Tempco calcs

Ian Fritz ijfritz at earthlink.net
Sat Nov 27 01:47:11 CET 1999


Harry et al. --

I just pulled apart a 600 Ohm reed relay I had laying around. It had 44
ga wire. Unfortunately the ends of the coil were embedded in the potting
material, so I couldn't unwind it. My idea is to wind a coil
non-inductively for use as a feedback resistor. 

So does anybody know a source for 48 ga magnet wire? I can make jigs or
mandrels or whatever and do the winding on my lathe.

  Ian

Harry Bissell wrote:
> 
> 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



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