RTD linearization

No uC at hand for this job? Maybe this helps:

But you don't have to use a Maxim opamp :-)

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I'm thinking I'll use 1K RTDs for the automation project, and lay out an interface board... easier than hand wiring. The little RS232 widget has a 10 bit ADC, so I should use opamps circuits to shict and span up the RTD signals to a nice fat swing for the ADC. One more resistor gives me linearization.

Turns out that about -2750 ohms linearizes a PT100 RTD around room temp. If one excites the RTD with a resistor, and not a current source, the -2750 can be changed to cancel the drive resistor loading, too, like in the Maxim circuit.

Or I could do it in the PC software, I suppose.

John

I think you wrote earlier that you'll just parallel in a 70F thermostat which provides an added layer of safety. If you'd control the furnace directly instead and the SW hangs you could end up with a humongous propane bill.

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Natural gas. But I'm going to put an HC123 one-shot in the relay drive path, so the PC has to keep banging it to keep the heat on.

John

Or use a 4060, then the PC doesn't have to bang that often and it avoids rapid on/off cycling which could cause it to go kablouie.

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I think John intends to hit it often enough that the on/off times will be quite long.

For a one-off, why not use a packaged window watchdog chip?

That will also reject too-fast kicking of the WDT.

Could use a second one (plus a *long* inhibit timer to allow boot-up) to reset the PC if it goes to la-la land (or use an industrial card with a built-in WDT).

Best regards, Spehro Pefhany

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1k RTDs are easier to interface. I used one to control my floor heating. 2k2 (IIRC) in series from 3.3V and then fed directly into an ADC. In a limited temperature range, the output is quite linear so there is not really a need for fancy math.

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On a sunny day (Sun, 13 Dec 2009 12:20:05 GMT) it happened snipped-for-privacy@puntnl.niks (Nico Coesel) wrote in :

And no need for a fancy RTD. A 1 cent Si diode will do, to keep room temp constant.

I thought RTD's were supposed to be linear. The 100 ohm resistance being at

0 Degrees C and a change of .385 ohms (for a 100 Ohm RTD) per Degree C. I read a description of instrumentation for RTD's once. They said they used a 1ma current source to the RTD and compared it to the voltage drop with 1ma in a 100 Ohm resistor. Low current, 1ma, was used to minimize the RTD heating up from power. I know there is a 3rd and sometimes 4th sense wire used to compensate for the lead resistance. I don't exactly remember the source but I read the information when looking up info for my Allen Bradley RTD input card for my PLC 5 rack.

RogerN

Just ran across this looking for the alpha number for RTD's.

RogerN

I'm thinking along these lines...

ftp://jjlarkin.lmi.net/RTD.jpg

All the 1Ks will be 0.1%.

The opamp will have a gain of about 8, input and output centered on

+2.5. This should be accurate to a fraction of a degree C, so we'll know how to dress. The Z-wave home automation systems have a combined motion detector and temperature sensor (no temp-only nodes I know of), but I don't trust their accuracy and they only go down to 40F, not good for outdoors.

I can run RG174 to the RTDs. No EMI/ESD hazards as with semiconductor sensors.

John

Platinum RTDs have a little curvature versus temperature, not bad around room temp. But if you excite them from a power suply and a series resistor, avoiding the current source, the resulting voltage has a bunch of curvature. The combination can be corrected very well, either analog (a positive feedback resistor) or in software, with a simple 2nd order polynomial, one line of Basic.

As noted in another post, a 100R platinum RTD becomes very linear if you parallel it with -2750 ohms.

John

Add a BAV99 to +5V and GND at C1 through C4, and 100ohms or so each from there to the inputs of the 4051. Otherwise, if you have a thunderstorm in Truckee like we had one yesterday (flash, followed by loud bang in a couple hundred milliseconds) your 4051 might go *PHUT*

Ain't that bad. Most semi sensors have a resistive output and you can bypass the dickens out of it.

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DIN-standard Pt100 sensors change 38.5 ohms between 0°C and 100°C, but it's not linear (although the error would be rather modest over a typical 'comfort heating' temperature range*). Using a taste of positive feedback you can make it very close to linear (the residual error is an S-curve). Typically we would use controlled current sources because it's hard to compensate for leadwire resistance in industrial setups otherwise.

The positive feedback could be set to make the reading dead on at a third point (eg. mid-scale) with zero and span getting the first two points, or one could minimize the error according to some cost function.

BTW, there is/was also a US standard of 39.2 ohms change from zero to

100 C, but it's about dead now- pretty much everyone uses the European standard.

Back the last time I worked with Pt RTD instrumentation design (a few weeks ago now) I simply implemented a binary search to invert the Callendar-Van Dusen equations to give T(Rx) (I needed to convert in both directions-- T(Rx) and Rx(T) for that application).

• Set it 'on' at 0C and 50C, and the error at 25C is less than 0.1C.

More of a problem in industrial situations.. set it on at 0C and 400C and the error at 200C is about 6 degees C.

Just as you'd expect for a parabolic error curve, it is proportional to the square of the span, eh?

Best regards, Spehro Pefhany

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On a sunny day (Sun, 13 Dec 2009 09:28:53 -0800) it happened John Larkin wrote in :

For a reasonable long cable to the sensor, a 4 wire system with current source would be better. Else you will have to include the variance of the resistance of the copper, plus the voltage drop in the cable. You will also have to take into account cut or shorted cables, and the sort of signals that wil lgenerate, and how to handle that. I find this a bit primitive.

I don't like this concept very much. Typically we'll run a 100R RTD at < 1mA for a total Pd of < 100uW (much much less in some applications). And usually the RTDs have a lot of added surface area because they're glumped into some kind of protection tube.

You have 1K RTDs running at 2.5mA for a power dissipation of around

6mW at 20C. That alone will result in an error of several degrees C with an un-housed thin film sensor in moving air, more in static air, natch.

Best regards, Spehro Pefhany

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Of course I've considered self-heating.

I was planning to use the largish Minco ceramic-slab parts (we have

1Ks in stock) and stick them in a plastic tube full of epoxy, to weatherize and reduce theta. The self-heat error should be small. I could epoxy them to a small strip of aluminum first, if I were compulsive. And I may as well calibrate the whole thing end-to-end against a good thermocouple.

I do have some scope shots that quantify 1206 surface-mount RTD transient self-heating under different mounting scenarios. I could post them if there were great popular demand.

John

would be better.

Too complex. My engineering budget (design, pcb layout) is one day. I can do the software up in the cabin during the holidays.

RG174 is about 40 milliOhms per foot, and I doubt I'll need to run over 25 feet to the outdoor sensor (I get to use my Makita hammer drill again!) That's about a 0.25C error, which I can mostly correct for anyhow.

signals

Why would my cables get cut or shorted? And it's not as if that would go undetected.

Simple is not primitive, it's good engineering.

Go to a store and look at electronic or mechanical thermometers on display. You may see a 5 degree C spread. I'll be happy with 1 degree true accuracy.

John

The RCs (like R1, C1) ahead of the mux should take care of that. Using grounded coax helps, too.

Accuracy is usually bad, 3C for many parts, and I'd need a 3-wire run. The RTDs are nice.

John