RTD linearization
in [Design]
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From: John Larkin on 12 Dec 2009 15:09 Does anybody remember the value of negative resistance that linearizes a 100 ohm platinum RTD? John
From: Joerg on 12 Dec 2009 15:46 John Larkin wrote: > > Does anybody remember the value of negative resistance that linearizes > a 100 ohm platinum RTD? > No uC at hand for this job? Maybe this helps: http://pdfserv.maxim-ic.com/en/an/AN3450.pdf But you don't have to use a Maxim opamp :-) -- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
From: John Larkin on 12 Dec 2009 17:40 On Sat, 12 Dec 2009 12:46:37 -0800, Joerg <invalid(a)invalid.invalid> wrote: >John Larkin wrote: >> >> Does anybody remember the value of negative resistance that linearizes >> a 100 ohm platinum RTD? >> > >No uC at hand for this job? Maybe this helps: > >http://pdfserv.maxim-ic.com/en/an/AN3450.pdf > >But you don't have to use a Maxim opamp :-) 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
From: Joerg on 12 Dec 2009 18:47 John Larkin wrote: > On Sat, 12 Dec 2009 12:46:37 -0800, Joerg <invalid(a)invalid.invalid> > wrote: > >> John Larkin wrote: >>> Does anybody remember the value of negative resistance that linearizes >>> a 100 ohm platinum RTD? >>> >> No uC at hand for this job? Maybe this helps: >> >> http://pdfserv.maxim-ic.com/en/an/AN3450.pdf >> >> But you don't have to use a Maxim opamp :-) > > 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. > 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. -- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
From: John Larkin on 12 Dec 2009 19:27
On Sat, 12 Dec 2009 15:47:57 -0800, Joerg <invalid(a)invalid.invalid> wrote: >John Larkin wrote: >> On Sat, 12 Dec 2009 12:46:37 -0800, Joerg <invalid(a)invalid.invalid> >> wrote: >> >>> John Larkin wrote: >>>> Does anybody remember the value of negative resistance that linearizes >>>> a 100 ohm platinum RTD? >>>> >>> No uC at hand for this job? Maybe this helps: >>> >>> http://pdfserv.maxim-ic.com/en/an/AN3450.pdf >>> >>> But you don't have to use a Maxim opamp :-) >> >> 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. >> > >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. 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 |