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BASIC THEORY

Resistance Value of Thermistor
Generally, the resistance value of Thermistor is the no-load resistance value at the standard temperature of 25℃.
The no-load resistance value is the direct current resistance value of Thermistor where there is no electrical heat dissipation at a determined temperature. In order to measure this value, heat dissipation of Thermistor must be limited in order that the change of direct current resistance value becomes within ± 0.01 %, even if heat is produced.
The temperature dependence of Thermistor resistance in no-load state of the indicated temperature can be roughly explained by the following algebraic formula.

NTC Thermistor is a semi-conductor element that has a high negative temperature coefficient. The relationship between resistance and temperature can be roughly explained by algebraic formula (1).

Ingredient constant of Thermistor, constant that expresses the size of change in resistance regarding temperature between a certain two temperatures (T₁, T₀) in resistance - temperature characteristic; increases by small amounts as temperature rises, and there are differences depending on ingredient and temperature range. The fact that B constant is large means that the resistance change regarding temperature is large, and it is generally expressed as the following.

If there are no special references, the B constant is calculated with resistance values of 25 ℃ and 85 ℃ [ B 25/85 ], B constant is the property value of the ingredient able to be obtained from Thermistor, and is determined in accordance with construction or sintering condition of the ingredients.

Resistance temperature coefficient α is the coefficient that expresses in what rate it changes as the resistance value increases by 1 ℃ of temperature from the randomly given temperature( T ), and the unit is % / ℃.
The formulaic expression of the resistance temperature coefficient α can be expressed with the relationship formula (3). Resistance value and temperature within a very limited temperature range can be roughly calculated with the algebraic formula (4) below.

Rated resistance (RN) and B constant influence the tolerance of Thermistor. Due to the tolerance of B constant, at the standard temperature layer, the resistance tolerance of thermistor becomes larger.
Such result is easily inferred from the approximate expression of resistance below. If the resistance tolerance is X and B constant tolerance is Y, the distribution of resistance value according to temperature can be expressed with approximate expression (5).

The heat dissipation constant is the constant that indicates the power required to increase the temperature of Thermistor itself by 1 ℃ by self-heating at thermal equilibrium state. It is calculated by dividing the consumed power of Thermistor by the increase in temperature of the element.
The unit of heat dissipation constant is ㎽/ ℃ and it can be expressed as the following.

If there are no special references, calculate by dividing the amount of power, required when increasing temperature up to 75 ℃ through self-heating from halted standby temperature of 25 ℃ inside the standardized test equipment that has minimum 1000 times or more than volume of the Thermistor, by temperature increase ( △ T ) 50 ℃.

Rated resistance (RN) and B constant influence the tolerance of Thermistor. Due to the tolerance of B constant, at the standard temperature layer, the resistance tolerance of thermistor becomes larger. Such result is easily inferred from the approximate expression of resistance below. If the resistance tolerance is X and B constant tolerance is Y, the distribution of resistance value according to temperature can be expressed with approximate expression (5).

Thermal time constant is the constant that expresses, when the temperature surrounding the Thermistor or the current that flows through the Thermistor rapidly changes, how fast the resistance value changes in response to that change, and the smaller the heat capacity against the surrounding conditions, the shorter the thermal time constant τ. The thermal time constant refers to the time taken for the temperature of the thermistor to change from the initial temperature to 63.2% of the difference between the initial temperature (T₁)) and the final temperature (T₀), and the value can change depending on the medium or storage method of the location of the Thermistor.

Rated resistance (RN) and B constant influence the tolerance of Thermistor. Due to the tolerance of B constant, at the standard temperature layer, the resistance tolerance of thermistor becomes larger. Such result is easily inferred from the approximate expression of resistance below. If the resistance tolerance is X and B constant tolerance is Y, the distribution of resistance value according to temperature can be expressed with approximate expression (5).

The maximum useable temperature the temperature that the characteristic of Thermistor can use with realibility during the period of use. This temperature is the result of internal or external heating or internal/external heating at the same time, and it must not exceed the indicated maximum value. The rated power of Thermistor is the maximum power that the characteristic of Thermistor can use during the period of use with reliability and for long-term.

Peak Voltage is the point at which the voltage has the highest value according to increase of current at Thermistor’s current – voltage characteristic and after that even if the current continues to increase, it does not increase any more and starts to decrease, namely where dE/ dI = 0.

The reliability of Thermistor is the ability of the Thermistor to maintain and hold the given characteristic after environmental or electrical testing.