Grant Richter grichter at execpc.com
Thu Nov 4 22:49:36 CET 1999

Thermistors generally have large and non-linear resistance changes
with temperature, perhaps as large as 2% per degree C.

A temperature compensating resistor (tempco) has a very small and
linear, temperature dependent resistance of +3500 parts per million (ppm)
or 0.0035% per degree C.

The actual math works out to about +3200 ppm but varies depending
on the transistor by perhaps 200 ppm on either side.
Some people prefer +3000 ppm instead of +3500 which is dependent
on what exact silicon you are using for the exponential converter pair.

You can custom order resistors of this type in various compensation
ranges in either positive or negative coefficients and steps of around 500
They are also used to compensate analog sensors over long wire lengths.
For instance if you have 500 feet of wire that changes resistance
a +3000 ppm, if you put a tempco with a negative coefficient of -3000
ppm in series with it, the resistance will remain more constant over
ambient temperature.

> From: Keith Winstanley <keithw at cix.compulink.co.uk>
> To: synth-diy at mailhost.bpa.nl
> Subject: Tempcos
> Date: Thursday, November 04, 1999 2:00 PM
> I am sorry if I am being a bit thick here but I am assuming tempco
> the same thing as thermistors?
> If not could someone point me at one in the CPC, Rapid, Farnell, RS or
> catalogue.I cannot find a referance to "Tempco's" anywhere
> ta..
> ....Keith Winstanley(filling small brain up)

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