Positive Temperature Coefficient (PTC) Thermistors are thermistors whose resistance is directly proportional to temperature (i.e. resistance increases with an increase in temperature or vice versa). They are fabricated using Silicon or Barium Titanate which are ceramic semiconductor materials that provide high resistance. PTCs are widely used to protect circuits against malfunction and damage from high temperatures. 

Types of PTC Thermistor

PTC thermistors are divided into two based on their material and structure:

Silistors: A Silistor PTC thermistor uses Silicon as its base material. It has a linear Resistance-Temperature relation where resistance increases proportionally with an increase in temperature.

Switching PTC Thermistors: This type of PTC thermistor has a nonlinear Resistance-Temperature relation. Initially, when the PTC is heated, its resistance starts decreasing, until a critical temperature (maximum allowable temperature) is reached. When the temperature rises above the critical temperature, the resistance of the PTC increases drastically. The switching of a PTC at a high temperature helps in protecting a circuit from damage by resisting the flow of current or preventing heavy voltage drop.

Resistance-Temperature (R-T) Graph

The graph above shows the R-T characteristics of both the Silistor and Switching type PTC. The red line represents the R-T characteristics of a Silistor and the blue curve represents the R-T characteristics of a Switching Type PTC. Tc denotes the Critical Temperature, R25 denotes the Resistance (base resistance) at 25°C (standard temperature) and Rmin denotes Minimum Resistance.

In the switching type, the resistance decreases till the standard temperature of 25°C and maintains its resistance till the critical temperature is reached. Once the critical temperature (Tc) is reached, the resistance increases drastically. In Silistors, there is a linear relationship between resistance and temperature.

Switching type PTCs are used as over-current limiters or resettable fuse as it offers protection to the circuit from damage, by providing high resistance at high temperature when an immensely large amount of current flows through the circuit while Silistors are typically used as temperature sensors because of its linear characteristics i.e., the rate of change of resistance is proportional to the rate of change of temperature.

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