm
    resistor, giving you 100mA test current. You will only be measuring
    20mV for an 0.2 ohm test resistor, but you'll still be way ahead of the
    game if you've got a 200mV DC range on your DVM, despite reduced
    voltmeter accuracy.

    By the way, don't be too surprised if your calculated ohms are quite a
    bit different than what the Wheatstone bridge led you to expect.
    You'll be getting a more accurate reading this way, though.

    If you have any questions or additional problems, don't hesitate to
    post back.

    Good luck
  4. Matty F

    Matty F Guest

    I'll have a look at the instrument tomorrow. It's a quality instrument,
    and I used an identical one 40 years ago at University. There are only
    two terminals. I do suspect self-heating in an internal resistor.
    I am being criticized for blaming the meter. Do you have any references
    for inaccuracy in measuring low resistance, i.e. around 0.2 ohm?
    The boss has already suggested doing something like that, and is going
    to bring his Avometer and other meters tomorrow. We have already put 30
    amps through another solenoid, and it seems to work fine. We have a
    large battery with a 100 amp meter and can change the current with a
    compressible pile of carbon slabs.
    All we want to do is to check whether all the solenoids are the same,
    and whether they have shorted turns. Measuring the precise resistance is
    not important. I was just wondering why it keeps changing.

    In use, the solenoids get a variable high current (maybe 30 or more
    amps) for a maximum of a minute or so.
    According to tables I've just read, 100 feet of 14 AWG is about 0.25
    ohms, which is in line with what I get.
  5. Chris

    Chris Guest

    Hi, Matty. The quality of the instrument doesn't matter if it's being
    used to measure something it's not made to do. I'd be fairly confident
    that if there are only two terminals, it's probably not made to
    accurately measure less than an ohm. So don't worry about blaming the
    instrument. From what you've said, that's the first place I'd look,

    Unless you have a resistance standard, you'll kind of have to wing it
    on bouncing low ohm measurements. The first quick check is actually
    the force current/measure voltage method. If you have a good handheld
    DC voltmeter like the Fluke 77 (around 0.1% DC Volts accuracy), your
    resistance measurement accuracy will be almost entirely dependent on
    the accuracy of your ammeter. If you can crank 1 amp through your Rx,
    another Fluke 77 measuring DC Amps will give you a resistance
    measurement accuracy of 1.5%, which should be fine for what you're
    doing. Inferring resistance by forcing a measured current and then
    measuring voltage is theoretically sound as well as practical. Your
    boss is making sense on this one. But in order to get more accuracy or
    give you confidence in your measurements, Ohmite, Dale and others make
    those aluminum-housed 50 watt 0.1% 100 milliohm Evanohm wirewound
    resistors, which can be convenient, especially if you just want a quick
    sanity check. If you thermal cycle the resistors in an oven several
    times to stabilize them, measure resistance with a quality calibrated
    instrument, and then make sure never to apply more than 1 amp of test
    current to them (<20% of rated wattage), you can be fairly confident of
    the results when you bounce instruments. I've got some myself in
    decade values from .1 ohm to 100K ohm, and keep them in reserve for
    when I need an ohms sanity check. All unofficial, of course.

    High currents are more difficult to measure accurately, though. A
    12VDC 1 amp unregulated wall wart, an LM317 with a heat sink, two
    resistors and two 10uF caps will get you the regulated 10 volts. Most
    of this stuff might even be in your junkbox or scrounge-able. Excess
    currents will just cause Rx heating, and make your life more difficult.
    You don't need and shouldn't use 30 or 100 amps to measure an 0.25 ohm
    copper resistor.

    Checking solenoids by checking resistance alone can be a bit of a
    problem. The tightness of winding causes variations in wire length
    which might account for your variations in resistance. Variations in
    drawn wire diameter can make resistance values easily vary by 10% or
    more. I think resistance _and_ inductance measurements might be a
    better way of getting where you want. A better wound coil would have a
    somewhat lower resistance, but a somewhat higher inductance. A coil
    with marginally smaller wire diameter would have higher resistance, but
    almost equal inductance. Missing or shorted windings would be more
    noticeable with both measurements. If you've got one of those handheld
    meters that measure inductance, that might help a lot.

    Another practical, valid method you might use if you're serious is
    putting a standard voltage across the solenoid, and measuring pull
    force. I'm not sure if that's practical, though, for a 4-off check.

