What is a Thermistor?

1. What is a thermistor?

Thermistor is a generic term for Thermally Sensitive Resistor, a semiconductor component whose resistance value changes significantly with changes in temperature.
Those with a negative temperature coefficient, whose resistance value decreases with increasing temperature (see Figure 1), are called NTC thermistors.

Figure 1 Graph of resistance vs. resistance / temperature chracteristics
The graph shows that NTC thermistors are thermally sensitive resistors that decrease in resistance value as temperature increases.

The term “thermistor” generally refers to an NTC thermistor.

Thermistors are ceramic semiconductors made mainly from metal oxides and sintered at high temperatures. Various characteristics can be obtained depending on the material used.
Because thermistor elements can be flexibly processed into a wide variety of shapes according to the application, they are widely used as temperature sensors for temperature measurement, temperature control, and temperature compensation in all kinds of equipment, including automobiles, home appliances, and medical devices.

2. What are the resistance / temperature chracteristics of thermistors?

The resistance value of thermistors varies greatly with temperature, and as shown in Figure 1, the relationship between temperature and resistance value varies exponentially.
The resistance / temperature chracteristics of a thermistor can be approximated by the relationship between resistance value and temperature over a certain temperature range as shown in Equation 1.

Formula for thermistor resistance / temperature chracteristics

T, Ta: Absolute temperature (K)
R, Ra: Zero power resistance at T, Ta (Ω)
B: B value (K)

3. What is zero power resistance?

Zero power resistance refers to the resistance of a thermistor measured at a constant temperature and at a sufficiently low power consumption that changes in resistance due to self-heating can be ignored, and is denoted “R25”. R25 indicates the zero power resistance at 25℃.

4. What is B value?

B value expresses the magnitude of change in resistance value obtained from the temperatures of any two points in the resistance / temperature chracteristics, and is expressed by Equation 2.

Formula for zero power resistance

Ta, Tb: Absolute temperature (K)
Ra, Rb: Zero power resistance at Ta, Tb (Ω)
B: B value (K)

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

Figure 2 Graph of resistance vs. resistance / temperature chracteristics
Graph of resistance vs. resistance / temperature chracteristics

B value is denoted “B25/85,” where B25/85 indicates the value calculated from the resistance value between two points, 25℃ and 85℃.
The larger B value, the larger the slope of the graph, which means that it is easier to detect small temperature changes and is more sensitive to temperature changes.

5. What is the heat dissipation constant?

The heat dissipation constant 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 a thermistor is P(mW), the heat dissipation constant is obtained by Equation 3.

Graph of heat dissipation constant

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)

The heat dissipation constant is determined by the material, structure, and size of the thermistor.
When using thermistors for temperature measurement, it is advisable to keep the applied power as low as possible to avoid errors in the measured temperature.

6. What is the thermal time constant?

The thermal time 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.

When the thermal time constant (τ) is multiplied by n, the rate of change of the temperature difference is as follows, which means that 95% of the temperature difference changes in about 3 times the thermal time constant.
τ= 63.2% 2τ = 86.5% 3τ = 95.0%

Figure 3 Graph of time required to change the temperature difference between the initial temperature (Tb) and the final temperature reached (Ta)

Graph of time required to change the temperature difference between the initial temperature (Tb) and the final temperature reached (Ta)

The smaller the thermal time constant, the faster the response speed with respect to temperature change.