Redundant Heating Circuit - How to fool control card

This open-loop method will only work if you can keep the 'temperature' reading within whatever bounds the computer uses. Without knowing how wide the bounds are, how much the temperature of the input card varies*, and how fancy the algorithm is that checks things, all anyone can say is "maybe" unless they're familiar with that specific system.

A theoretically much more reliable method would be to take the control for that heating zone and apply it to the temperature feedback; i.e. give the thing a 'temperature' to regulate.

No matter what you do I can practically guarantee that you'll be screwing around with it, and your potential for success ranges from stunning to stunning -- stunningly good to stunningly bad, that is.

• Thermocouples only read temperature differences, so thermocouple reading circuits have a cold junction compensator. If you put a fixed voltage into a thermocouple input the actual temperature read will be a fixed offset from the cold junction temperature.

Disconnect both the cold-junction compensator and the thermocouple, replacing them with a short. Adjust the setpoint to 0C, and the system should be quiescent.

Jerry

That would work if (a) each thermocouple channel has it's own cold-junction compensator, (b) the cold-junction compensator in question can be disconnected (what if it's synthesized on the board?), and (c) the controller looks for the temperature to be close to the setpoint rather than looking for integrators being out of saturation (which would happen after a while if there is any bias or drift in the electronics).

You could get around (a) and (b) by jumpering one of the thermocouple channels from a working sector to the one you want to cut out of the system, but you still can't get around (c) (which may or may not be a problem). However, you may introduce a problem with the two thermocouple inputs 'fighting' each other -- this wouldn't happen with some circuits, and would happen with others, so it may or may not be a workable solution.

I'm curious to know why the controller's output can't be ignored and any associated alarm disabled. No power need be wasted if no heater is connected.

Jerry