A Guide for Online Information About:

Thermistors

by Rick Prescott

A thermistor, or thermally sensitive resistor, is a resistor that changes resistance with a change in temperature. They are used in a variety of applications, including temperature measurement.

A thermistor is a piece of semiconductor made from metal oxide, pressed into a small bead, disk, wafer, or other shape, sintered at which temperatures, and finally coated with epoxy or glass. There are two basic types of thermistors, PTC and NTC. A PTC (positive temperature coefficient) device will increase resistance with an increase on temperature. An NTC (negative temperature coefficient) device will decrease resistance with an increase in temperature. Both PTC and NTC devices are manufactured in a broad selection of styles and resistance values. However, NTC thermistors are more commonly used than PTC thermistors, especially for temperature measurement applications.

Thermistors offer some advantages over other temperature measurement devices:

• high sensitivity—temperature coefficients can be of the order of –10 kilohms/degree Celsius
• fast response—thermistor beads can be small physically and, therefore, are quick to react to temperature change
• relatively high resistance—high resistance diminishes the effect of inherent resistance in the lead wires

Thermistors have drawbacks as well:

• highly nonlinear output—makes interfacing more difficult (a plot of resistance - temperature curve of a thermistor)
• relatively limited operating range—typically limited to around 350°C
• high-quality constant current or voltage source is required

More information can be obtained at:
National Instruments- Measuring Temperature with Thermistors, a Tutorial
- Thermistor Overview
- Resistance/Temperature Characteristics of Thermistors
- Thermistor Measurement Circuits
- Signal Conditioning Systems for Thermistors
- SCXI Signal Conditioning Systems
- Thermistor Unit Conversion Routines

More general thermistor information from National Instruments.

Quality Thermistor, Inc.
- NTC thermistors are the ideal solution for low-cost temperature measurement between the range of –55ýC to 150ýC. Their small size allows for PC board mounting, as well as for potting into enclosures.

- QTMC—Radial leaded thermistors are bare leaded NTC thermistors. The ideal solution for low-cost temperature measurement between the range of –55ýC to 150ýC. Their small size allows for PC board mounting, as well as for potting into enclosures.
- QTLC—Interchangeable thermistors. The name "interchangeable" is given to thermistors that have ultra-tight tolerances guaranteed to track a specific resistance-temperature curve.
- QT0805—SMD thermistors. Surface Mount thermistors are the ideal solution for temperature sensing functions found in temperature compensation networks. The patented design features "Leach Guard" terminations that are able to withstand many of today's modern automated assembly and soldering processes. Standard EIA sizing allows for tape and reel packaging designed for automated placement equipment.
- QTHT—High temperature thermistors. High-temperature thermistors are ideal where measurements must be made at extreme temperatures. Constructed with materials designed for high-temperature operation, they have exceptional time response and can withstand harsh chemical environments at elevated temperatures. Typical applications include electric motor protection, high-speed laser printers, automotive under-hood applications, food, and consumer products industry.

Thermistors can be used in a variety of ways. Here are a few typical applications from Omaga.

ISA Standards: SP1.3, Resistance Thermometers

Training courses offered my VASA

Below are a few companies that manufacturer thermistors:

Catalogue of the manufacturers and suppliers of analytical instrumentation and laboratory equipment.

National Instruments-

Quality Thermistor Inc.

BALHE Corporation is specialized in PTC ceramic thermistor and heaters.
Products available: PTC ceramic heater/thermistor (Exporter)
Sell : PTC ceramic thermistor/heaters

Mitsubishi Materials Corporation
MMTL's advanced products division manufactures surge absorbers, chip thermistors, and other electronic devices.

Xian Radio Factory #2 is able to manufacture many kind of PTC thermistors:

• degaussing thermistor
• motor starting thermistor
• thermistor for over-current protection
• thermistor for temperature control
• thermistor for temperature compensation
• PTC thermistor for self-controlled heating
• intelligent thermistor for electronic ballast

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