[sdiy] resistor temperature coefficient measurement

[Ian Fritz]

Hi Jim --

Good point about thermal EMFs.  I spent many years doing low-level 
electrical measurements (for semiconductor materials characterization) 
where thermal EMFs and also contact resistance effects had to be carefully 
accounted for.  Thermal EMFs are usually in the uV range and for high 
resistances can be swamped out by measuring the resistance at reasonably 
large voltage levels.  For low resistances they may be a big problem, but 
they can be eliminated by reversing the current direction and averaging the 
measurement.

Martin's measurements may have been at a large enough resistance level that 
the thermal EFMs were small enough to ignore, but we would need details 
about the resistance value and also the levels the meter was using to be sure.

   Ian

At 08:13 AM 2/14/2003, patchell wrote:

>     But the big problem I used to have was not so much with the tempco of the
>parts, but rather the Thermal EMF's that were generated by the parts...back
>when I used to do this work, wirewound resistors are what you used if you
>wanted Low TC and precision (I used to use a lot of Jordan J-120 0.01%
>resistors).  These had a built in thermal couple where the resistance wire was
>
>bonded to the wire lead.  As long as the part had no temperature gradients so
>that both ends were generating the same voltage, everything was OK, but if you
>
>took a soldering iron and heated up just one end of the resistor, things would
>
>really go hay-wire...so to speak (the circuits I was working with were doing
>things at DC, but down below a microvolt).  Vishay Bulk Metal resistors are
>better choices these days, they were just coming out, or I should say, making
>themselves known, at about the time I moved on to other things...