PIC based Pt100 RTD temp sensor circuit
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From: MarkMc on 5 Jul 2005 12:31 Can anybody give me any hints ideas or circuits to allow me to read the temperature from a Pt100 RTD probe in to a PIC microcontroller? Regards, Mark
From: Dan Hollands on 5 Jul 2005 13:23 There are 3 issues 1. convert the change in resistance into an analog signal 2. digitize the analog signal and get the digital value into the microcontroller 3. convert the digitized value into real units Which are you concerned about? 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:1120581097.596235.247300(a)f14g2000cwb.googlegroups.com... > Can anybody give me any hints ideas or circuits to allow me to read the > temperature from a Pt100 RTD probe in to a PIC microcontroller? > > Regards, > Mark >
From: John Popelish on 5 Jul 2005 13:46 MarkMc wrote: > Can anybody give me any hints ideas or circuits to allow me to read the > temperature from a Pt100 RTD probe in to a PIC microcontroller? > > Regards, > Mark > The signal from an RTD is very small, so you need a low offset, low drift amplifier to bring the signal up to a range that is appropriate for the PIC A/D converter. If you are determined to roll your own amplifier, I suggest you visit the various op amp manufacturers' sites and look for application noted for platinum RTDs. For example: http://pdfserv.maxim-ic.com/en/an/AN3450.pdf http://www.national.com/nationaledge/dec04/article.html
From: MarkMc on 6 Jul 2005 07:45 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 £20 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 09:41
MarkMc wrote: (snip) > 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? Except for the transposition of LM, yes. It is a precision 2.5 volt reference regulator. In combination with the opamp, it forces the voltage drop across the grounded 2.5K resistor to be a constant 2.5 volts, so the RTD current is forced to be a constant 1 mA. > 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? Most PICs have an internal 10 bit (1024 level) ADC. You just need to amplify and shift the signal into the ADC measurement range. You may also want to reference the ADC full scale voltage to a precision reference, instead of using the 5 volt supply as the full scale value, or else the accuracy of the PIC supply gets involved in the measurement. > 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. You might use a distilled water ice bath and boiling point as a two point calibration. Otherwise, you buy a high accuracy liquid thermometer and use it in a stirred water or oil bath as a temperature reference. > 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? You can add an arbitrary offset to the resistance measurement by connecting R1 in figure 1 to an adjustable voltage, instead of to ground. This voltage should be a low impedance source (not just a pot hooked up between two voltages) like an opamp follower, in order to not upset the subtracter's ability to measure the voltage across the RTD. The subtracter circuit is A4 and the resistors around it. You can use a pot between the 2.5 volt reference and ground to feed the follower. When you have zero degrees just above ground and 100 degrees just below the ADC positive reference voltage (full scale voltage) you are ready to calibrate and linearize the circuit with stored constants in the program. > Regards, > Mark > |