How to Select an NTC Thermistor

Negative Temperature Coefficient (NTC) thermistors are made of sintered metal oxide. They display large decreases in resistance in proportion to small increases in temperature.

Their resistance is calculated by passing a small and measured direct current (DC) through the thermistor and measuring the voltage drop produced.

 Applications

  • Temperature Measurement
  • Temperature Compensation
  • Temperature Control

5 Essential Considerations When Selecting an NTC Thermistor

  • Temperature Range
    • When choosing a temperature sensor, the first consideration should be the temperature range of the application.
    • Since NTC thermistors perform well in an operating range between -50°C and 250°C, they are well suited for a wide range of applications in many different industries.
  • Accuracy
    • Of the basic sensor types, an NTC thermistor’s ability to achieve the highest accuracy is within the -50°C to 150°C range, and up to 250°C for glass encapsulated thermistors.
    • Accuracy ranges from 0.05°C to 1.00°C.
  • Stability
    • Stability is important in applications where long-term operation is the goal. Temperature sensors can drift over time, depending upon their materials, construction, and packaging.
    • An epoxy-coated NTC thermistor can change by 0.2°C per year while a hermetically sealed one changes by only 0.02°C per year.
  • Packaging
    • Packaging requirements are dictated by the environment the sensor will be used in.
    • NTC Thermistors can be customized and potted into various housings dependent on application requirements. They can also be epoxy coated or glass encapsulated for further protection.
  • Noise Immunity
    • NTC Thermistors offer excellent immunity to electrical noise and lead resistance.

More Considerations

  • NTC thermistors have specific electrical properties:
    • Current-time characteristic
    • Voltage-current characteristic
    • Resistance-temperature characteristic
  • Type and Size of Product
    • The thermistor user will usually know what is needed in terms of size, thermal response, time response, and other physical features that go into the configuration of the thermistor. It should be easy to narrow down the choice of NTC thermistors even when data is lacking, but a careful analysis of the intended application of the thermistor must be made.
  • Resistance-Temperature Curves
    • Ametherm data sheets contain a table or matrix of resistance ratios versus temperature for each of their NTC thermistor products. Coefficients α and β are also provided for particular equations to help the user or designer to translate resistance tolerance in terms of accuracy in temperature, as well as calculate the temperature coefficient for each curve.
    • There is quite a wide range of materials that can be used to manufacture thermistors, but there are limitations involved depending on the size, operating and storing temperature range and nominal resistance values.
  • Nominal Resistance Value
    • The next factor to consider is whether the application needs to be curve matched or point matched. This will allow for the calculation of the needed nominal resistance value at a given temperature.
      Ametherm offers an entire range of nominal resistance values for their NTC thermistors. The standard reference temperature is 25°C, but buyers and designers can request different temperatures.
    • A word of caution: if the desired resistance is not available in the combination of product type and material component, then a decision must be made as to which characteristic takes priority: product type/size, material preference or resistance ratio.
  • Resistance Tolerance
    • When looking at product specification sheets, Ametherm provides standard tolerances. For example, disc or chip thermistors usually have a zero-power resistance distribution of ± 1% to ± 20%.
    • To save on costs, Ametherm recommends specifications of the broadest possible tolerance that is relevant to intended use.

Types of NTC Thermistors

  • Disc and Chip: They come configured with or without coating with bare or tinned copper leads. There are thermistors for a broad span of resistance values to suit
    every situation.
  • Epoxy: Epoxy dip coated and soldered between jacketed Teflon/PVC wires. Their small dimensions allow for easy installation, and they can be point or curve matched.
  • Glass-Encapsulated: An excellent choice when dealing with extreme environmental conditions and when stability is of the utmost importance. Configurations include
    radial leaded or axial leaded thermistors.
  • Probe Assemblies: Available in a variety of
    housings depending on application requirements.
  • Surface Mount: Configuration options include Bulk, Tape & Reel, Two-Sided, and Wrap-Around with Palladium Silver Terminations. Made with Nickel Barrier, these thermistors work great in precision circuits.

 Calculations You May Need

  • α – constant (%/°C)
    The temperature coefficient of resistance is the ratio at a specified temperature, T, of the rate of change of zero-power resistance with temperature to the zero-power resistance of the thermistor.
  • β – constant (°K)
    The material constant of an NTC thermistor is a measure of its resistance at one temperature compared to its resistance at a different temperature. The reference temperatures used in the following formula for Ametherm’s thermistors are 298.15°K and 348.15°K.

You can calculate the resistance of NTC Thermistors at a given temperature using beta as shown above, but there is an even more accurate way to do this using the Steinhart & Hart Equation.

You can measure a temperature range of an NTC Thermistor with a Wheatstone Bridge.

Additional Resources

  • What is an NTC Thermistor
    Summary: Explains what an NTC thermistor is and its capabilities as a temperature sensor. Ametherm’s NTC thermistors and probes are described, as well as the terminology used.
  • NTC Thermistors – Temperature Measurement With Wheatstone Bridge
    Summary: The Wheatstone Bridge is one of the easiest ways to measure temperature and explains how it is calculated using a specific example with certain variables. A chart of temperature versus volts is also provided.
  • NTC Thermistors – Calculate Beta Value For NTC Thermistors
    Summary: Explains why the beta value, although often used, is not as accurate as using the Steinhart and Hart equation. The Steinhart and Hart equation uses three temperatures over a given range.
  • NTC Thermistors – Steinhart and Hart Equation
    Summary: This equation is arguably the best to use when determining the resistance temperature relationship of NTC thermistors and NTC probe assemblies, given that the equation uses three temperatures. This article tells you which equation to use in your particular application.