Temperature Measurement with NTC Thermistors

Thermistors (thermal resistors) are temperature dependent variable resistors. There are two types of thermistors, Positive Temperature Coefficient (PTC) and Negative Temperature Coefficient (NTC). When the temperature increases, PTC thermistor resistance will increase and NTC thermistor resistance will decrease. They exhibit the opposite response when the temperature decreases.
Both types of thermistors are used in a variety of application areas. However, here the focus will be on using NTC thermistors to measure temperature in microcontroller based applications. Thermistor Specifications
The following NTC thermistor parameters can be found in the manufacturer’s data sheet.

    • Resistance
      This is the thermistor resistance at the temperature specified by the manufacturer, often 25°C.


    • Tolerance
      Indicates how much the resistance can vary from the specified value. Usually expressed in percent (e.g. 1%, 10%, etc). For example, if the specified resistance at 25°C for a thermistor with 10% tolerance is 10,000 ohms then the measured resistance at that temperature can range from 9,000 ohms to 11000 ohms.


    • B (or Beta) constant
      A value that represents the relationship between the resistance and temperature over a specified temperature range. For example, “3380 25/50″ indicates a beta constant of 3380 over a temperature range from 25°C to 50°C.


    • Tolerance on Beta constants
      Beta constant tolerance in percent.

    • Operating Temperature Range
      Minimum and maximum thermistor operating temperature.


    • Thermal Time Constant
      When the temperature changes, the time it takes to reach 63% of the difference between the old and new temperatures.


    • Thermal Dissipation Constant
      Thermistors are subject to self-heating as they pass current. This is the amount of power required to raise the thermistor temperature by 1°C. It is specified in milli watts per degree centigrade (mW/°C). Normally, power dissipation should be kept low to prevent self-heating.


    • Maximum Allowable Power
      Maximum power dissipation. It is specified in Watts (W). Exceeding this specification will cause damage to the thermistor.


  • Resistance Temperature Table
    Table of resistance values and associated temperatures over the thermistors operating temperature range. Thermistors operate over a relatively limited temperature range, typically -50 to 300°C depending on type of construction and coating.

Thermistor Response to Temperature
As with any resistor, you can use the ohmmeter setting on your multi meter to measure thermistor resistance. The resistance value displayed on your multi meter should correspond to the ambient temperature near the thermistor. The resistance will change in response to temperature change.

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