Characteristic parameters of temperature sensors

Characteristic parameters of temperature sensors

1. Classification of Thermistors

Thermistors are divided into two categories: PTC (Positive Temperature coefficient) and NTC (Negative Temperature coefficient). The PTC resistance increases with increasing temperature. The resistance is mainly composed of BaTiO3, SrTiO3, and PbTiO3, with trace amounts of Nb, Ta, Bi, Sb, Y, and La added to make it semiconductive. The NTC resistance decreases with increasing temperature. Resistors are mainly made of metal oxides such as manganese, cobalt, nickel, and copper using ceramic technology.

These metal oxide materials have semiconductor properties because they are completely similar in conductivity to semiconductor materials such as germanium and silicon. When the temperature is low, these oxide materials have fewer charge carriers (electrons and holes), so their resistance values are higher; As the temperature increases, the load
As the number of streamers increases, the resistance value decreases.

2. NTC Key Characteristic Parameters

The key parameters of the chip introduced below are determined by the material system of the chip.

(1) Conductivity of the chip

　
According to the formula R=& Rho* L/S can deduce conductivity under the condition of knowing the length, width, and height of the chip.

　
The larger the cross-sectional area (chip size) of a chip with the same conductivity, the smaller the resistance value of the chip.

　
From this, we can preliminarily infer the resistance range of the chip by looking at its size.

(2) Zero power resistor R (25)

When the zero power resistor is applied to the voltage and current at both ends of the chip in a linear relationship, the chip itself has no self-heating error. Unless otherwise specified, it is the design resistance value and nominal resistance value of the thermistor. For example, 102=1K=1000 ohms @ 25C or 545=5400000 ohms @ 25C

The process of chip selection is the first parameter to consider. Select a chip according to customer requirements, and by changing the length, width, and height of the chip, the resistance value can be changed to achieve the target value.

3. Two temperature points of the chip; Beta; value

&Amp; Beta= (T * T0)/(T0-T) * ln (RT/RT0)

&Amp; Beta; Representing the slope between two temperature points on R-T,& Beta; The larger the curve, the steeper it is, and the more sensitive it is to changes in resistance caused by changes in unit temperature. In a certain situation,& Beta; It can also indicate the material of the chip.

4. The ratio of two temperature points on a chip

Ratio: The ratio of chip resistance at two temperature points, and the ratio of resistance at low temperature points to resistance at high temperature points.

5. Thermal time constant; Tau;

The thermal time constant, also known as the thermal response time, refers to the time required for a thermistor to change from the initial temperature to 63.2% of the final temperature when the ambient temperature suddenly changes at zero power. Generally, 75C and 25C are selected as the initial and final temperature points. Thermal time constant; Tau; It is directly proportional to the thermal capacity C of the thermistor and is proportional to the dissipation coefficient; Delta; In inverse proportion. Generally, the larger the constant, the better the performance of this thermistor. Medical products have requirements for the reaction time of thermistors.

6. Dissipation coefficient; Delta;

It represents the power required to increase the resistance temperature by 1C. Within the operating temperature range,& Delta; Varies with changes in ambient temperature. The ratio of the rate of change in dissipated power of a thermistor to its corresponding temperature change at a specified ambient temperature. Generally, 75C and 25C are selected as the initial and final temperature points.

In practical chip applications, the dissipation coefficient is used to determine the maximum test current of the chip (when testing at zero power). Or calculate the self-heating error of the chip at a specific temperature point. It can also be used to measure other parameters.

For example, the zero power resistance R (25) of a certain chip is 10K, and the test current is 1MA; Delta= 1mw/℃, from which the self heating temperature difference can be calculated.

Solution: Dissipated power=I2R=(0.001) 2 * 10000=10mw

　 　
Self heating temperature difference P/& Delta= 10mw/1mw/℃=10 ℃

7. Heat capacity C

The thermal capacity refers to the energy required to change the temperature of the thermistor by 1 Kelvin, expressed in J

Thermal capacity=thermal time constant * dissipation coefficient, i.e. C=& Delta/& Amp; Tau;