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 power resistance at T, Ta (Ω)
B: B value (K)
Zero power resistance
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 power resistance.
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.
Resistance / temperature coefficient
A coefficient expressing the rate of change of zero power resistance per 1℃ at a given temperature, expressed in Equation 3.
α: Resistance / temperature coefficient (%/K)
T: Any absolute temperature (K)
R: Zero power resistance at T(K) (Ω)
B: Constant B (K)
Thermal 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: Thermal 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
Maximum electrical power (mW) which can be applied continuously to a thermistor at a given rated temperature (usually 25 ˚C).
When applying electrical power to a thermistor the thermistor will self heat and reach a thermal equilibrium with the environment temperature.
However, in case of excessive electrical power thermal runaway may occur which can destroy the thermistor’s electrical characteristics.
The maximum rated power value for each thermistor is determined as the value at which the thermistor self heats 5 ˚C
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.
