need EE help for a control circuit
 

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 ?

Maybe a Maxim digital potentiometer (one wire bus)?

Here's how to use Maxim 2 wire stuff over a serial port with labview vis:

http://korea.maxim-ic.com/appnotes.c...te_number/1190

Not sure if the 1845 or 1847 digital potentiometer would work for you but if so this should be cheaper than $400

what can I say

procooling.com is truly a super resource, thanks guys

- and why I support this site's activities (well, ph's testing anyway)

Well, at least phaestus is pretty good resource anyways!

plenty of other savvy members have also helped over time
the thanks was general

I get back many times what I put in here as far as electronics help goes. I happened to be looking at Maxim/Labview stuff today with an eye towards integrating my diode reader into the new test system so I had that link handy.

One wire stuff is TOO cool; you can put together a little weather station without a lot of effort or cost and with good accuracy.

Just interested in where this goes so I'll add three related links as an excuse to subscribe to the thread.

maxim

Mouser

An interesting article I remembered reading and bookmarking worth reading for those who can use it. :eek: amazed that I still had it.

beep, beep, beep, beep
information overload

jeez, good article - but now I know that I know even less than before
back to the books

Good to see I could do that to you, since you've done it to me alot more than once.

Nice to see I'm not the only one reading ED. :D

ok, the difficulty with digital pots - if I am understanding the specs:

the smallest is 10k, I'm using a 5k
the number of (programmable ?) steps is 256, yielding a resolution of 39 ohms with a 10k pot

as I presently make adjustments of several ohms, I think this may not work
even running 2 in parallel I'm still at 19 ohms

am I missing the obvious ?

Try Analog:
http://www.analog.com/IST/SelectionT...n_table_id=214

the real question is, what interface?

http://www.analog.com/IST/SelectionT...n_table_id=214

1kohm/256 steps
5kohm/256 steps
25kohm/1024 steps

will I2C or SPI interface with labview?

<edit> bigben beat me to it, and out of curiosity, are any of these compatible with the SM-Bus/SpeedFan?
<edit again> I2C and SM-BUS appear to be the same thing, would SpeedFan recgonize one of these, out of curiosity? If not, what chip can I use as a digital pot?

for billa, it looks like the 1k/256 or 5k/256 would work best, using the maxim translator chip to go from RS-232 to I2C 2-wire serial.


Everyone, correct me if wrong

I don't know enough about SpeedFan to advise you, but if it's on the bus, it's accessible.

SpeedFan's site:
http://www.almico.com/speedfan.php

Knew this was going to be a good thread

I appears that Analog provides rudamentary drivers with it's design tools, so either I2C or SPI should be usable.

Most likely through the LPT port

Make friends with DLPortIO then :)

Hmm, good link!

doesn't have to be speedfan, but any program that will auto-adjust fanspeed to temperatures...like increasing speed as processor load increases.

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

R3 is your fixed point. The image is blurred, so I'm guessing here: 1.27K with 1% tolerance.

The pot (R1) above is showing as a 500 ohm variable resistor. Your "factory adjustment"?

R2 would be the pot (multiturn) that you "relocated". It reads 5k.

What's the highest temp you can set this to, and what's your target resolution (obviously you've already done the math).

Yes, it is possible to drive 2 * AD5231.
http://www.analog.com/UploadedFiles/...66174AN582.PDF


Where's our EE when we need him anyways?

To me it looks like:

R1 - factory adjustment (to adjust for actuall chamber temp)

R4, R6 - 22 deg set point fixed (the two fixed resistors set the voltage drop)

R2, R5 - the switch position selects these two for the voltage drop setting the chamber temp

Have to go to work very soon. Will look further into this. I suspect that by changing R5 we can find a simple solution ie a single digital pot. Just a matter of keeping the ratios correct.

Bill, do you know what the 5k pot you are using is set at? and the range that you normally adj?

I don't quite understand the problem. When you use your pot, you ARE changing the set point. Does the thermostat not hold the chamber at that temp? Or is it a matter reproducibly setting various chamber temperatures?

More info on the chamber would be good. What kind of cooling does it have? Pelt? Phase-change? If phase, is the compresser variable speed or does it cycle on/off? What are the connections to the terminals on your schematic?

Anyway, if you really want to control it from LabView, I'd bypass their entire temperature comparator section and feed a logic signal from your virtual PID into optoisolator U4. Maybe even scrap their board entirely.

I concur with Murray13 on the resistor uses.
It would be useful to change the setpoint using your 5k pot, measure the value on the pot and then the temp setpoint in the chamber. Since we know that 22 C is R6/(R4+R6), this would give you a second point to establish the linear reltionship on the resistances. Then you could change R2 and R5 to give you the precision and range needed with the digital pot.

I'll be concluding a run in an hour, so I will disassemble the box and take some measurements

Groth
The unit is a blood platelet incubator with a compressor running flat out. The evaporator is in a passage that is the back of the chamber, and above it is about 4' of coiled nichrome wire which is the heater used to moderate the cool air to the desired temp. A fan at the top maintains a constant forced circulation, top to bottom. As an environmental chamber its good to about a 400W load.

no, for me (my skills) its easier to figure out how to 'feed' their circuit, its control is excellent (I can hold ±0.05°C manually, a LabVIEW PID should improve that)