What is a thermistor? A thermistor is a temperature sensitive resistor, they are often used as a temperature sensor. The term thermistor is a contraction of the words "thermal" and "resistor". All resistors have some dependency on temperature, which is described by their temperature coefficient. In most cases for (fixed or variable) resistors the temperature coefficient is minimized, but in the case of thermistors a high coefficient is achieved. Unlike most other resistors, thermistors usually have negative temperature coefficients (NTC) which means the resistance decreases as the temperature increases. These types are called NTC thermistors. Thermal resistors with a positive temperature coefficient are called PTC thermistors (Positive Temperature Coefficient). Thermistor definition A resistor whose resistance changes significantly with a change in temperature. Types and applications Thermistors are ceramic semiconductors. In most cases they are composed of metal oxides, which are dried and sintered to obtain the desired form factor. The types of oxides and additives determine their characteristic behavior. For NTC's cobalt, nickel, iron, copper or manganese are common oxides. For PTC's barium, strontium or lead titanates are commonly used. NTC thermistor The NTC type is used when a change in resistance over a wide temperature range is required. They are often used as temperature sensors in the range of -55°C to 200°C, although they can be produced to measure much lower of higher temperatures. Their popularity can be accounted to their quick response, reliability, robustness and low price. PTC thermistor The PTC type used when a sudden change in resistance at a certain temperature is required. They exhibit a sudden increase in resistance above a defined temperature, called the switch, transition of "Curie" temperature. The most common switching temperatures are in the range of 60°C to 120°C. They are often used for self-regulating heating elements and self-resetting over-current protection....
There are a lot of different resistor types, all with their own applications, characteristics and construction. This page lists the different resistor types which are described on the resistor guide. Fixed resistors The fixed resistor type is the most common resistor, when people talk about a resistor they most probably mean a fixed resistor. The picture to the right shows an axial carbon film resistor, the most common type. Fixed resistors have a constant resistance value. Different resistor materials are used for fixed resistors. For all resistor types the used materials has influence on the resistor properties like the tolerance, cost and noise. Fixed resistors are available in axial and SMD packages. Variable resistors There are several variable resistor types, their general property is that the resistance value is adjustable. Most variable resistors are adjusted by mechanical movement (linear or rotary). When they are used as a variable voltage divider, they are called potentiometers. When they are used as a variable resistance to control the current in a circuit, they are called rheostats. Digital potentiometers are controlled electronically instead of by mechanical action. In general variable resistors have a higher tolerance of 20% compared to fixed resistors where 5% is the most common. Thermistors Thermistors are resistors of which the resistance changes significantly when temperature changes. Different thermistor types exist, the two most common types are the NTC and PTC thermistor. NTC thermistors decrease in resistance when the temperature rises, while PTC thermistors increase in resistance when the temperature rises. Thermistors are often used as temperature sensors or thermal protection devices. Varistors Varistors have a non-linear resistance which is dependent on the voltage over the varistor. Above a certain threshold voltage the resistance of a varistor drops very fast. Different varistor types exist, but nowadays the MOV varistor is the...