Author Topic: Sorting resistors with a wheatstone bridge  (Read 4143 times)

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Offline cosmicray

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Sorting resistors with a wheatstone bridge
« on: February 13, 2015, 11:03:49 pm »
A few weeks back I acquired a full reel (~5000 pcs) of 392 ohm 1% 1/4w 50ppm axial resistors. It was one of those odd deals that happens. One percent is still ± 3.92 ohms.

I would like to pull a few hundred off the reel, and see if I can organize them in sets of matched values. While I can use the 2000 ? scale on the DMM, I want to give it a try matching them via a wheatstone bridge.

The plan is to rough bin them with the DMM, then try to match them via the wheatstone bridge. The amp-meter in the schematic would be the DMM on the 2000 uA scale. If I can get a zero reading on that, then swap R1 with R4 and still get a zero current flow, that should indicate I have a 4-way matched set. Am I understanding this correctly ?

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Offline zapta

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Re: Sorting resistors with a wheatstone bridge
« Reply #1 on: February 14, 2015, 03:16:36 am »
>> that should indicate I have a 4-way matched set

How do you plan to use those 4-ways matched sets?
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Offline andtfoot

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Re: Sorting resistors with a wheatstone bridge
« Reply #2 on: February 14, 2015, 03:38:01 am »
I would have thought you would use a voltage measurement, not current.
You are effectively creating two voltage dividers and comparing the junctions.
 

Offline cosmicray

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Re: Sorting resistors with a wheatstone bridge
« Reply #3 on: February 14, 2015, 04:05:40 am »
I would have thought you would use a voltage measurement, not current.
You are effectively creating two voltage dividers and comparing the junctions.
It works either way. I can flip the cheap DMM to 200 mV or to 2000 uA scale. I can see difference readouts that way, that I can not see on the 2000 ohm scale. I'm powering it with a cast off wall-wart. Rating says 6V, but my load is so low I'm getting 8V w/filtering. It took me a while to get all the bits organized, but I'm beginning to see a few results. When one resistor shows a center differential of 1.2 mV, but swapping it with one that shows 0.6 mV tells me that I working towards a set. Once I can move all 4 in any direction, and hold 0.0 mV across the middle that says I have a matched set (or something very close). At that point, I can use one position in the wheatstone as a very sensitive test point. Thus allowing me to more finely bin the others. The rough bin only allowed dividing into 2 piles, mostly 390 and a few that said 389. The DMM is not so good at fine resistance measurements.

With a complete set, measuring the current into the top point, may actually permit me to compute the true resistance of the 4 matched resistors (assuming I can trust the current reading on the DMM). True resistance may be based on ambient temp tho.

This setup is so sensitive that touching both sides of a resistor with finger tips, causes a change in the mV reading across the center.
it's only funny until someone gets hurt, then it's hilarious - R. Rabbit
 

Offline cosmicray

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Re: Sorting resistors with a wheatstone bridge
« Reply #4 on: February 14, 2015, 04:22:43 am »
>> that should indicate I have a 4-way matched set

How do you plan to use those 4-ways matched sets?
I don't have a precise use for them yet, but fine-binning them may actually give me more than 4 of a matched value. There has to be a good use for matched resistors somewhere. :)
it's only funny until someone gets hurt, then it's hilarious - R. Rabbit
 

Offline zapta

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Re: Sorting resistors with a wheatstone bridge
« Reply #5 on: February 14, 2015, 05:24:08 am »
I don't have a precise use for them yet, but fine-binning them may actually give me more than 4 of a matched value. There has to be a good use for matched resistors somewhere. :)

It's a very specific kind of matching were you have two voltage dividers with the same ratio. Nothing else.  If you don't have a use for it, why bother?

Matching pair of resistors to have the same value is more useful IMO, but then again, why bother if you don't have a use for it and you don't even know what resistance you will need for your future circuit.

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Offline cosmicray

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Re: Sorting resistors with a wheatstone bridge
« Reply #6 on: February 14, 2015, 10:26:39 pm »
I don't have a precise use for them yet, but fine-binning them may actually give me more than 4 of a matched value. There has to be a good use for matched resistors somewhere. :)

It's a very specific kind of matching were you have two voltage dividers with the same ratio. Nothing else.  If you don't have a use for it, why bother?

Matching pair of resistors to have the same value is more useful IMO, but then again, why bother if you don't have a use for it and you don't even know what resistance you will need for your future circuit.
Why ? Mostly educational, and I'm forcing myself to learn something I have never done before. When it's all over, I may find a use for some of these matched sets.

I believe I've achieved a matched set of 4. I have traded the resistors in the opposite corners, with no change. The DMM is has been observing the difference points for 30 minutes. It drifts a very small amount, at the minimum resolution of the DMM, from -0.0 to 0.0 to (less frequently) 0.1 mV. By my calculation the drift is +/- 0.0025%. That is so low, that I'm willing to assign it to minute changes in the test jig holding the resistors, internal heat dissipation in the resistors (approx 40.8 mW each), the cheap DMM, etc. This appears to be a stable reference set of 4. If I had a better test jig, I would try shoving them into the freezer, to see what happens at 0c. Room temp at the workbench is 25c. I do not know the actual resistance of this group. To know that I would need something traceable to NIST, unless someone has a way to infer a reasonable approximation.

With a stable reference set, I can now take other likely sets, and see how close they are to one another. My medium binning produced 30 brackets to work from.
it's only funny until someone gets hurt, then it's hilarious - R. Rabbit
 


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