SV: Re: SV: Re: [sdiy] Compensating PTC and NTC resistors
dalenkarl at yahoo.se
Mon Mar 17 23:46:41 CET 2008
Hmm,perhaps this have been discussed in past years?
--- René Schmitz <uzs159 at uni-bonn.de> skrev:
> Hi Karl and all,
> >> I think the idea is that the change of the carbon resistor is so small
> >> compared to the 3900TCR that it could be considered ~linear~
> >> So what you do is to make up a resistor where one portion of the total
> >> resistance is the TCR, and the other fixed... so the overall tempco is
> >> reduced. Its a ~mixture~ problem... maths left to the interested :^)
> > Yes, but still, where do one get the -600ppm resistor?
> > The user cant compensate a +3900ppm with a +/-600ppm resistor! Nor are the
> > part fixed with temp, nor linear even manufacturers want to claim that.
> > papers on the subject suggest xxxppm but they let the sign out! Elby papers
> > example does not suggest any sign nor do Rene's! So do the user use a
> > mass (hehe), carbon film, metal film, metal oxide, plastic, ceramic
> > or any of the myrriad of new resistor techs thats available today.
> Thats not how it works, you can't just add and subtract tempcos like
> that. The idea is more that when you change the resistance by adding a
> series resistor (of assumed zero TC, don't bother with minute second
> order effects, and use MFR), the tempco of the whole deal becomes lower.
> The maths is basically a rule of three.
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