Negative Temperature Coefficient (NTC) thermistors are the thermistors whose resistance is inversely proportional to the temperature. This means at high temperatures they will have a low resistance and at low temperatures they will have a high resistance. They are fabricated using semiconductor materials, usually oxides of metals like cobalt, manganese and nickel.

NTC thermistors are used for measuring temperature, protecting devices and controlling current.

• Measuring Temperature: When paired with an electrical circuit, NTC Sensors' ability to change resistance with temperature gives a way to constantly measure temperature. 
• Protecting Devices: NTC thermistors are capable of protecting the circuits/devices by providing high resistance from surge current.
• Controlling Current: NTC thermistors are used as inrush current limiters, for controlling surge flow of current to enter into the circuit when it is first turned on.

One of the most common uses of an NTC thermistor is as an inrush current limiter. An inrush current limiter, limits surge current from damaging a circuit. An example circuit diagram is shown below:

NTC based Inrush Current Controller

When the circuit is turned on, the temperature of the circuit is initially low. At this stage an NTC provides high resistance thus decreasing the inrush current that will enter the circuit.  However, once the circuit starts operating, its temperature rises over time, which gradually decreases the resistance of the NTC thermistor. Thus, during the steady state condition of the circuit, the NTC provides low resistance. This makes using an NTC thermistor for inrush current limiting better than other components like series resistors and in-line filters that are prone to damage at high temperatures. 

The most common disadvantage of an NTC thermistor is self-heating as temperature rises. To reduce overheating of an NTC, we use a larger value of effective series resistance (Rs, as seen in the figure above) which helps in lowering the biasing current through the thermistor, hence reducing heat dissipation. This helps reduce self-heating of the NTC component keeping it safe.

In the graph below shows the relation between Resistance and temperature of an NTC. The X-Axis shows Temperature and the Y-Axis shows Resistance.

R-T Characteristics of NTC

The graph depicts the gradual decrease in resistance with increase in temperature. This is usually plotted by measuring the resistance of an NTC over a temperature range. The reference value of the resistance of an NTC is usually measured at standard temperature of 25°C.

Types of NTC

Based on the type of their shape and fabrication, NTCs are classified into the following types: Disk and Chip, Bead, Epoxy and Glass Encapsulated.

Disk and Chip: Disk and Chip NTC thermistors are cost effective thermistors that are generally used for temperature measurement. These types of NTCs are generally larger in size and thus can dissipate more heat. 

• Disc type NTCs are fabricated at a high temperature by pressing metal oxide blends of cobalt, nickel or manganese. 
• Chip type NTCs are fabricated by tape-casting ceramic material using a ceramic slurry (Semi-liquid form of ceramic material) and then laying out a thick film of it, sintering and cutting it into small pieces.

Bead Type: Bead Type NTC thermistors are lead wires made up of platinum alloy which is directly sintered into the ceramic case. They have high stability and can be used at very high temperatures compared to Disc and Chip NTC thermistors. A major disadvantage of bead type NTCs is that they can get easily damaged, so they are often encased into a glass frame.

Epoxy: Epoxy type NTCs are polymers of epoxides that are soldered in encased Teflon/PVC wires, used for protecting circuits by providing electrical insulation. They are generally built-in small sizes and can be easily mounted in a circuit.

Glass Encapsulated Thermistors: As the name suggests, these NTC thermistors are encapsulated/sealed within a glass body. They are designed to function at temperatures above 150°C. The encapsulation of the thermistor gives it better stability and a longer life.

How to Select an NTC

The following parameters must be considered when selecting an NTC for use in a circuit.

• Reference Resistance: The reference resistance value of an NTC is its resistance at room temperature (usually calculated at 25°C).
• Temperature Coefficient of Resistance (𝛼): It is the measure of change in the resistance with change in temperature. It is measured in ppm/°C (1 ppm = 0.0001%).
• Resistance Tolerance: This is a measure of the variation in resistance of the NTC at particular temperature. This usually +/- 1% to +/-1.5%.
• Temperature Range: Based on electronic fabrication variations, NTCs are capable of functioning between a temperature range of -50°C to 150°C (can go up to 200°C for glass encapsulated NTC thermistors).
• Packaging/ Casing:  Based on the requirement, NTC thermistors come in a variety, be it sintered bead type, disc and chip, epoxy or glass encapsulated.

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