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Mechatronics Zone
Six-Wire Technique Nulls Stray Resistance
11/15/2011

A DMM that offers six-wire measurements lets you guard an unknown resistance with another resistance path that conducts current. The drawing on the left shows an equivalent test fixture that might include resistive paths.

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Gold
six wire technique
11/26/2011 10:59:11 AM
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This is very useful idea for ICT development.

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Platinum
Six wire technique nulls stray resistance
11/17/2011 10:51:04 PM
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A whole bunch of years ago I designed and improved system for checking the fuel injection system s tverify that all 4 of the 1.2 ohm injectors were connected.  I replaced a resistance measuring systemthat had problems with one that used a constant current source. Because the injectors were relatively high power devices I was allowed to run 100Ma through the harness as part of the test. I used a constant current regulated source, and so the connected harness assembly yielded 100 millivolts per ohm, entirely adequate resolution. My application only required four wires to provide the needed accuracy. It worked well and saved our company a lot of money, and it made our customer happy as well. On top of that, it gave us a very short product lead time.

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Iron
Clever technique
11/16/2011 9:35:28 AM
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Simple & elegant.  Just watch your OpAmp's feedback polarity.  Your schematic as drawn doesn't work ;)

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Re: More complexity, more work?
11/15/2011 11:05:50 AM
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Got it. Thanks for clarifying, Jon.

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Re: More complexity, more work?
11/15/2011 10:40:22 AM
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Hi, Beth.

If you have a resistance (A)  in a network of other resistances, Ohm's Law dictates the resistance you would measure directly across resistance A.  To accurately measure resistance A by itself, you either isolate it from the circuit (removing it would do the job) or electrically null the other resistances so no current flows through them.  That's what the 6-wire technique does.  Automatic test equipment (ATE) uses a similar technique to electrically isolate resistances.  By the way, a similar technique would for capacitances, too.

The need to electrically isolate components does require extra test equipment, but the 6-wire technique offers the only practical way to make accurate in-circuit resistance or capacitance measurements.--Jon

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More complexity, more work?
11/15/2011 6:40:16 AM
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So if I understand it correctly, this six-wire technique works best when there's a network of resistors and you're challenged with measuring the resistance of only one? It also makes sense when measuring resistance with test equipment because of the possible interferences by dirt, grim, etc.? I'm wondering how much more difficult this technique is compared with standard practices?

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