# Tempco calcs

Rob cyborg_0 at iquest.net
Sat Nov 27 04:01:43 CET 1999

```What about tungsten? Doesnt it have a particularly high resistance? You
could literally take it out of a new lightbulb, and its usually wound, so
that would give you perhaps a little significant length..

Rob

----- Original Message -----
From: Ian Fritz <ijfritz at earthlink.net>
To: <synth-diy at mailhost.bpa.nl>
Sent: Friday, November 26, 1999 1:40 PM
Subject: Re: Tempco calcs

> Hi All --
>
> More musings on tempcos.
>
> I had a chance the other day to review the theory for electrical
> resistivity of metals. For pure elemental metals (Cu, Pt, W, Na, Pb,
> etc.) theory shows -- and experiments confirm -- that the resistivity of
> every element is described by a single universal function that has just
> one parameter, which is different for each metal. It's really a
> beautiful sight to see the data from 17 different metals falling exactly
> on the same single theoretical curve!
>
> At very low temperatures the resistivity follows a T^5 law. At higher
> temperatures the resistivity is linear with temperature, with accurate
> linearity being seen over hundreds of degrees. Ordinary room
> temperatures are in the linear region of the curve for almost all
> metals.
>
> As discussed before, the ideal tempco resistor for compensating
> exponential converters has an R(T) characteristic that is linear near
> room temperature with this linear region extrapolating through the
> origin: R_extrap(0K) = 0. For a pure metal, the linear resistance region
> extrapolates to a negative resistance, R_extrap(0K)<0, because of the
> T^5 regime at low temperatures. Therefore, an ideal tempco can be made
> from a composite of a pure metal in series with a
> temperature-independent resistance equal in magnitude to the
> extrapolated resistance: R_series = -R_extrap(0K). This correction
> resistor can be fairly small. As an example, for Pt the series
> resistance is 8% of the resistance at T=273K (0C).
>
> It's interesting to realize that a good tempco could be made from
> copper. (The series resistance needs to be about 20% of the 273K
> resistance in this case.) Unfortunately, the resistivity of copper is so
> low that a large amount of very fine wire would be required. This may
> not be practical: but does anyone know of a source of very fine copper
> wire? What's the smallest diameter you can get?
>
>   Ian
>

```