Thermistor

Temperature characteristic

The relationship between resistance value and temperature over a given temperature range, which is approximated by Equation 1.
T, Ta: Absolute temperature (K)
R, Ra: Zero load resistance value at T, Ta (Ω)
B: Constant B (K)

Temperature characteristic

Zero load resistance value

A thermistor measures resistance value at a constant temperature, and the resistance value when measured at a sufficiently low power consumption where the change in resistance value due to self-heating can be ignored is called the zero load resistance value.

Constant B

It is a constant that expresses the magnitude of change in resistance value obtained from the temperatures of any two points in the resistance-temperature characteristic, and is expressed by Equation 2.

When this characteristic is graphed with IogR and 1/T, it can be expressed almost linearly.

Constant B

Temperature coefficient of resistance

A coefficient expressing the rate of change of zero load resistance value per 1℃ at a given temperature, expressed in Equation 3.

α: Temperature coefficient of resistance (%/K)
T: Any absolute temperature (K)
R: Zero load resistance value at T(K) (Ω)
B: Constant B (K)

Temperature coefficient of resistance

Heat dissipation constant

A constant that represents the power required to raise the temperature of a thermistor element by 1℃ in thermal equilibrium through self-heating. It is obtained by the ratio of the power consumption of the thermistor to the temperature rise of the element.

If the power consumption of the thermistor is P(mW),
P = δ(Tb-Ta), then
δ= P/(Tb-Ta) = I2R/(Tb-Ta)
P: Thermistor power consumption (mW)
δ: Heat dissipation constant (mW/℃)
Ta: Ambient temperature of thermistor (℃)
I: Current flowing through thermistor (mA)
Tb: Temperature of the thermistor when the thermistor rises in temperature and reaches thermal equilibrium (℃)
R: Thermistor resistance value at Tb(℃) (Ω)

Thermal time constant

This constant expresses the time required for the temperature of the thermistor element to change by 63.2% between the initial temperature and the final temperature attained when the ambient temperature of the thermistor is suddenly changed under a zero load condition.

The thermal time constant (τ) multiplied by n is as follows:
τ = 63.2% 2τ = 86.5% 3τ = 95.0%.

Self-heating

Since thermistors are resistors, they generate heat when power is applied to them.
This is called self-heating because the thermistor appears to be warming itself.
If the temperature rise is ΔT (℃), ΔT = P/σ (℃)
Because thermistors are small, the unit for power P is mW, and the unit for σ is mW/℃.

Rated power

Power value at which the temperature rises 5℃ above the thermistor’s self-heating

SMD

Abbreviation for Surface Mount Device, an electronic component manufactured so that it can be mounted on the surface of a printed circuit board only by soldering.

NTC Thermistor

Abbreviation for Negative Temperature Coefficient Thermistor, a thermistor whose resistance value decreases with increasing temperature.

PTC Thermistor

Abbreviation for Positive Temperature Coefficient Thermistor, a thermistor whose resistance value increases with increasing temperature.

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