[sdiy] Tempco 2k +3350ppm Group buy

[Ryan Williams]

HA.Synth at t-online.de wrote:
> Thanks Ryan,
> 
> looks like I should go for +3500. But I have one questons. If i look at the 
> tracking chart of PT146-35 it says 3440ppm at 25 and 3500ppm at 100degC.
> In your paper you wrote +3369 at 25C and +3430 at 100C.
> If I look at RES/TEMP CURVE at their site I can read +3369 at 25C and +3420 at 
> 100C.
> What am I missing here? Is the tracking chart wrong?

It's been several months since I wrote that. I was just now looking at 
the website trying to figure out why I used that equation for the TCR 
curve instead of the tracking chart (i used the graph). Now I remember, 
it was based on this comment on the site:

"if you are looking for a systems offset of +3350 ppm/°C. to +3450 
ppm/°C, we are confident you can achieve dramatic results with our 
standard off-the-shelf compensators. The actual TCR of a standard 3500 
element is very close to +3350 ppm/°C. at 25°C. and lower than +3450 
ppm/°C. at 100°C."

I just use that provided equation as an example, but they make this 
claim that its correct. I think their chart is also just an example. 
they will have some tolerance but i'm not sure how much. If you really 
want to do it right, you might measure and adjust the circuit after. 
Even if the ones you measure did have a TCR closer to 3440, you can 
decrease that TCR in the circuit with a series resistor. It a bit more 
difficult to increase a TCR (see Ian Fritz website).

-Ryan Williams