[NewCandle] Measuring low resistances

[Horace Heffner]

google (measuring low resistances)

There are various articles on the 4-wire Kelvin method.  Notable  
articles:

http://www.allaboutcircuits.com/vol_1/chpt_8/9.html

http://archive.evaluationengineering.com/archive/articles/ 
1106/1106the_art.asp

One method I have seen to extend the Kelvin method beyond the range  
of a typical multimeter is to create a low "standard" resistance  
element Rs that is later used as one leg of a bridge.  Measure its  
resistance using the Kelvin 4-wire technique with your ordinary  
multimeter.  Then you can measure up to about an order of magnitude  
lower test resistance Rt using a bridge.  Across the middle of the  
bridge use a bipolar op-amp IC to sense when there is a zero voltage  
condition while adjusting R1 and/or R2.

power---o---Rs----------o--------Rt-------o----G  <---- test side of  
bridge
         |               |                 |
         |               |                 |
         |              IC                 |
         |               |                 |
         |               |                 |
         o---R1----------o--------R2-------o  <---- potentiometer  
side of bridge


You now have:

    Rs/Rt = R1/R2

    Rt = Rs*R2/R1

You can use this method to create yet another lower resistance  
standard. Lots of worries about how carefully Rs and Rt are connected  
though.

Best regards,

Horace Heffner
http://www.mtaonline.net/~hheffner/




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