Alpha (α)

Temperature coefficient of resistance (alpha, α).

Beta (ß)

Material constant (Beta, ß).

Curie point

Switch temperature (Ts).

Current-time characteristic

The current-time characteristic is the relationship at a specified ambient temperature between the current through a thermistor and time, upon the application or interruption of voltage to it.


Dissipation constant (D.C. or delta, δ).

Delta (δ)

Dissipation constant (D.C.) or delta, δ).

Dissipation constant (D.C. or delta δ)

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.


Heat capacity (Hc)


Maximum continuous current (Icc).


Maximum steady-state current (Imax).

Inrush current

Inrush current is the initial surge of current that results when power is first applied to a load having a low starting impedance, such as a discharged capacitor, a cold lamp filament, or a stopped motor’s winding.

Inrush current limiter

Specially designed and constructed NTC thermistors may be used as inrush current limiters. RTI Surge-GardT inrush current limiters are available in a wide range of current handling and zero-power resistance value combinations.


Minimum switching current (Is).

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.

b = ln (R @ T2/R @ T1) / (T2-1 – T1-1)

Maximum continuous current (Icc)

The maximum continuous current is the amount of current, normally expressed in amperes (A), that a standard PTC thermistor must be capable of conducting without switching into its high resistance state.

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 power rating

The maximum power rating of a thermistor is the maximum power, expressed in watts or milliwatts (W or mW), which a thermistor will dissipate for an extended period of time with acceptable stability of its characteristics.

Maximum steady-state current (Imax)

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 surge current

The maximum surge current is the maximum permissible surge current in a circuit and, in conjunction with the maximum peak voltage, determines the minimum required zero-power resistance of the Surge-GardT thermistor required to limit it adequately. See inrush current.

Maximum operating voltage (Vmax)

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.


MIL-T-23648 is the U.S. military’s general specification for thermistors.

Minimum switching current (Is)

The minimum switching current is the minimum amount of current, normally expressed in amperes (A), that, when conducted by a standard PTC thermistor, is required to cause it to switch to its high resistance state.


Mini-SensorT is the trademark name for RTI miniature glass encapsulated thermistors.

Negative temperature coefficient (NTC)

A NTC thermistor is one whose zero-power resistance decreases with an increase in temperature.


Negative temperature coefficient (NTC).

Positive temperature coefficient (PTC)

A PTC thermistor is one whose zero-power resistance increases with an increase in temperature.


Positive temperature coefficient (PTC) Maximum continuous current (Icc)

Recovery time

The recovery time of a thermistor is the approximate time required for it to cool sufficiently after power is removed and allow it to provide the characteristics required when power is reapplied.

Resistance at maximum current (Rlmax)

The resistance at maximum current is the approximate resistance of an inrush current limiting thermistor, expressed in ohms (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.


Resistance at maximum current (Rlmax).


Zero-power resistance (Rt or Ro).


Zero-power resistance (Rt or Ro).

Silicon PTC thermistor

A silicon PTC thermistor is a type PTC thermistor that has an approximately linear resistance-temperature characteristic and a temperature coefficient of resistance of approximately +0.7%/°C. Silicon PTC thermistors are distinguished from standard PTC thermistors.


The stability of a thermistor is the ability of it to retain specified characteristics after being subjected to designated environmental or electrical test conditions.

Standard PTC thermistor

A standard PTC thermistor is a type of PTC thermistor that has a switch temperature. Standard PTC thermistors are distinguished from silicon PTC thermistors.

Standard reference temperature

The standard reference temperature is the thermistor body temperature at which nominal zero-power resistance is specified and is usually 25°C.

Static voltage-current curve

The static voltage-current (V/l) curve defines the relationship between voltage and current at any point of equilibrium for a standard PTC thermistor.


Surge-Gard™ is the trademark name for RTI inrush current limiting thermistors.

Switch temperature (Ts)

The switch temperature is the temperature of a standard PTC thermistor at which its resistance begins to increase very rapidly. The typical specification for RTI standard PTC thermistors rates their resistance at their switch temperature as two times their zero-power resistance at 25°C. Switch temperature is sometimes also identified as transition temperature or Curie point.

Tau (τ)

Thermal time constant (T.C. or tau,τ).


Thermal time constant (T.C. or tau, τ).

Temperature coefficient of resistance (alpha, α)

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).

aT = (dRT)/(dT)


Switch temperature (Ts).

Temperature-wattage characteristic

The temperature-wattage characteristic of a thermistor is the relationship at a specified ambient temperature between the thermistor temperature and the applied steady-state wattage.

Thermal time constant (T.C. or tau, τ)

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.


A thermistor is a thermally sensitive resistor whose primary function is to exhibit a change in electrical resistance with a change in body temperature.

Transition temperature

Switch temperature (Ts).


Maximum operating voltage (Vmax).

Zero-power resistance (Rt or Ro)

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.