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

[Ian Fritz]

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 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.  Or tuned in 
with the dial-a-tempco.
http://home.comcast.net/~ijfritz/sy_cir9.htm

   Ian