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<div><font class="Apple-style-span" face="Courier" size="4"><span class="Apple-style-span" style="font-size: 14px;">google (measuring low resistances)</span></font></div><div><font class="Apple-style-span" face="Courier" size="4"><span class="Apple-style-span" style="font-size: 14px;"><br></span></font></div><div><font class="Apple-style-span" face="Courier" size="4"><span class="Apple-style-span" style="font-size: 14px;">There are various articles on the 4-wire Kelvin method. Notable articles:</span></font></div><div><font class="Apple-style-span" face="Courier" size="4"><span class="Apple-style-span" style="font-size: 14px;"><br></span></font></div><div><font class="Apple-style-span" face="Courier" size="4"><span class="Apple-style-span" style="font-size: 14px;"><a href="http://www.allaboutcircuits.com/vol_1/chpt_8/9.html">http://www.allaboutcircuits.com/vol_1/chpt_8/9.html</a></span></font></div><div><font class="Apple-style-span" face="Courier" size="4"><span class="Apple-style-span" style="font-size: 14px;"><br></span></font></div><div><font class="Apple-style-span" face="Courier" size="4"><span class="Apple-style-span" style="font-size: 14px;"><a href="http://archive.evaluationengineering.com/archive/articles/1106/1106the_art.asp">http://archive.evaluationengineering.com/archive/articles/1106/1106the_art.asp</a></span></font></div><div><font class="Apple-style-span" face="Courier" size="4"><span class="Apple-style-span" style="font-size: 14px;"><br></span></font></div><div><font class="Apple-style-span" face="Courier" size="4"><span class="Apple-style-span" style="font-size: 14px;">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. </span></font></div><div><font class="Apple-style-span" face="Courier" size="4"><span class="Apple-style-span" style="font-size: 14px;"><br></span></font></div><div><font class="Apple-style-span" face="Courier" size="4"><span class="Apple-style-span" style="font-size: 14px;">power---o---Rs----------o--------Rt-------o----G <---- test side of bridge</span></font></div><div><font class="Apple-style-span" face="Courier" size="4"><span class="Apple-style-span" style="font-size: 14px;"> | | |</span></font></div><div><font class="Apple-style-span" face="Courier" size="4"><span class="Apple-style-span" style="font-size: 14px;"> | | |</span></font></div><div><font class="Apple-style-span" face="Courier" size="4"><span class="Apple-style-span" style="font-size: 14px;"> | IC |</span></font></div><div><font class="Apple-style-span" face="Courier" size="4"><span class="Apple-style-span" style="font-size: 14px;"> | | |</span></font></div><div><font class="Apple-style-span" face="Courier" size="4"><span class="Apple-style-span" style="font-size: 14px;"> | | |</span></font></div><div><font class="Apple-style-span" face="Courier" size="4"><span class="Apple-style-span" style="font-size: 14px;"> o---R1----------o--------R2-------o <---- potentiometer side of bridge</span></font></div><div><font class="Apple-style-span" face="Courier" size="4"><span class="Apple-style-span" style="font-size: 14px;"><br></span></font></div><div><font class="Apple-style-span" face="Courier" size="4"><span class="Apple-style-span" style="font-size: 14px;"><br></span></font></div><div><font class="Apple-style-span" face="Courier" size="4"><span class="Apple-style-span" style="font-size: 14px;">You now have:</span></font></div><div><font class="Apple-style-span" face="Courier" size="4"><span class="Apple-style-span" style="font-size: 14px;"><br></span></font></div><div><font class="Apple-style-span" face="Courier" size="4"><span class="Apple-style-span" style="font-size: 14px;"> Rs/Rt = R1/R2</span></font></div><div><font class="Apple-style-span" face="Courier" size="4"><span class="Apple-style-span" style="font-size: 14px;"><br></span></font></div><div><font class="Apple-style-span" face="Courier" size="4"><span class="Apple-style-span" style="font-size: 14px;"> Rt = Rs*R2/R1</span></font></div><div><font class="Apple-style-span" face="Courier" size="4"><span class="Apple-style-span" style="font-size: 14px;"><br></span></font></div><div><font class="Apple-style-span" face="Courier" size="4"><span class="Apple-style-span" style="font-size: 14px;"><span class="Apple-style-span" style="font-family: Times; font-size: 16px; "><div><font class="Apple-style-span" face="Courier" size="4"><span class="Apple-style-span" style="font-size: 14px; ">You can use this method to create yet another lower resistance standard. Lots of worries about how carefully Rs and Rt are connected though. </span></font></div><div><br></div></span></span></font></div><div> <font class="Apple-style-span" face="Courier" size="4"><span class="Apple-style-span" style="font-size: 14px;">Best regards,<br class="Apple-interchange-newline"></span></font><div><font class="Apple-style-span" face="Courier" size="4"><span class="Apple-style-span" style="font-size: 14px;"><br class="khtml-block-placeholder"></span></font></div><div><font class="Apple-style-span" face="Courier" size="4"><span class="Apple-style-span" style="font-size: 14px;">Horace Heffner</span></font></div><div><font class="Apple-style-span" face="Courier" size="4"><span class="Apple-style-span" style="font-size: 14px;"><a href="http://www.mtaonline.net/~hheffner/">http://www.mtaonline.net/~hheffner/</a></span></font></div><div><br class="khtml-block-placeholder"></div><br class="Apple-interchange-newline"><br class="Apple-interchange-newline"> </div><br></body></html>