I need to build a thermocouple simulator for the test bench. It will be for type K and will simulate 32 F to 1600 Deg F or there abouts. It is NOT a calibrator, just to be able to simulate some adjustable inputs. I have to build a box with 24 pots in it for 24 channels.
No whiz at all on this so give me hand if I'm getting in trouble. I'm probably doing the hard way. My supply is from a 7805 so I was going to take 5 volts to each pot like this..
5v ---10K----100---- Ground
Between the 10 k resistor and the 100 ohm resistor, I would put a 10 K pot. so that one end of the 10 pot went to ground and my center tap would be an adjustable 0 to 50 millivolt output.
Is there a better arrangement of resistors to use?
Thanks. Richard
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Hey, I'm just now finishing up the firmware for a 16-channel t/c simulator module, J K E T R S B N, rtd reference junction sensors, floating outputs. Pretty serious pita.
Don't forget reference junction compensation. If you were to do...
How do you plan to simulate 32°F if your minimum voltage output is equalt to or greater than 0V? You'll need to go negative by a mV or so depending on your ambient temperature.
That will give a pretty high source impedance (from almost 0 to about
2.5K). Are you sure that will be okay with whatever it is you're going into? Typically thermocouple instruments detect sensor breaks by passing a small current thorough the sensor.
With wirewound pots, there's typically a trade-off between resolution and resistance value.
Best regards, Spehro Pefhany
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I used to calibrate thermocouple potentiometers in a cal lab (late 60's) with a voltage standard such as the Fluke 332. If you have access to a lab voltage standard, all you need is a temperature-voltage table for the thermocouple types you need to simulate. Set the voltage standard to the voltage corresponding to the desired temperature and there you are.
Using your method, I can see a problem immediately. The circuit that you are going to drive with the output of your resistor string will need to have a very high impedance in order to avoid loading problems. A better way, if you insist on using this method, is to follow the resistor string output with a quiet, low-drift op amp that can easily drive the circuit that will measure the voltage. You can find a good assortment of those at Linear, Analog Devices, Maxim, etc. Make sure that you select a model that has offset voltage adjust facilities, or provide another method to adjust the output offset to zero volts for zero volts input. You also would need to use a bipolar power source for the op amp.
A third method would be to use a stable power source into a 15- or 25-turn wirewound pot. Follow the output of the pot with a stable op amp with suitable gain to drive a 3 1/2- or 4 1/2-digit digital panel meter to give you a scaled reading. Use the voltage table to adjust to the voltage you need.
Whichever method you decide to use, you need to pay close attention to the quality of the components. Use low tempco resistors and pots. Use low-drift op amps. Use very stable voltage sources. Otherwise, all the detail you put into your simulator will be wasted by temperature drift and instabilities.
All that said, if you're doing this for your company, I'd suggest that buying a simulator might be cheaper than building one. Omega has a cheap one at
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It doesn't have the accuracy of the higher-priced models, but you didn't state that requirement in your post. Googling for "thermocouple simulator" returned a lot of possibilities.
Cheers!!!
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Thanks for the input, we have several calibration and simulator tools, this is not about precision or dift or anything else.
It's just a rough way for us to hook up a panel and dial some temps in above our shutdowns, and be able to slightly adjust them in order for our panel to stay in run mode without flagging an error.
Pots are what we are using for all the analog inputs and although they are scrathy and noisy, they do fine for the intent.
Thanks, Richard
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Gosh, Fred, I'm starting to lose hope of ever pleasing you.
It has 16 channels, each with an isolated dc/dc supply and isolated data paths. Each has a 16-bit dac with programmable ranges from +-12.5 volts to +-25 mV full-scale. Outputs are protected to +-35 volts forced differentially, +-750 common-mode. There are four 24-bit RTD acquisition channels for external reference junction boxes and one internal temperature sensor. Each channel has a relay that can be programmed to switch it to the D9 cal connector for in-crate calibration check. There are 85 distinct power rails on the board.
Net 65 parts per channel ain't too bad, and I think we can maybe delete about 100 parts from the next rev if we can simplify the dc/dc stuff; we always figured that would be the toughest part, and it was.
Our initial customer will use it for testing and certifying jet engine control computers; two other vendors tried and screwed up one way or another, so they asked us to do this ASAP... nobody can do decent analog stuff any more! I think we have a helicopter developer sold, too, and maybe a couple other aerospace apps like JSF.
Next project will be the complement, a 16-channel isolated analog/thermocouple input board, using the same module mechanics and uP/control stuff.
Sadly, the density of this board doesn't leave room for much "circuit design"... so we have to use ICs wherever we can. More and more, we just connect boxes. We did look into polyfuses and transzorbs, but the polys are very sloppy parts and we couldn't make the numbers work.
We used these...
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which is, actually, an oldish idea using cascaded enhancement mosfets.
nfet pfet in __________________ _____________ __________ out ----- ----- --- --- | | | | v+ v-
which works great until you blow the gates out.
Also, the final pole of the output amp loop is a biggish cap directly across the output. This allow microfarads of external caps to not destabilize the loop, and serves as the gross ESD protector.
Dang, time to take the girls to the opera, "Dr Atomic."
We did propose a couple of different architectures, with my favorites promptly shot down, and settled on this one in a meeting with our prime customer in Hartford. This is what they want to buy, so I can't argue. Their test cells run to high channel counts, so 16 dacs on a board isn't especially inefficient.
The mezzanine board thing sounds good (PMC, IndustryPak, several others) but in real life it winds up with higher cost and lower density than dedicated single boards, and the mez things are usually a programming and packaging nightmare.
The scanned s/h made more sense when 16-bit dacs were very expensive, but they're dirt cheap now, and isolating the s/h thing (flying caps?) is nasty, too. Done that, don't expect to do it again.
I guess you could put the digital part of a delta-sigma dac in an fpga, optoisolate the 1-bit result, and get a floating dac with just a switched reference and a lowpass on the isolated side. If we'd had more time, we might have investigated that, but I think it would be inherently slow, and we want the option to have a fastish version.
How did you achieve that +/-35V differential protection- if it's not proprietary- I like this clever scheme from LT, (they omit the pull-up emitter SD because they can diffuse for high Veb breakdown): View in a fixed-width font such as Courier.
Highest speed development would be to modularize into say 4-channel self-contained boards depending on economics and available parts- then making multiple smaller boards is much easier than one humongous thing- the control part is easily scaled. I have seen schemes where a S/H is refreshed by a central DAC to drive multiple channels like that but
16-bits might be pushing it, at least for you, best to stay out of unknown territory when you're in a rush:-)
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