# Tempco calcs

Harry Bissell harrybissell at prodigy.net
Sat Nov 27 02:59:25 CET 1999

I think you can get 60ga wire... a lot of guitar pickups use wire in the
40-60 ga. range. It is beastly hard to work with unless you make some
special jigs to do the winding. An old technique was to modify turntables to
hold a bobbin and hand wind.

A thread (sorry, forgot name but not idea...) suggested using the coils of
reed relays for the "copper" content... you can get nominal 1K resistances
easily. Maybe thats the way to go...

:^) Harry

Ian Fritz wrote:

> 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