[sdiy] PTC Tempco?

[Jay Schwichtenberg]

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%.

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

Previous resistance. I'll write this in simple C code since we don't have
math symbols.

assume T >= Tb and temps are integer values.

for ( R = Rt, Ti = Tb; Ti != T; Ti++ )
{
  R = R + R*alpha;
}

Also can someone point to some good links on PTCs? I haven't found anything
out there on using PTCs in exp/log compensation. It's mainly been power
control and fluid sensing.

Thanks much.
Jay