Tempco calcs

Donald Johnson djohnson at mdsi.bc.ca
Sat Nov 27 02:18:36 CET 1999


Date sent:      	Fri, 26 Nov 1999 17:59:25 -0800
From:           	Harry Bissell <harrybissell at prodigy.net>
To:             	ijfritz at earthlink.net
Copies to:      	synth-diy at mailhost.bpa.nl
Subject:        	Re: Tempco calcs


Anders Sponton's "Cloned Analog Gear" site:
   http://omega.tellus.vallentuna.se/anders/
under "Synthesizers" has an ETI magazine article in the Transcendent
2000. The article suggests using a regular resistor (about 130 ohm)
and a copper wire-wound resistor (about 870 ohm) to create
a 1K tempco. They do also suggest using the Tel Labs Q81 too.
This tempco is not in a feedback loop so the inductance of
the resistor shouldn't affect the VCO. Has anyone tried using
a reed relay?

don.

Harry Bissell <harrybissell at prodigy.net> 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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