PIC based Pt100 RTD temp sensor circuit
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From: MarkMc on 6 Jul 2005 09:38 I'll have to digest some of this to understand it fully, but one thing which I'm not too sure how to do is the two point calibration. IIRC the MAX197 app notes suggest calibrating with a highly accurate 100R and 300R resistors. Say I do this and note the voltage or binary value from the ADC, what do I then do with these two values? I see how I can apply a correction factor, but this doesn't take both calibration points in to consideration. Cheers, Mark
From: Dan Hollands on 6 Jul 2005 11:08 A design to do what you want is a big challange to one of your experience. You will probably spend considerable money and a great deal of time and still not achieve your goal There are a multitude of RTD digital panel meters that do what you want available in the UK Just buy one - it will work and probably be cheaper than you can do it yourself Search internet for RTD temperature panel meter UK Dan -- Dan Hollands 1120 S Creek Dr Webster NY 14580 585-872-2606 QuickScore(a)USSailing.net www.QuickScoreRace.com "MarkMc" <mmcnospam(a)yahoo.co.uk> wrote in message news:1120650307.753396.12880(a)g43g2000cwa.googlegroups.com... I had a read of the MAXIM application notes, which use the MAX197 IC and a couple of op amps for amplification. This seems to be perfect, but the only problem is that it costs ý20 for a single 28-pin DIN MAX197, which makes it a bit unviable for me. It is a one-off personal project, but others on my brewing forum may well want to build units themselves. I think I understand what's required at a high level, but my electronics isn't up to it at the low-level. I need a range of 0C to 100C and 0.5C accuracy/resolution for my application. High level things I think I need (high-level block diagram) 4-wire PT100 RTD "Something" to generate the excitation current - I think this needs to be extremely accurate and invariable. Something to amplify the small voltage across the RTD in to the range of an ADC. Looks like NS ML4140A-2.500 is used for this in the links above? An ADC to convert the analogue signal in to a format for my PIC to use. Are there not PIC's which have on-board ADC's that I can use for this purpose or is this not a good idea? I think I know how to convert from the digital voltage in to a temperature - divide the known voltage range up in to chunks based on the bit resolution of the ADC. Say - 0-5v ADC and 10-bit, I would then 5/((2^10)-1) to get the voltage increment per bit. Then map the 'voltage' reading back to resistance (known excitation current), and use a look-up table to 'linearise' and then interpolate to give a reasonable approximation of temperature. One thing I'm not sure about is how to calibrate this process. Another is that IIRC 0C is 100R and 100C is 138.5R (for Platinum PT100) which @ 1mA gives a voltage reading range of 100mV-138.5mV. I'm sure I can scale this up so that the 100C resistance maps somewhere near 5 volts, but the 0C will not be near 0v. so some of the 10-bit signal is wasted. Could I not get better accuracy (perhaps it's not needed with 10-bits) by translating the 0C voltage/resistance somewhere near 0v and then scale/amplify so that 100C is near 5v so I'm using a larger portion of the bit range? Regards, Mark
From: John Popelish on 6 Jul 2005 11:48 MarkMc wrote: > I'll have to digest some of this to understand it fully, but one thing > which I'm not too sure how to do is the two point calibration. > > IIRC the MAX197 app notes suggest calibrating with a highly accurate > 100R and 300R resistors. Say I do this and note the voltage or binary > value from the ADC, what do I then do with these two values? I see how > I can apply a correction factor, but this doesn't take both calibration > points in to consideration. The two point calibration I was referring to is just interpolating 0 to 100 degrees, linearly between the two A/D values you get with an ice bath and a boiling bath, using your RTD and amplifier. The linearity errors over that small temperature range are not very big. If you want to correct them, you need a table or polynomial fit to the RTD curve, and fit that to your zero and 100 degree points.
From: MarkMc on 6 Jul 2005 12:33 Had a look, and they seem to be about £200. Not what I'd call cheap for hobby use. What would be so expensive to make? I have the PIC microcontrollers and programming hardware + software etc. The way I see it I just need to find a good OpAmp and ADC? Half the fun is the journey with this hobby I think. Yep, right now, I don't know too much, but I think I'm getting somewhere - doesn't seem too scarey. Regards Mark
From: MarkMc on 6 Jul 2005 12:34
Ah, I see. The calibration I had in mind was to pull things in to line with the look-up tables. Cheers, Mark |