Heat Tracing Questions

"The PTC tape will therefore be APPROXIMATLY a constant wattage load at any reasonable voltage. An ALMOST constant wattage requires more current at a lower line voltage.


Experiment and observation beats argument, buy a small PTC heating element and take measurements at different voltages and at different temperatures."


I completely agree with you regarding experiment and I wasn't actually arguing but was puzzled by the statement regarding the increase in current with reduction in voltage and following on the idea that Ohm's Law didn't apply.

I think Roger has explained more clearly that the power generated as heat reaches a balance (almost constant wattage) as a result of the composite material changing resistance in a negative feedback loop. Ohm's Law will therefore always apply and the measured current is a product of line voltage applied and the resistance of the cable and hence power is produced (V²/R). There is I expect a drop in power production at some temperature point when reducing voltage as the cable resistance no longer reduces dramatically with lowering temperature depending on the composition/mixture as determined during manufacture.

I may take up your suggestion and get some tape as it could very very useful to stop the pipes freezing.?

Andrew, the difficulty you are having is because Ohms law does not apply directly to PTC materials. The thing controlling the resistance is temperature only, given a certain minimum value. These materials have a kink in the ohmic value, known as a knee. It is fairly sharp and if you plot resistance against temperature you will see a fairly linear response to some temperature and then a fairly sharp increase in resistance to temperature slope, which may get to be almost constantly very high. This means that the only thing controlling the circuit current at the operating temperature is the temperature, Ohms law goes out of the window. Draw a little sketch graph of resistance v temperature as described to see this clearly.

Here is a fleabay supplier of cheap self regulating heating tape, if anyone wishes to buy some and then observe the input current at different voltages and at different temperatures.

fleabay link

Hi Dave.

Many thanks for the reply and explanation. This is a great example of how there's always something new to learn! I was viewing Ohm's Law in a very simplistic way as a straightforward arithmetic equation. A quick look at Wikipedia put me straight in defining it as I∝V which is a linear relationship. Obviously non linear materials (and of course semiconductor devices) do not follow this relationship and I'm familiar with the V/I curves of various diodes etc. I had in my head this image from Z's post of the current increasing with decreasing voltage at a fixed temperature which would require the resistance to drop without temperature change! Now the fog has cleared somewhat I do appreciate that the non linear (nonohmic) behaviour means in this case I is not proportional to V.

The actual power developed in the resistance of the cable can still of course be calculated using Ohm's law providing we know the material characteristics and therefore resistance at a specified temperature and the voltage applied.

Thanks guys for your patience.

Cheers Andy

I know that on the odd few occasions  that I have installed heating tape the makers have always insisted on a remote stat feeding its supply. Purely mini ladder type I would imagine