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

[karl dalen]

--- Ian Fritz <ijfritz at comcast.net> skrev:

> At 04:49 PM 3/17/2008, karl dalen wrote:
> >So are Elbys calculations right or wrong?!
> >http://www.elby-designs.com/datasheets/tempco.pdf
> 
> Unfortunately, they are not quite right.  He assumes that the coefficient 
> that needs to be corrected is 3500 ppm/K.  But 3500 is actually the 
> *average* tempco between 25 and 100 deg C.  The tempco *at* 25 deg  C is 
> 3440 ppm/K, as can be seen immediately from the table, and as is discussed 
> explicitly by the manufacturer.  This is what needs to be corrected (to 
> 3360 ppm/K).  The proper correction resistor Rc is given by
> 
> (3440R + 100Rc)/(R + Rc) = 3360,
> 
> (assuming a 100 ppm/K metal film compensator).  For a 1k tempco the correct 
> compensator is easily seen to be 25R.

But again is it +100 or a -100ppm figure? I have a data sheet here 
from a reputable resistor vendor who specify +/-100ppm or +/250ppm etc,
among the very few who i have found that actually publish a ppm chart for
the entire temp range and it looks awful, not even near anything that
resembles log or line, it widens, narrownesses, then goes straight, then 
looks like a bumpy hillside etc. It basically wanders all over the place
and some times are - then later + sign or both! Compare this issue to
carbon mass resistors of +5000ppm. Electrolytic capacitors, +10 000ppm
they appears to have a defined sign.

The issue here is like saying: This MFR has only + 5% tolerance
or -5% in its resistive tolerance, we all know in reality its both,
so what about the TC % figure is it + or - or both?

> But a further complication is that there is a significant uncertainty in 
> the coefficient itself.  In the end, the correct compensator needs to be 
> determined by careful measurements on the converter itself. 

Are you talking about the PTC coefficient or that one im talking about
the PTC compensator coefficient?(the additional resistor).

KD 



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