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

[karl dalen]

--- Neil Johnson <neil.johnson97 at ntlworld.com> skrev:

> 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.

Well, speaking of wild-ass-guessing Neil!
TC of MFR appears in some cases to come in +/-. Still you talk about
200ppm which one, the - or the + ,clearly you assumes the - sign but
how do you know the MFR are? 

Futher, about guessing, im not guessing im trying to figure out whats
going on by puting up some questions here, and also by digging trough
various sources this ppm thing in resistors are far from of one issue
only there are plenty of quite delicate things as i have understood,
also it differs between technology used, manufacturing process and
application and between manufacturers and even mounting of the darn
resistors.  
 
> And yes, this has been discussed many times in past years.  A search  
> of the various archives might prove worthwhile.

Not completely just in fragments, since at that point in time few understood
the complete picture. Still people don't understand the whole picture and i 
*gladly* admit i don't understand either but other more qualified then me
don't either.

So regarding the Rapid resistor you mentioned if you had actually read the
Vishay datasheet you would have found a spec that says +/-ppm , there is
not one single place in that sheet that speaks about -or + ppm only.

As an example straight out of their sheet:

:Ordering Example
:The ordering code of a MBA 0204 resistor, value 47 k  and
:TC 25 with ± 0.1 % tolerance, supplied on bandolier in a box
:of 5000 units is: 2312 906 74703.

Actually, yes, in tiny fragment of their datasheet (how to order parts)
Vishay actually says one can order the resistor in -25ppm tolerance!!
How peculiar! Question arises,can one also order them in +25ppm?
Which ppm has Rapid stocked them in???

KD


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