SV: Re: SV: Re: [sdiy] Compensating PTC and NTC resistors

[karl dalen]

--- 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
> ppm
> > part fixed with temp, nor linear even manufacturers want to claim that.
> Most
> > papers on the subject suggest xxxppm but they let the sign out! Elby papers
> for
> > example does not suggest any sign nor do Rene's! So do the user use a
> carbon
> > mass (hehe), carbon film, metal film, metal oxide, plastic, ceramic
> compound,
> > 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.

So how does it works then if its just an idea? Asumed TC of zero are not
a viable reality not even with MFR, it clearly states +/-100ppm, whare do
you find MFR of 0 TC?. Even Tony A suggests teasing down the tempco of PTC
by a serial resistor although he didn't say which one. You suggests
paralleling to reduce tempco of NTC's.

Do you mean that Elby adds a resistance just by random chance and by random
fabrication of the resistor they use? http://www.elby-designs.com/

It would be nice once and for all a in depth clarification of this ppm
compensation of NTC's and PTCs.

Reg
KD


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