[sdiy] tempco resistors

[Simon Brouwer]

Neil Johnson schreef:
> David,
>
> The Farnell part number is 1174306.  Looking this up it is a TYCO 
> ELECTRONICS - LT300014T261K0J temp.co resistor with 3000ppm/'C slope.  
> Here's the Tyco product page:
>
> http://catalog.tycoelectronics.com/TE/bin/TE.Connect?C=1&M=BYPN&TCPN=1624329-2&RQPN=LT300014T261K0J 
>
>
> which has a link to the PDF spec page.
>
> Not quite the 3900ppm/'C that is usually used, but a lot better than 
> nothing!
Actually the ideal tempco resistor has a resistance that is proportional 
to absolute temperature T, as it has to compensate for an effect in the 
expo pair that is inversely proportional to T.
This means at 25 degrees centigrade, i.e. 298.15 K, a tempco of 
1/(298.15) = 3354 ppm/'C.

Therefore a resistor with 3900ppm/'C is more off than one with 
3000ppm/'C by itself. However, you can combine it with a series or 
parallel resistor to lower the tempco. This easy method of correction is 
not possible if you have a lower tempco to begin with.

-- 
Vriendelijke groet, 
Simon Brouwer. 

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