[sdiy] tempco questions

[Jim Patchell]

    You basically got it.  3000ppm means 3000 parts per million, and in
this case, one should really add "per degree C" to make it a complete
sentence.  Another way of saying this is 0.3% per degree C (3000/1000000
* 100), so for every decgree C the temperature rises, the resistance will
increase by .3%.  The magic number you are looking for 3300ppm/C, as this
is how much the transconductance of a transistor changes with temperature
(3000 close enough).

    Technically, yes, these are thermistors.  You have to watch yourself
here, however.  You can have positive and negative tempco.  In generally,
positive tempco resistors are used to compensate our exponential
converters.  Some list members have used negative tempco parts as well.
Negative tempco parts are easy to come by.  Positive tempco parts are a
lot more rare.  It is nice that TomG is making these availiable through
his web site for a very good price.  Real TelLabs Q81's that you see in a
lot of old schematics are no longer availiable, and also were very big.
KRL still sells them as well, but, on a custom basis (i.e. you have to
purchase a lot of them, and they are not very cheap).

    -Jim

Reverend djbluE wrote:

> so my question is... what is a tempco?  I think it's something like a
> resistor that changes resistance depending on the temperature.  Is
> this something like a Thermistor?  If not, what is a thermistor?
> what do the "3000ppm" numbers mean?  is it a higher the better/lower
> the better/ need a certain range/number for a certain application?
>
> thanks all..