Derived from the term “thermally sensitive resistor,” thermistors are accurate and cost-effective sensors for measuring temperatures. Thermistors are available as negative temperature coefficient (NTC), and positive temperature coefficient (PTC), with NTC thermistors being the more commonly-used. NTC thermistors have their resistance decrease as their temperature increases, while PTC thermistors’ resistance increases as their temperature increases.
Thermistors are made from metallic oxides, binders, and stabilizers, pressed into wafers and cut into chip size, left in disc form, or made into another shape. The precise ratio of these materials governs their resistance/temperature “curve.” This is closely controlled, as this determines how the thermistor will function.
Thermistors are comprised of materials with a known resistance. As the temperature increases, an NTC thermistor’s resistance will increase in a non-linear fashion, following a particular curve. The shape of this resistance vs. temperature curve is determined by the properties of the materials that make up the thermistor.
Thermistors are available with a variety of base temperatures and resistance vs. temperature curves. Low-temperature applications generally use lower-resistance thermistors, while higher temperature applications use higher-resistance thermistors. Thermistors must be accurate, durable, long-lasting, and inexpensive. They are often chosen for applications where ruggedness, reliability, and stability are important, where environmental conditions are extreme, and where electronic noise is present. Thermistors with epoxy coatings are used in lower temperature applications (-50 to 150 degrees Celsius), while glass-coated thermistors are used in higher temperature applications (-50 to 300 degrees Celsius).
Thermistors are available in several configurations. The three most commonly-used are hermetically-sealed flexible (HSTH) thermistors, bolt-on/washer, and self-adhesive surface-mounting. HSTH thermistors are sealed within PFA (plastic polymer) jackets to protect the sensing element from moisture and corrosion. They are used to measure temperatures of liquids like oils, chemicals, and food. Thermistors with bolt or washer-mounted sensors are installed to standard-sized threaded holes or openings. Lastly, surface-mounted thermistors have adhesive exteriors and can be stuck in place on flat or curved surfaces, and can be removed and re-mounted at will to serve their various commercial and industrial applications.