Thermistors are temperature sensors that are made from a variety of metal-oxide semiconductor materials. The semiconductor material used determines the temperature range, sensitivity and resistance ranges involved in its application. The resistance of the semiconductor varies with a change in temperature. That change in resistance is interpreted by an instrument that in turn displays a temperature reading. Thermistors have a negative temperature coefficient. That means that as the temperature increases, the thermistor's resistance value decreases.
Sensitivity
Sensitivity is a function that describes how much resistance changes with a change in temperature. According to Johnson, resistance changes of 10 percent per degree Celsius are not uncommon. Therefore, if a thermistor has a nominal value of 10,000 ohms at laboratory temperature (20 degrees Celsius), a 1 degree Celsius change in temperature could cause a 1,000 ohm change in resistance.
Construction
Thermistors are most commonly formed into the shape of beads, disks, and rods. Their size can vary from as small as 1 mm and as large as several cm in diameter. A smaller thermistor will react faster to temperature changes. Different materials, such as epoxy, are used to surround and protect the thermistor from damage.
Temperature range
The temperature range of a thermistor is limited. The resistance-to-temperature curve is extremely non-linear. At lower temperatures, the resistance increases rapidly. At higher temperatures, the resistance decreases to nearly zero. Therefore, most thermistors are intended to be used between -80 degrees Celsius to 300 degrees Celsius.
Response time
Response time is how quickly a thermistor responds to a change in temperature. A 1-mm thermistor bead in an oil bath can react as quickly as half a second. The same bead in open air may have a response time of nearly 10 seconds. As the thermistor's size increases, so does its response time. If a thermistor is being used in a temperature control system, a faster response time results in better temperature control.
Signal conditioning
Due to a thermistor's non-linearity, it is difficult to use alone as a temperature measurement device. The thermistor is usually connected to a divider circuit with a fixed resistor of the same nominal value. The measurement instrument then senses the change in resistance of the thermistor and converts it to a temperature reading.
