Thermistors

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The Software is the Instrument

NATIONAL INSTRUMENTS

Application Note 065

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Measuring Temperature withThermistors – a Tutorial
David Potter

Introduction
Thermistors are thermally sensitive resistors used in a variety of applications, including temperature measurement. This application note discusses this application of thermistors, including basic theory and how to interface thermistors to modern data acquisition systems.

Thermistor Overview
A thermistor is a piece of semiconductor made from metal oxides, pressed into a small bead, disk, wafer, or other shape, sintered at high temperatures, and finally coated with epoxy or glass. The resulting device exhibits an electrical resistance that varies with temperature. There are two types of thermistors – negative temperature coefficient (NTC) thermistors, whose resistance decreases with increasing temperature, and positive temperature coefficient (PTC) thermistors, whose resistance increases with increasing temperature. NTC thermistors are much more commonly used than PTC thermistors, especially for temperature measurement applications. A main advantage of thermistors for temperature measurement is their extremely high sensitivity. For example, a 2252 Ω thermistor has a sensitivity of -100 Ω/°C at room temperature. Higher resistance thermistors can exhibit temperature coefficients of -10 kΩ/°C or more. In comparison, a 100 Ω platinum RTD has a sensitivity of only 0.4 Ω/°C. The physically small size of the thermistor bead also yields a very fast response to temperature changes. Another advantage of the thermistor is its relatively high resistance. Thermistors are available with base resistances (at 25° C) ranging from hundreds to millions of ohms. This high resistance diminishes the effect of inherent resistances in the lead wires, which can cause significant errors with low resistance devices such as RTDs. For example, while RTD measurements typically require 3-wire or 4-wire connections to reduce errors caused by lead wire resistances, 2-wire connections to thermistors are usually adequate. The major tradeoff for the high resistance and sensitivity of the thermistor is its highly nonlinear output and relatively limited operating range. Depending on the type of thermistors, upper ranges are typically limited to around 300° C. Figure 1 shows the resistance-temperature curve for a 2252 Ω thermistor. The curve of a 100 Ω RTD is also shown for comparison.

_____________________________ Product and company names are trademarks or trade names of their respective companies. 340904B-01 © Copyright 1996 National Instruments Corporation. All rights reserved. November 1996

10 M 1M Resistance (Ω) 100 k 10 k 1k 100 10 -200 -150 -100 -50 0 50 100 150 200 250 300 350 400 Thermistor (2252 Ω at 25 ˚C)

RTD (PT 100 Ω)

Temperature (˚C)

Figure 1. Resistance-Temperature Curve of a Thermistor

The thermistor has been used primarily for high-resolution measurements over limited temperature ranges. The classic example of this type of application is medical thermometry. However, continuing improvements in thermistor stability, accuracy, and interchangeability have prompted increased usage of thermistors in all types of industries.

Resistance/Temperature Characteristic of Thermistors
The resistance-temperature behavior of thermistors is highly dependent upon the manufacturing process. Therefore, thermistor manufacturers have not standardized thermistor curves to the extent that thermocouple or RTD curves have been standardized. Typically, thermistor manufacturers supply the resistance-versustemperature curves or tables for their particular devices. The thermistor curve, however, can be approximated relatively accurately with the Steinhart-Hart equation : 1 a0 + a1 ln( R T ) + a2 ln( R T )

T(°K ) =

[

]3

Where T(°K) is the temperature in degrees Kelvin, equal to T(°C) + 273.15, and RT is the resistance of the thermistor. The coefficients a0, a1, and...
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