The resistance changes of thermistors vary widely when the temperature ranges to some extent. To narrow the variation, the thermistor is linearized. Linearization includes a voltage mode in which a positive output voltage is obtained with respect to a temperature change, and a current mode in which a negative output voltage is obtained.
R1 can be obtained from the output voltage at the operating temperature for both the voltage mode and the current mode. Following demonstration will be using the voltage mode (series connection). The conditions are as follows. ・Input Voltage：Vin ・Series resistance value：R1 ・Resistance value of thermistor：RT Output voltage Vout is calculated from the following formula.
Calculate R1 value suitable for the operating temperature range. The resistance and output voltage of the thermistor at the operating temperature of lower limit, intermediate, and upper limit are as follows. ・Lower limit temperature RTL, VoutL ・Intermediate temperature RTM, VoutM ・Upper limit temperature RTH, VoutH Calculate output voltage at each temperature.
Since VoutM-VoutL = VoutH-VoutM is suitable for linearization, we can obtain the formula 2 VoutM = VoutH+VoutL. Substituting the equation for the output voltage at each temperature into this equation and obtain R1 value from the following equation.
Conditions ・Vin 1V ・Operating temperature range 0～100℃ ・B constant of thermistor 3800 ・Resistance value of thermistor R0 at 25℃ 22kΩ From the thermistor characteristics for resistance and temperature R=R0 exp {B(1/T – 1/T0)} and T(K)=273℃+t℃, we can obtain thermistor resistance value at 0℃, 50℃, and 100℃ is calculated as follows; RT0=70.637kΩ, RT50=8.207kΩ, RT100=1.698kΩ From these values, R1 is calculated to be 6.326 kΩ. Plotting the Vout graph from this R1 value results in:
Linearized at 0 to 100℃
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