What are Thermistors?

Thermistors are a type of sensitive components. They are divided into positive temperature coefficient thermistors (PTC) and negative temperature coefficient thermistors (NTC) according to different temperature coefficients. The typical characteristic of a thermistor is that it is sensitive to temperature and exhibits different resistance values at different temperatures. Positive temperature coefficient thermistors (PTC) have higher resistance values at higher temperatures, and negative temperature coefficient thermistors (NTC) have lower resistance values at higher temperatures. They both belong to semiconductor devices.

The main characteristics of the thermistor are:

The thermistor will be inactive for a long time

Nominal

Heat-sensitive materials can generally be divided into three types: semiconductors, metals and alloys, which are briefly described below ^[1]

When testing, use the multimeter's ohm range (depending on the nominal resistance value to determine the range, generally R × 1), which can be divided into two steps: First, the room temperature test (the indoor temperature is close to 25 ° C), and the alligator clips instead of the test leads Measure the actual resistance of the two pins of the PTC thermistor and compare it with the nominal resistance. It is normal for the difference between the two to be within ± 2. If the actual resistance is

If you plan to use a thermistor temperature sensing device over the entire temperature range, designing the device can be challenging. The thermistor is usually a high impedance, resistive device, so when you need to convert the resistance of the thermistor into a voltage value, this device can simplify one of the interface problems. However, the more challenging interface problem is how to use a linear ADC to digitally capture the non-linear behavior of the thermistor.

The term thermistor is derived from the generalization of the description of heat-sensitive resistors. Thermistors include two basic types, namely positive temperature coefficient thermistors and negative temperature coefficient thermistors. The negative temperature coefficient thermistor is very suitable for high-precision temperature measurement. To determine the temperature around the thermistor, you can use the Steinhart-Hart formula: T = 1 / (A0 + A1 (lnRT) + A3 (lnRT3)). Among them, T is the Kelvin temperature; RT is the resistance value of the thermistor at temperature T; and A0, A1, and A3 are constants provided by thermistor manufacturers.

The resistance of the thermistor changes with temperature, and this change is non-linear. The Steinhart-Hart formula shows this. When making temperature measurements, you need to drive a reference current through the thermistor to create an equivalent voltage that has a non-linear response. You can use a reference table equipped on the microcontroller to try to compensate for the non-linear response of the thermistor. Even if you can run such algorithms on microcontroller firmware, you still need a high-precision converter for data capture in extreme temperature conditions.

Alternatively, you can use "hardware linearization" technology and a lower-precision ADC before digitizing. (Figure 1) One technique is to connect a resistor RSER in series with a thermistor RTHERM and a reference voltage or power supply (see Figure 1). The PGA (programmable gain amplifier) is set to 1V / V, but in such a circuit, a 10-bit precision ADC can only sense a very limited temperature range (about ± 25 ° C).

1. The thermistor symbol is PTC,
   The resistance value changes with the change of temperature. There are positive temperature type and negative temperature type.
   2. Varistor resistance value changes with pressure change,
   High, medium and low voltage

   Thermistor alloys have begun to be used more and more widely for temperature monitoring and collision making. For example, it has been widely used in environmental monitoring, long-term storage of food, biological engineering, and cutting-edge military engineering ^[1] .

   Thermistor alloys generally have higher resistivity and temperature coefficient of resistance, so they can be made into highly compact temperature sensors with high sensitivity. For example, a foil-type strain gauge temperature sensor is an ideal structural component temperature measurement element. In addition, thermistor alloys have also been used in high temperature aircraft total temperature sensors and large passenger aircraft temperature sensors. It can be seen that the superiority of the thermistor alloy will become increasingly significant ^[1] .