Temperature measurement is widely used, not only the production of temperature control technology, some electronic products are required for their temperature measurement, such as computer to monitor the temperature of the CPU motor controller, power driver to know the temperature of IC and so on, the introduction of several commonly used temperature sensor.

Temperature is often used in the actual application of the test parameters, from steel manufacturing to semiconductor production, many processes have to rely on temperature to achieve, temperature sensor is the bridge between the application system and the real world. In this paper, a brief overview of different temperature sensors, and introduces the interface between the circuit system.

Thermistor

There are many kinds of sensors used to measure the temperature. Many thermistors have a negative temperature coefficient (NTC), that is to say, when the temperature drops, the resistance will rise. In all passive temperature sensors, the sensitivity of the thermistor (i.e., the change of resistance at each change of temperature) is the highest.

Typical performance parameters of a NTC thermistor.

These data are measured on the Vishay-Dale thermistor, but it also represents the overall situation of the NTC thermistor. The value of resistance to a given ratio (R/R25), the ratio represents the current temperature resistance and 25 degrees of the resistance ratio, usually with a series of thermistor temperature curve with similar characteristics and the same resistance /. In Table 1 the thermistor series as an example, 25 C values of 10K resistor, at 0 DEG C 28.1K resistors, 60 DEG 4.086K resistors; similarly, 25 C resistance when 5K thermistor resistance at 0 DEG C for 14.050K.

Thermistor temperature curve, you can see the resistance / temperature curve is nonlinear.

Although the thermistor data here is 10 degrees C, some thermistors can be increased at a temperature of about 5 DEG C or even at a temperature of 1. If you want to know the resistance between the two points at a temperature, you can use this curve to estimate, you can directly calculate the resistance value, the formula is as follows:

T refers to the Kelvin absolute temperature, A, B, C, D is a constant, according to the characteristics of the thermistor is different, these parameters are provided by the manufacturer of thermistor.

Thermistor generally has an error range, used to specify the consistency between samples. Depending on the material used, the error is usually between 1% and 10%. Some thermistor designed applications are interchangeable, can not be used for on-site regulation occasions, such as an instrument, user or site engineer only to replace the calibrated thermistor, the thermistor is much higher than ordinary precision, is also much more expensive.

Figure 2 is a typical circuit for measuring temperature using a thermistor. Resistor R1 the thermistor voltage to the reference voltage, it is generally consistent with the reference voltage of ADC, so if the reference voltage of ADC is 5V, Vref will also be 5V. The thermal resistance and the resistance are connected in series to produce the partial pressure, and the change of the resistance makes the voltage at the node also change.

Self heating problem

Because the thermal resistor is a resistor current flowing through it generates heat, so the circuit designer should ensure that the resistance pulled large enough to prevent excessive heat from the thermistor, otherwise the system measurement is a thermistor heat, rather than the ambient temperature.

The influence of the energy consumed by the thermistor on the temperature is expressed by the dissipation constant, which means that the temperature of the thermistor is increased by 1 MW higher than the ambient temperature. The dissipation constant is not the same as that of the thermistor, the pin size, the sealing material and other factors.

The allowable heat and current limiting resistance of the system is determined by the measurement precision. The measurement accuracy is 5, and the measurement system is more than the accuracy of the thermistor.

It should be noted that the resistance of the resistor must be calculated in order to limit the total thermal power consumption in the whole temperature range. Given the resistance value, because of the change of the thermistor resistance, the dissipated power is also different at different temperatures.

Sometimes it is necessary to calibrate the thermistor input to obtain the appropriate temperature resolution. Figure 3 is a circuit that extends the temperature range from 10 to 40 5V to the entire input range of ADC to 0.

The operational amplifier output formula is as follows:

Once the thermistor input calibration is completed, you can use the diagram to show the actual resistance and temperature corresponding. The thermistor is nonlinear, so it is necessary to use graph system, to know how much is corresponding to each temperature value of ADC meter accuracy is 1 DEG C for specific incremental or to 5 degrees increments according to the specific application to.

Cumulative error

When the thermistor is used to measure the temperature, the sensor and other components should be selected in the input circuit to match the required precision. Some occasions require a precision of 1% resistance, and some may require a precision of resistance of 0.1%. In any case, a table is used to calculate the cumulative error of all components on the measurement accuracy.

If the requirements of high precision and want to spend a little money, it is necessary to build the system after it is calibrated, because the circuit board and thermistor must be replaced at the scene, it is generally not recommended to do so. Where the equipment cannot be replaced on site or the engineer has other party