[sdiy] 1.0 kohm "tempco" for VCO / sub octave question

[René Schmitz]

Hi Thomas,

> *The 1.0 kohm "tempco" to be thermally bonded to the CA3046 - the design
>  specifies the Precision Resistor Co. PT146, which has proven difficult to
>  find here (if anyone's got just TWO pieces to spare from their personal
>  stash, please let me know...)
>   ELFA, however, stocks a Philips part which I wonder if might
>   do the trick:
> 
>   KTY 81-120 (SOD-70 package)
>   1000 ohms +/- 2%
>   Temperature coefficient: +0.75%/K (isn't that like saying 7500ppm/K?)
> 
>   The PT146 has a postive temp. coefficient of 3350ppm, less than half
>   that of the Philips KTY81-120 - but are they otherwise (functionally)
>   the same kind of part? I have never worked with PTCs like these so I
>   have to ask, the part is to be in the feedback loop of a TL084 amp
>   at the CV input of the VCO, so I assume it must be a pure resistance
>   seen from the op-amp... Might the Philips KTY81-120 work?

I have been using these for quite some time now. They wok fine if you 
adjust the circuit a bit, you get a different value than 1k, since you 
need to lower the tempco by a series (or parallel) resistor.

Have a look here:
http://www.uni-bonn.de/~uzs159/vco3.html
http://www.uni-bonn.de/~uzs159/expo_tutorial/index.html

Cheers,
  René

-- 
uzs159 at uni-bonn.de
http://www.uni-bonn.de/~uzs159