Thermistor Low-Cost Temperature Sensor

One of the most common types of temperature sensors on the market is the thermistor, a shortened version of "thermally sensitive resistor." Thermistors are low-cost sensors that are very rugged and robust. The thermistor is the temperature sensor of choice for applications that require high sensitivity and good accuracy. Thermistors are limited to a small operational temperature range applications due to their non-linear response to temperature.

Construction

Thermistors are two wire components made of sintered metal oxides which are available in several package types to support a variety of applications. The most common thermistor package is a small glass bead with a diameter of 0.5 to 5mm with two wires. Thermistors are also available in surface mountable packages, discs, and embedded in tubular metal probes. The glass bead thermistors are quite rugged and robust, with the most common failure mode being damage to the two lead wires. However, for applications which require a greater degree of ruggedization, the metal tube probe style thermistors provide greater protection.

Benefits

Thermistors have several advantages, including accuracy, sensitivity, stability, quick response time, simple electronics, and low cost. The circuit to interface with a thermistor can be as simple as a pull-up resistor and measuring the voltage across the thermistor.

However, a thermistors response to temperature is very non-linear and they are often tuned to a small temperature range which limits their accuracy to the small window unless linearization circuits or other compensation techniques are used. The non-linear response does make thermistors very sensitive to changes in temperature.

Also, the small size and mass of a thermistor gives them a small thermal mass which allows a thermistor to respond rapidly to a change in temperature.

Behavior

Thermistors are available with either a negative or positive temperature coefficient (NTC or PTC). A thermistor with a negative temperature coeffecient becomes less resistive as the temperature increases while a thermistor with a positive temperature coeffecient increases in resistance as its temperature increases. PTC thermistors are often used in series with components where current surges could cause damage. As resistive components, when current runs through them, thermistors generate heat which causes a change in resistance. Since thermistors either require a current source or voltage source to work, self-heating induced resistance change is an inevitable reality with thermistors. In most cases, self-heating effects are minimal and compensation is only need when high accuracy is required.

Operational Modes

Thermistors are used in two operational modes beyond the typical resistance vs temperature mode of operation. The voltage-vs-current mode uses the thermistor in a self-heating, steady state condition. This mode is often used for flow meters where a change in the flow of a fluid across the thermistor will cause a change in power dissipated by the thermistor, its resistance, and current or voltage depending on how it is driven.

A thermistor can also be operated in current-over-time mode where the thermistor is subjected to a current. The current will cause the thermistor to self-heat, increasing the resistance in the case of an NTC thermistor and protecting a circuit from a high voltage spike. Alternatively a PTC thermistor in the same application can be used to protect from high current surges.

Applications

Thermistors have a wide range of applications, with the most common being direct temperature sensing and surge suppression. The characteristics of NTC and PTC thermistors lend themselves to applications including:

  • Liquid level indicators
  • temperature compensation
  • Flow measurement
  • Vacuum Gages
  • Thermal Protection
  • Ampifier Gain Control
  • Time Delay Circuits
  • Thermal Switches

Linearization

Due to the non-linear response of thermistors, linearization circuits are often required to deliver good accuracy across a range of temperatures. The non-linear resistance response to temperature of a thermistor is given by the Steinhart-Hart equation which provides a good resistance to temperature curve fit. However, the non-linear nature results in poor accuracy in practice unless high resolution analog to digital conversion is used. Implementing a simple hardware linearization of either a parallel, series, or parallel and series resistance with the thermistor drastically improves the linearity of a thermistors response and extends the operational temperature window of the thermistor at a cost of some accuracy. The resistance values used in linearization circuits should be chosen to center the temperature window for maximum effectiveness.