Resistance thermistors

These thermistors consist of a semiconductor material in which resistance decreases as temperature increases. Key benefits of thermistors are high-resolution measurements over limited ranges and low cost. Instead of tracking resistance in metals, thermistors track it in semiconductors. The base material is a mixture of metal oxides pressed into a bead, rod, disk, wafer, or other shape. The bead, with embedded leadwires, is sintered at high temperatures and often coated with epoxy or glass. Beads may be quite small, down to 0.01 in. diameter in some cases.

The design, construction, and characteristics of thermistors vary widely among manufacturers.

Typical properties of thermistors are as follows:

1. 

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   Thermistors exhibit very large resistance changes, but usually in a direction opposite to resistance thermometers; resistance drops as temperature rises. This is called negative temperature coefficient of resistance (NTC).

2. Base resistances, commonly specified at 25° C, range from thousands to millions of ohms. Thermistor sensitivity dwarfs that of resistance thermometers.

3. Resistance/temperature curves deviate widely from linearity, except over narrow ranges.

4. Thermistors tend to drift more than resistance thermometers, although they stabilize over time.

5. Temperature ranges are moderate, with 300° C as the common upper limit.

The combination of nonlinearity, high sensitivity, and instability has generally limited thermistors to high-resolution measurements over limited ranges. A classic example is medical thermometry. Physicians are only concerned with a small range around 98.6° F and thermistors can be chosen to provide a large, fairly linear signal in this area. One-point calibration is simple and sufficient.

Some manufacturers of thermistors offer special models with these characteristics:

• Positive temperature coefficient (PTC) models, which are used more for current limiting in electronic circuits than for temperature measurement; as current increases through the bead, self-heating drives up resistance dramatically, throttling the current.

• Thermistors that are interchangeable over a specified range.

• Linearized thermistors or thermistor sets that produce a highly linear output over a limited range.

• Glass-coated specially aged thermistors that maintain high stability over moderate temperatures — for example, ± .005° C per year between 0 and 100° C.