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 R L. For instance, lowpass filters are useful for removing the 60 Hz power line noise that is prevalent in most laboratory and plant settings. Thermistors are manufactured from metal oxide semiconductor material encapsulated in a glass or epoxy bead. The first, more common, has a resistance that decreases with increasing temperature while the latter exhibits increased resistance with increasing temperature. Two-Wire Connection The voltage difference across the resistor is read as a temperature. This source must be constant and precise. Common Symbol for Thermistor Each sensor has a designated nominal resistance that varies proportionally with temperature according to a linearized approximation.
Thermistors come in either two-, three-, or four-wire configurations, and they can be connected as depicted in Figure 2. . Considerations Because the nominal resistance of a thermistor is very high, you need a source that can output low currents accurately. In other words, you must connect both the +ve and —ve terminals of the analog input channel across the thermistor. 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. 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.
This, along with their high nominal resistance, helps to provide precise measurements in lower-temperature applications. The relationship between voltage across a resistor and temperature is not perfectly linear. Though they exhibit a fast response rate, thermistors are limited for use up to the 300 °C temperature range. You take temperature measurements by connecting the thermistor differentially to an analog input channel. 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.
Signals emitted by thermistors are typically in the millivolt range, making them susceptible to noise. 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. The easiest way to connect a thermistor to a measurement device is with a two-wire connection see Figure 3. Lowpass filters are commonly used in thermistor data acquisition systems to effectively eliminate high-frequency noise in thermistor measurements.
With this method, the two wires that provide the thermistor with its excitation source are also used to measure the voltage across the sensor. . . . . .
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