Redundant Heating Circuit - How to fool control card

[woodglass]

I have a very old computer controlled system controlling a number of heating zones.

I need to permanently remove/disable one of the zones, and fool the computer into thinking that the redundant zone is functioning ok.

The system doesn’t allow me to ‘disable’ the redundant heating zone.

What I need to do is program a temperature setpoint at the computer, disconnect the thermocouple, & apply a voltage at the thermocouple input to the control card such that the computer thinks that the redundant heating zone is at the required setpoint.

I was thinking of ‘tying’ one of the thermocouple terminals to 0V, & , using a potential divider applying a voltage to the other thermocouple terminals. The +ve side of the potential divider would be connected to the regulated supply of the control card.

Will this work ?.

 

[benta]

Can't you just heat the thermocouple with a resistor?

Benta.

 

[OperaHouse]

Many systems already apply a small voltage via a resistor to detect an open thermocouple. A small resistor at the terminal may provide that simulation, though it might be in the opposite direction. Why not just use a thermocouple at the computer. That should stay in a normal region.

 

[mcgyvr]

So you are going to have
Heating zone 1
Heating zone 2 (Disabled)

Why not just use the thermcouple from zone 1 to feed into zone 1 thermcouple input and zone 2 thermocouple input.
Just tap into each wire and bring to zone 2 input. It might mess with the accuracy slightly.. but it doesn't sound like high accuracy is that necessary anyways

 

[itsmoked]

Yes it would mess the the accuracy ... Possibly badly.

woodglass; Why can you not just disconnect the output so there is nothing to drive and let that zone do what ever it wants?

Why do you have to have that zone seem happy?

Can you just stick a 1 inch thermocouple on the input and set that zone to room temperature?

Can you not just move its thermocouple over next to another zone's thermocouple and set the zone to the same temperature as the existing zone's then disconnect any output from this superfluous zone?



Keith Cress
kcress -

http://www.flaminsystems.com

 

[Warpspeed]

Just measure the resistance of the thermistor at some nominal temperature, and fit a fixed value resistor of that resistance.

That will fool the control system into thinking the temperature in that zone is constant.

Then as you have already suggested, adjust the temperature set point to the temperature indicated by the "fake" thermistor.

Usually these systems have a dead band, so the temperature does not continuously hunt. If the "fake" thermistor, and set point temperatures are close, the system should remain in the dead band and try to neither to heat or cool that zone.

 

[itsmoked]

Cept he said thermocouples..

Keith Cress
kcress -

http://www.flaminsystems.com