Basic Thermistor Glossary

Basic Thermistor Glossary

Alpha (): See Temperature coefficient of resistance (alpha,).

Beta ( ß ): See Material constant (Beta, ß).

Dissipation constant (D.C. or delta ): The dissipation constant is the ratio, normally expressed in milliwatts per degree C (mw/°C), at a specified ambient temperature, of a change in power dissipation in a thermistor to the resultant body temperature change.

Heat capacity (Hc): The heat capacity of a thermistor is the amount of heat required to increase the body temperature of it by one degree centigrade (1°C). Heat capacity is a common rating of standard PTC thermistors and is expressed in watt-second per cubic inch per degree C (watt-sec/in³/°C). The heat capacity per unit volume relationship of standard PTC thermistors is approximately 50 watt-sec/in³/°C.

Material constant (Beta , ß ): The material constant of a NTC thermistor is a measure of its resistance at one temperature compared to its resistance at a different temperature. Its value may be calculated by the formula shown below and is expressed in degrees kelvin (°K). The reference temperatures used in this formula for determining material constant ratings of RTI thermistors are 298.15°K and 348.15°K.

ß = ln (R @ T2/R @ T1) / (T2^-1 - T1^-1)

Maximum operating temperature: The maximum operating temperature is the maximum body temperature at which a thermistor will operate for a extended period of time with acceptable stability of its characteristics. This temperature is the result of internal or external heating, or both, and should not exceed the maximum value specified.

Maximum steady-state current (I_max): The maximum steady-state current is the rating of the maximum current, normally expressed in amperes (A), allowable to be conducted by an inrush limiting NTC thermistor for an extended period of time.

Maximum operating voltage (V_max): The maximum operating voltage is the maximum rated voltage, either direct current or 60 Hz RMS alternating current, expressed in volts (VDC or VAC), that a standard PTC thermistor will continuous withstand for an extended period without affecting its normal characteristics.

Negative temperature coefficient (NTC): A NTC thermistor is one whose zero-power resistance decreases with an increase in temperature.

Resistance at maximum current (Rl_max) The resistance at maximum current is the approximate resistance of an inrush current limiting thermistor, expressed in ohms, when it is conducting its rated maximum steady-state current.

Resistance ratio characteristic: The resistance ratio characteristic identifies the ratio of the zero-power resistance of a thermistor measured at one temperature to that resistance measured at a different temperature. The resistance ratio characteristic specified in military specification MIL-T-23648 is the resistance measured at 25°C divided by the resistance measured at 125°C.

Resistance-temperature characteristic: The resistance temperature characteristic is the relationship between the zero-power resistance of a thermistor and its body temperature.

Temperature coefficient of resistance (alpha,): 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. The temperature coefficient is commonly expressed in percent per degree C (%/°C).

Thermal time constant (T.C. or tau,): The thermal time constant is the time required for a thermistor to change 63.2 percent of the total difference between its initial and final body temperature when subjected to a step function change in temperature under zero-power conditions and is normally expressed in seconds.

Zero-power resistance (R_t or R_o): The zero-power resistance is the direct current resistance value of a thermistor measured at a specified temperature, T, with a power dissipation by the thermistor low enough that any further decrease in power will result in not more than 0.1 percent (or 1/10 of the specified measurement tolerance, whichever is smaller) change in resistance.

Consult factory for more detailed Thermistor Terminology.