Post by thread:[PICLIST] [EE] Thermistor resistance calculation

I'm using an NTC thermistor to detect an overheat condition within my project, the device is specified as; 10k Ohms (+/-2%) at 25 degrees C Beta value 25 to 85 degrees C = 3977K(+/-0.75%) My question is - how do I calculate the resistance of the thermistor for a given temperature (say 75 degrees)? I'm sure I should know this but it's been a long time since College! Thanks in advance. Nigel -- http://www.piclist.com hint: The list server can filter out subtopics (like ads or off topics) for you. See http://www.piclist.com/#topics

At 02:05 PM 1/8/01 -0000, you wrote: >I'm using an NTC thermistor to detect an overheat condition within my project, >the device is specified as; > >10k Ohms (+/-2%) at 25 degrees C >Beta value 25 to 85 degrees C = 3977K(+/-0.75%) > >My question is - how do I calculate the resistance of the thermistor for a >given temperature (say 75 degrees)? > >I'm sure I should know this but it's been a long time since College! I never learned anything as practical as this in University! Here is the basic approximation:- R = R0 * exp(B/T0 - B/T) , where B is assumed to be constant, T0 is the reference temperature for R0. All temperatures are absolute, so add 273 to the temperatures in degrees C, the units of Beta are Kelvins. So, at 75 degrees C, R = 10K / exp(3977/(25+273) - 3977/(75+273)) = 1470 ohms. This is probably not accurate enough for your purposes, so we can use something like the Steinhart and Hart equation:- R = R25 * exp( a + b/T + c/T^2 + d/T^3), the manufacturer should supply the coefficients a..d, or you can curve fit it given a table of values vs. temperature. Best regards, (& sorry about the extra copy, Nigel) =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-= Spehro Pefhany --"it's the network..." "The Journey is the reward" spam_OUTspeffTakeThisOuTinterlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com Contributions invited->The AVR-gcc FAQ is at: http://www.bluecollarlinux.com =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-= -- http://www.piclist.com hint: The list server can filter out subtopics (like ads or off topics) for you. See http://www.piclist.com/#topics