[sdiy] PTC Tempco?

[Forbes, William - EE - UK/Leamington]

Hi All,

> -----Original Message-----
> From: Jay Schwichtenberg [mailto:jays at aracnet.com]
> Sent: 07 February 2002 18:14
> To: Synth-DIY
> Subject: [sdiy] PTC Tempco?
> 
> 
> Howdy,
> 
> While were on the subjuct of MATs and matching transistors I've got a
> question on the PTC tempcos we use in our expo/log converters.
> 
> So the question is: what's the resistance at a temperature?
> 
> To me there are two ways that I can think of calculating 
> this. One as a
> linear function and the other based on the previous 
> resistance. Let me try
> to explain.
> 
> Tb = base temp. Usally 25 C.
> Rt = resistance at a base temperature. Usally 1K or 2K.
> alpha = percent per degree C change. Ideally 3.3% but we 
> usally use 3.2% or
> 3.5%.

I think you'll find is more like 0.33% (3300 parts per million per
degree C

Thus for a 1k resistor, it will change by 3.3ohms per every
degree change in temperature.

> 
> For a linear function:
> R(T) = Rt + Rt*(T - Tb)*alpha
> 

Thats correct
If Rref = Refrence resistance (1k)
   Tref = Refrence temperature (25 degree C)
   alpha = 0.0033 (3300/1000000)

Then 

R(T) = Rref + Rref * ( T - Tref ) * alpha
or
R(T) = Rref * ( 1 + ( T - Tref ) * alpha )

> Previous resistance. I'll write this in simple C code since 
<snip>

> 
> Thanks much.
> Jay

Hope this helps,
>