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

Neil Johnson neil.johnson97 at ntlworld.com
Tue Mar 18 00:06:10 CET 2008


On 17 Mar 2008, at 22:22, karl dalen wrote:
> 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.

Maths.  Wonderful thing.  Allows you to "work things out" rather than  
blindly wild-ass-guessing all over the place.

For example, in this case the formula to apply, given two resistors  
R1 and R2 in series with temperature coefficients of T1 and T2, we get:

	Rs = R1 + R2

	Ts = ( R1 x T1 + R2 x T2 ) / ( R1 + R2 )

So if you have a 1k0 3900ppm thermistor you can add a 150R 200ppm  
metal film resistor in series to get a 1150 resistor with a combined  
temp.coeff of about 3417ppm/C.

And yes, this has been discussed many times in past years.  A search  
of the various archives might prove worthwhile.

Neil
--
http://www.njohnson.co.uk






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