Ok...

It worked.

I've tried twice now to submit a reply to this thread from an email link over the last couple of days and both times lost all of what I had written. Bother.

Thank you all for getting back. All good stuff.

I am familiar of course with the temperature controllers out there. They do work great... no question. I guess I just want to re-invent the wheel. There... I said it! :-)

There are a lot of off the shelf modules out there. Every one of them will do MOST of what I want... but never quite all of what I want. And then to do what I want requires that the operator be able to go through multiple layers of menus. Not good for the business I am in... heatsealing high temperature fluoropolymers. The only controller that would appear to come the closest is the ROPEX controller and it's expensive... about $2k.

What makes my heat control dream somewhat unique is that I want it to be easily configured to different material heater ribbons, of different shapes, of different thickness and lengths. Then I want the controller to control the current to the heater ribbon, bringing it's temperature from room to 700 degrees F (or any temperature in between) in 8 to 10 seconds and hold it there for an operator chosen dwell time. When the cycle is over, the current is shut off, the temperature drops and the mechanical jaws open at 150 degrees F to allow the sealed materials to be removed from the machine.

All this control, in a small package, built on the cheap (say under $200 US) with no more than two-layer-deep operator configurable control over heat time and pressure. And of course... I only need two or three of them. HA HA HA HA HA!

Ahhh... but those small problems don't quench my insatiable desire to stick my neck out and say "LET HER GO!"

The ribbon I am currently designing around is 303 stainless... .005" thick x .75" wide x 17" long. That works out to about .12 ohms of resistance at room temperature. I honestly haven't worked out what the resistance is at 700 degrees F. I keep burning my fingers (kidding!).

My current controller is using two pic processors. One is dedicated to doing only one thing... waiting for a zero-cross pulse, checking an eight bit port and then outputing a pulse to turn on an alternistor after a pauseus (based on the port data input. It repeats this every half cycle at 60hz (120 times a second). And it is rock solid. No complaints there.

The other processor polls a max 6675 for a thermocouple reading. Based on this reading, it raises or lowers the pauseus data for the second controller. But the 6675 is slow at 220ms between conversions. So I can poll it about 4 times a second.

I want it to be faster! :-)

I'm banging a lot of amps (75 to 100) or so at 1-2 volts ac for microseconds through this ribbon. It heats up quick and cools down in roughly the same time.

That's the story... I'll fill in more details as you guys thrash me behind the dumpster.

Thanks for the great input. So... how would you read the ribbon resistance between current pulses? Any thoughts??????

Ross