    Good luck. Feel free to post again to let us know how you're doing.
  6. Matty F

    Matty F Guest

    For the record I have found the following references saying that the
    simple Wheatstone Bridge is unsuitable for measuring resistances under 1

    Two of the more common types of bridges are the Wheatstone Bridge, which
    is used to measure resistances of 1 ohm to 100,000 ohms, and the Kelvin
    Double-Bridge, which is used to measure resistances in the range of
    ..0001 ohms to 1 ohm.

    A Kelvin bridge is recommended for measuring resistances lower than 1 ohm.

    There are lots of "Wheatstone Bridge" circuits and the simplest of these
    are NOT suitable for measuring low resistances below about 1 Ohm. So for
    a typical resistance wire investigation the simple Wheatstone Bridge can
    not be used.

    Test: Winding DC Resistance Measurements
    Detects: Broken strands, loose connections, bad tap changer contacts.
    Tool: Wheatstone Bridge (1 ohm and greater), Kelvin Bridge
    micro-ohmmeter (Less than 1 ohm)
    I have analogue meters up to 100 amps that seem to work fine, and as
    much DC power as anyone could want. There might even be a Kelvin Bridge
    around here somewhere. Clearly we need something like that. I am only
    trying to compare all the solenoids. I've already measured the pull
    force for one solenoid and that's OK.
    There's more than a dozen to check.
  7. Chris

    Chris Guest

    Hi, Matty. A 100 amp analog meter certainly isn't going to be good
    enough here. Their accuracy is specified as a percentage of full
    scale. Even a very good 100 amp meter might be specified as +/-2%,
    which would be +/- 2 amps. You're not going to get there from where
    you are. That meter will make you want to push higher current to get a
    more accurate reading, which will cause Rx heating and ruin your

    The same is true for DC current applied. Don't think large -- you'll
    ruin your measurement. Think small current, like 1 amp.

    Get yourself a couple of cheapie $10 USD DVMs from the hardware store
    if you have to. They'll be far more accurate for what you're doing
    than a 100 amp analog meter.

    And don't fall into the trap of thinking that unless you're reading
    ohms directly off a brand name meter, the measurement can't be as good.
    If you can accurately measure an appropriate DC current being forced
    through Rx, and you can accurately measure the volts being impressed
    across Rx, you _have_ measured Rx. It's ohm's law.

    R = V / I

    It works. The rest is just using common sense to avoid changing the
    resistance too much with your measurement.

    Good luck
  8. Matty F

    Matty F Guest

    I have fed current to the solenoid and measured the current and voltage
    with Avometers. The current was 1.45 amps (on the 1.5 amp scale) and the
    voltage 0.38 volts, which works out to 0.26 ohms, which is the same as
    what the Wheatstone bridge measured initially yesterday, and again
    today. I've avoided pressing the test button too much this time.

    The Wheatstone brand is "Pontavi" or some name like that. The name has a
    strange font so I can't be certain it's right. I cannot see that name on
    the Net. I'm told it has a 9 volt battery in it. If the lowest range
    used two 5 ohm resistors in the bridge, when measuring a 0.25 ohm
    resistance it would be trying to push nearly 2 amps through the bridge
    resistors, which could heat them up to have the effect I got before when
    the bridge readings kept going down for each measurement.
    There is no "sensitivity" switch (which is badly needed).
    There is no switch to put power on the resistors before the meter is
    used, so the large impedance of the solenoid makes the meter move when
    the test button is pressed.

    Everybody but me in the workshop says that the Wheatstone bridge is fine
    to measure resistances of way under 1 ohm. So clearly you and I and all
    the references that I gave before are wrong. All my University work in
    physics measurements and statistics was clearly wasted. :)
    Anyway, thanks for your help.
  9. Chris

    Chris Guest

    Hi, Matty. Electronics is a specialized branch of physics. The trick
    with the resistance is just a trick. You're on the right track here.

    Two of the best electrical engineers I ever worked with started out
    with degrees in physics.

    You might want to check around at hamfests and such, and see if you can
    find the manual for your "Punt-avi" bridge. It will tell you what you
    need your coworkers to know.

    Age doesn't necessarily bring wisdom, but it does bring experience.
    Experience has taught me to RTFM (read the manual). If they don't have
    the manual, get it.

    It's funny sometimes (or so the War Department tells me) how much the
    wisdom of age actually looks like just being tired. ;-)

    Good luck
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