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MEASURING TEMPERATURES USING THERMISTORS

by Jonathan Valvano

Start ý Alternative Transducers ý Resistance vs. Temperature Calibration ý Dissipation Constant ý Low-Cost Embedded Temperature Measurement ý High-Precision Temperature Measurement ý Sources and PDF

When faced with the need to measure temperature for a design, you must first consider some parametersýtemperature range, resolution, and accuracy, and time constant. The temperature range might be just a few degrees Celsius if you are building an oral thermometer, or it might be hundreds of degrees Celsius in some industrial applications. The temperature resolution is the smallest temperature change that can be reliably detected by the system. Resolution can usually be improved by using a more sensitive transducer and by reducing noise. The temperature accuracy is the difference between the measured and true temperatures. Accuracy can usually be improved by using a more stable transducer and by careful calibration. The time constant is the time it takes the temperature measuring system to reach 0.632 of its final value after a step change in temperature. The speed can usually be improved by using a faster transducer.

In this article, Iýll begin with some basic information about thermistors, discuss effective ways to calibrate, and conclude with three specific designs, illustrating various approaches to interfacing these temperature probes to a microcomputer.

THERMISTOR CONSTRUCTION

The thermistor is a temperature transducer made from a ceramic-like semiconductor. The thermistor resistance is sensitive to temperature. This sensitivity is the slope of the resistance temperature response. Thermistors come with both positive (PTC) and negative (NTC) temperature coefficients. Although PTC thermistors are more sensitive, NTC thermistors are typically used because they are more stable.

Thermistors are made from combinations of metal oxides of manganese, nickel, cobalt, copper, iron, and titanium. A mixture of milled semiconductor oxide powders and a binder are first shaped into the desired geometry. Next, the mixture is dried and sintered (under pressure) at an elevated temperature. Lastly, the wire leads are attached, and the combination is coated with glass or epoxy. This coating provides mechanical strength and electrical resistance. Because the electrical resistance of the epoxy/glass shell is high, the only noise pickup occurs through capacitive coupling.

By varying the mixture of oxides, a range of resistance values from 30 ohms to 20 megaohms (at 25ýC) is possible. Spherical thermistor beads come in diameters ranging from 0.3 to 2 mm (see Figure 1). The speed of a temperature transducer is related to its surface area, so you can design a system with a fast response time by using a small thermistor.

Figure 1ýHere you can see a mechanical cut-out drawing of a Thermometrics P60 glass-coated thermistor. This spherical bead has a diameter of 0.06ý, or 1.4 mm.

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