From cc5dfb1ddd35f8e59151877808e6762041c0430b Mon Sep 17 00:00:00 2001 From: "Mark D. Studebaker" Date: Wed, 17 Mar 1999 03:51:03 +0000 Subject: [PATCH] overview of math with thermistors and diodes. added link to the NTC Thermistor guide to the useful addresses. git-svn-id: http://lm-sensors.org/svn/lm-sensors/trunk@317 7894878c-1315-0410-8ee3-d5d059ff63e0 --- doc/temperature-sensors | 69 +++++++++++++++++++++++++++++++++++++++ doc/useful_addresses.html | 4 +++ 2 files changed, 73 insertions(+) create mode 100644 doc/temperature-sensors diff --git a/doc/temperature-sensors b/doc/temperature-sensors new file mode 100644 index 00000000..709a05cc --- /dev/null +++ b/doc/temperature-sensors @@ -0,0 +1,69 @@ +Termistor and Transistors as Temperature Sensors +------------------------------------------------ +Temperature measuring chips that use external sensors +generally are designed to use either thermistors or +transistors. + +Exceptions are the Winbond W83782D and W83783S which claim +to be able to use both, however it isn't clear how +to configure these to use transistors. + + + +Thermistors +----------- + +(summarized from http://www.thermometrics.com/htmldocs/ntcapp.htm - + most thermistor companies don't have much on their web sites but + Thermometrics has a comprehensive guide) + +NTC (negative temperature coefficient) thermistors +have a parameter B (beta), units are K (kelvin). +B is the "material constant" of the thermistor and +is a measure of the change of resistance with respect to temperature. +It is the slope of the line graphing (ln R) vs. (1/T). + +B values for three common termistor materials are +approximately 3400K, 3900K, and 4300K. + +The other measure of thermistors is the temperature coefficient 'a' (alpha). +This is the percentage change in resistance for a given change +in temperature. This varies with temperature. + +T = temp in Kelvin (C + 273) + +A = (dR/dT) / R + +A = - B / T**2 + +So by the last equation, a thermistor with a Beta of 3900 will change +resistance about 4.4% for 1 degree change at 25C (298K). +For a beta of 3423 as recommended in the 782D data sheet the change is 3.8%. + +For thermistors, resistance (= voltage for a constant current) +is exponentially related to temperature. + + + +Transistors +-------------- +To use a transistor as a sensor, hook the base of an NPN transistor +(such as the 3904) to the collector. + +This is from Richard Dorf, "Electrical Engineering Handbook". + +For a diode, the voltage Vd = Vt ln(Id/Is). +Vt = kT/q where k = Boltzman's constant 1.38 e-23; + q = elementary charge 1.6 e-19; + T = temp in kelvin. + Is is reverse saturation current. + +(more math and greek letters I don't understand completely) + +dVd/dT works out to, at room temperature, about 2.1 mV/degree +or a temperature coefficient of about 0.32%/degree. + +For diodes, voltage is linear with temperature. + + + diff --git a/doc/useful_addresses.html b/doc/useful_addresses.html index 9a7a87bf..f910c947 100644 --- a/doc/useful_addresses.html +++ b/doc/useful_addresses.html @@ -93,6 +93,10 @@ Make considered Harmful
  • Building a temperature sensor using a thermistor An article on the 3D Hardware site. +
  • +NTC Thermistor Guide +
    A comprehensive description of the physics and mathematics +of NTC Thermistors