NTC Thermistors: The Basics

Thermistors: An Overview

First, thermistors are semiconductor components made of ceramics that contain a mix of several metal oxides.

Second, thermistors are used in temperature compensation, voltage regulations, circuit protection, time delay and volume control.  These make them highly versatile electronic components.

Third, thermistors are available as small beads and flakes measuring less than 25 micrometers and also as disks, rods and washers.  Large disks and washers register heating capabilities as high as a few watts per degree Celsius.

Fourth,  there are two types of thermistors:  NTC (negative temperature coefficient) and PTC (positive temperature coefficient).  NTC thermistors are mostly used for temperature sensing purposes while PTC thermistors are used to control electric current.

This article will discuss NTC thermistors.

Unlike PTC thermistors whose resistance increases with an increase in temperature, the resistance of NTC thermistors goes down as the temperature increases.

The Steinhart-Hart Equation

This equation is mentioned frequently when thermistors are discussed, especially when a thermistor’s resistance in relation to its temperature is measured.

Two things to remember about thermistors:

  • the equation, at best, is a mathematical approximation.  Additional calculations are required as well as a comparison of the resulting temperature or resistance values with published resistance/temperature data.  These resistance/temperature data are based on actual measurements.  There will be a minor variation in the values arrived at using the Steinhart-Hart equation and the published data, but this variation is typically less than +/- 0.01 °C.
  • while it is a very useful equation in determining the thermistor’s resistance versus its temperature, the correlation obtained with actual measurements assumes ideal measurement conditions.

Commercial and Military Grade NTC Thermistors

Commercial grade NTC thermistors come in two models:  surface mount and leaded.

Surface mount thermistors are used when temperature sensing functions are carried out.  The thermal time constant and dissipation constant will vary depending on the type of mounting.  There are also surface mount thermistors that can measure temperature in a defined and closed area, an example of which is the area containing crystals in oscillator circuits.

Surface mount thermistors are designed to accommodate soldering process and tape and reel packaging.

Commercial grade leaded NTC thermistors, on the other hand, come in a variety of types: you have the interchangeable NTC thermistor, which simply means that they have extremely tight tolerances that can trace a resistance-temperature curve.  Most of these come with insulated leads.  Another type – the bare-leaded thermistors- are suitable for low cost temperature measuring  at  -55°C and  +150°C.

Military grade NTC thermistors are used for critical applications in the fields of temperature measurement, temperature and power control, interfacing of micro-processors and stabilization of amplifiers.  They are particularly useful when hybrid circuitry involves bonding functions.  Resistance and curve combinations are available for customization purposes.