Re: Wheatstone bridge problem



Matty F wrote:
> 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
> temperature!

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
mind.

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 www.tech-chat.de

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
Chris

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