1. Thermistor Overview
|Thermistors, like RTDs, are thermally sensitive semiconductors whose resistance varies with temperature. Thermistors are manufactured from metal oxide semiconductor material encapsulated in a glass or epoxy bead. Also, thermistors typically have much higher nominal resistance values than RTDs (anywhere from 2,000 to 10,000 Ω) and can be used for lower currents.||
Figure 1. Common Symbol for Thermistor
Each sensor has a designated nominal resistance that varies proportionally with temperature according to a linearized approximation. Thermistors have either a negative temperature coefficient (NTC) or a positive temperature coefficient (PTC). The first, more common, has a resistance that decreases with increasing temperature while the latter exhibits increased resistance with increasing temperature.
You can use PTC thermistors, or posistors, as current-limiting devices for circuit protection (in place of fuses) and as heating elements in small temperature-controlled ovens. Meanwhile, NTC thermistors, the topic of this article, are used mainly to measure temperature, and are widely present in digital thermostats and in automobiles to monitor engine temperatures.
Thermistors typically have a very high sensitivity (~200 Ω/°C), making them extremely responsive to changes in temperature. Though they exhibit a fast response rate, thermistors are limited for use up to the 300 °C temperature range. This, along with their high nominal resistance, helps to provide precise measurements in lower-temperature applications.
2. How to Make a Thermistor Measurement
Because thermistors are resistive devices, you must supply them with an excitation source and then read the voltage across their terminals. This source must be constant and precise.
You take temperature measurements by connecting the thermistor differentially to an analog input channel. In other words, you must connect both the +ve and –ve terminals of the analog input channel across the thermistor.
Thermistors come in either two-, three-, or four-wire configurations, and they can be connected as depicted in Figure 2.
Figure 2 .Two-, Three-, and Four-Wire Connection Diagrams
When there are more than two wires, the additional wires are solely for connecting to the excitation source. A three- or four-wire connection method places leads on a high-impedance path through the measurement device, effectively attenuating error caused by lead-wire resistance (RL).
The easiest way to connect a thermistor to a measurement device is with a two-wire connection (see Figure 3). With this method, the two wires that provide the thermistor with its excitation source are also used to measure the voltage across the sensor. Because thermistors have a high nominal resistance, lead-wire resistance does not affect the accuracy of their measurements; thus, two-wire measurements are adequate for thermistors, and two-wire thermistors are the most common.
Figure 3 .Two-Wire Connection
The voltage difference across the resistor is read as a temperature. The relationship between voltage across a resistor and temperature is not perfectly linear. The NI-DAQmx driver scales the resistance of a thermistor to a temperature using the Steinhart-Hart thermistor third-order approximation:
where T is the temperature in Kelvin, R is the measured resistance, and A, B, and C are constants provided by the thermistor manufacturer.
Because the nominal resistance of a thermistor is very high, you need a source that can output low currents accurately.
If you cannot dissipate extra heat, heating caused by the excitation current can raise the temperature of the sensing element above that of the ambient temperature, causing an error in the reading of the ambient temperature. You can minimize the effects of self-heating by lowering the excitation current.
Signals emitted by thermistors are typically in the millivolt range, making them susceptible to noise. Lowpass filters are commonly used in thermistor data acquisition systems to effectively eliminate high-frequency noise in thermistor measurements. For instance, lowpass filters are useful for removing the 60 Hz power line noise that is prevalent in most laboratory and plant settings.
Getting to See Your Measurement: NI LabVIEW
Once you have configured the system properly, you can acquire and view the data using the LabVIEW graphical programming environment (See Figure 7).
Figure 7. Thermistor Reading in LabVIEW Front Panel