Thermistors

A thermistor is a semiconductor device formed from metal oxides. The principle of temperature measurement with a thermistor is that its resistance changes with temperature. Most thermistors differ from normal resistors in that they have a negative coefficient of resistance, this means that the resistance decreases with an increase in temperature. Negative (NTC) thermistors are the more common although positive (PTC) are also available. Thermistors were traditionally quite unstable, and their performance was unpredictable. However, improvements in technology and the selection of oxides now means that thermistors are produced that have characteristics that are well defined. A thermistor is a bulk semiconductor device, and as such can be fabricated in many forms. The more common include discs, beads and rods. Size does vary from a bead of 1mm to a disc of several centimetres in diameter and thickness. Wide ranges of thermistors (both resistance and temperature) are supplied by manufacturers. This is done by varying the doping and semiconductor materials.

Advantages

- Small size

- Fast response

- Very high sensitivity (Select range)

- No cold junction compensation

- Inexpensive

- Polarity insensitive

- Wide selection of sensors

Disadvantages

- Unstable due to drift and decalibration (especially at high temperatures)

- Not easily interchangeable

- Non linear

-Narrow span

- Fragile

- High resistance, noise problems