RTD linearization
in [Design]
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From: Joerg on 12 Dec 2009 20:43 John Larkin wrote: > 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. > 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. -- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
From: Spehro Pefhany on 12 Dec 2009 21:54 On Sat, 12 Dec 2009 17:43:02 -0800, the renowned Joerg <invalid(a)invalid.invalid> wrote: >John Larkin wrote: >> 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. >> > >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. 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? http://focus.ti.com/lit/ds/symlink/tps3813i50-q1.pdf 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 -- "it's the network..." "The Journey is the reward" speff(a)interlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com
From: Nico Coesel on 13 Dec 2009 07:20 John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> 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 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. -- Failure does not prove something is impossible, failure simply indicates you are not using the right tools... "If it doesn't fit, use a bigger hammer!" --------------------------------------------------------------
From: Jan Panteltje on 13 Dec 2009 07:38 On a sunny day (Sun, 13 Dec 2009 12:20:05 GMT) it happened nico(a)puntnl.niks (Nico Coesel) wrote in <4b24da35.319313656(a)news.planet.nl>: >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. And no need for a fancy RTD. A 1 cent Si diode will do, to keep room temp constant.
From: RogerN on 13 Dec 2009 10:50
"John Larkin" <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote in message news:p1u7i5thbjmtjvqcj63b291l19rf7ktllp(a)4ax.com... > > > Does anybody remember the value of negative resistance that linearizes > a 100 ohm platinum RTD? > > John 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 |