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Originally Posted by unregistered
I have an environmental chamber which has an RTD thermostat which will maintain the temp at 22°C
- with a toggle switch I can select 'manual control', and then by means of a pot set the temp manually
- I substituted a 10 turn pot and relocated it, works fine
the problem:
I would like to be able to manually change the set point to something other than 22°, and then have the thermostat maintain that temp
2 apparent means to do this:
1) modify the control circuit to be able to externally change the (now fixed) set point, or
2) using a PID controller set up in LabVIEW, read the temp and provide a (variable) resistance output in place of the present pot
1) am talking to Forma Scientific now, but this is not how the chamber was designed to be used
- and I have only an interconnect wiring diagram, not the pc board schematic
2) what do I use to obtain a variable resistance output ?
(a Valhalla GPIB controlled resistance unit went for over $400 on eBay - and that is the ONLY such I've seen)
any ideas ?
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well, I'm making progress - s l o w l y
re #1, now have the board schematic, and have been told the 22°C fixed setpoint is by means of a fixed resistor - but the specific resistor was not identified (I don't think the tech knew, and was not sympathetic given my use of the incubator)
the board schematic is
here if someone cares to look (while I was told it was a fixed resistor, I suspect that R1 is the factory adjustment - make any sense ?)
re #2, a bit snowed under by a bunch of stuff that I know nothing of; i.e. digital pots
but I (think I) understand that I can find a 5k digital pot (though I have not yet seen such), but I have a problem with only 256 steps
- I've seen reference to 1024 steps, but no 5k pot having such
I suspect such a pot would work, closest I've found is a AD5231, 10k, 1024 position
- can these be 'stepped' in parallel ?
for reference, my voltage controllers on a 0-10V or 0-1V scale have a resolution of (binary) 2^12 = 4096 steps