AW: Re: Tempco Resistors

[Matt Haines]

>> Bernie Hutchins mentioned on one issue of the Electronotes Newsletter that
>> it was entirely possible to use the proper sign (+ or -) Thermistor in
>place
>> of a Tempco resistor as long as it had the proper temperature specs (3000
> -
>> 4000 ppm/C).  If the Thermistor was off the opposite sign, then that could
>> be dealt with by the opamps involved.  (Inverting or not).
>
>The only question is if it is *linear* enough. If this is the case, every
>factor above 3300 should work - just combine it with an ordinary
>resistor. If you should find a 1k type with 6600ppm, just put an
>ordinary 1k resistor in series, and you have your 2k / 3300ppm
>stuff. You get the idea. The series resistor would even *linearize*
>the thermistor a bit - so maybe if you find a 200R / 33000ppm thing,
>you wouldn't have to care much about linearity, just combine it
>with a 1k8 resistor. Well, that's just theoretical, of course - I don't
>have any clue if such things do exist.
>


Since folks have had difficulty finding tempcos these days, I opened up the
Mouser catalog and they've got thermistors. The Positive Temperature
Coefficient (PTC) versions all have a transition temperature for when they
kick in. The 2k versions listed have a trans-temp of 100°C. That's a bit
warm...the VCO would never get to that point.

The NTC versions on the other hand are continuously variable over room
temperature-and-higher ranges. There's a 2k with a 'B-constant' of 3800
(I'm assuming this is the same thing as 'ppm'? It's in the thousands, so
I'm guessing). Alas there's no such thing as a 200R/33000ppm version. As
the TC increases, so does the resistance. 200R versions have a TC of +3100.

So is this worthwhile investigating?


.       .       .       .       .       .       .       .       .
    .       .       .       .       .       .       .       .
.       .       .       .       .       .       .       .       .
Matt Haines  haines at quick.net       .       .       .       .
control-X:to:abort:transmission .       .       .       .       .
    .       .       .       .       .       .       .       .
.       .       .       .       .       .       .       .       .
    .       .       .       .       .       .       .       .
There's only one break that breaks the unbreakable.