interpolating ribbon temperature by resistance

[Archangel]

Hi rossfree,
I was just wondering if you had tried sandwitching 2 of these SS strips together with some heat sink compound and (mica ?) insulators so you could heat one with current and measure the hot resistance of the other one ? It seems to me you could then allow adequate time for heat saturation to get an accurate idea of the hot resistance of the strip. Other methods might be to insert strip into a heat treat oven while wired to an ohmmeter. If you do not have one, I'll bet your local junior college does and pizza seems to be their official currency .
JS

[Acetronics2]

Hi, Ross

Depending on the voltage applied, the trick could be simple:

for DC supply, just apply Heating power by PWM ... and allow a continuous current flowing into your resistor.

So simple ... just measure the voltage across your heating element during the "0v" part of the PWM ...

For AC supply, Low voltage ... the PWM will be replaced by burst mode ...and constant DC current only applied when no AC provided.

AC, High voltage ... a probe really seems the simplest way !!!

Alain

[rossfree]

Thank you for the feedback...

Right now I am pulsing a 2KVA 48 Volt CT transformer using 110VAC line and an alternistor. One pic does the timing and sends the pulse to the alternistor to turn it on at the right time in the half cycle. The other pic runs the program, picking up the temperature and passing timing information to the first pic.

Depending on the ribbon size and length (it changes all the time) I may require 50 or so amps of current pulsed 120 times / second for very short duration to achieve the ramp and dwell time I require to bring the temperature of the ribbon up to the required temperature and sit there soaking heat into the film to melt it.

The voltage across the ribbon would never exceed 48VAC.

I like the idea of applying a constant current across the ribbon in series with an external resistor. Then reading the voltage across the resistor and interpolating the ribbon temperature with a look-up table or something like that. But I'm not to good at figuring out how to decouple the 48VAC transformer such that it does not affect the results. And forgive my ignorance... but I also don't know how the pulses would affect the constant current circuit. Boy... I wish I had a mind for this stuff!

My electronics buddy suggested injecting a 100khz signal across the ribbon and external series resistor and using a capacitor to decouple from the 48vac transformer. But he said that would likely get into some parts and require some tweaking.

And lastly... I have again revisited using a thermocouple... but instead of the MAX6675 Thermocouple to Digital converter (I am currently using)... I would use a thermocouple amplifier/conditioner like the AD597 which ouputs 10mv/degree C analog in real time. I could feed that into the A/D pin of the pic and convert it to Farenheit for display. This would be much faster than the MAX6675.

So I have choices... but still... I would really like to know the simplest... most precise way of reading the ribbon resistance. That would be the cats meow!

Do you smell it???

That's my brain.

Ross