THERMISTORS – GREAT TINY SENSORS FOR TEMPERATURE CONTROL

The term ‘thermistor’ is a short form of ‘Thermally sensitive Resistor’. It is a highly sensitive, temperature-dependent semiconductor resistor whose electrical resistance changes significantly and predictably with surrounding temperature (or temperature of body). Being a semiconductor, its resistance lies between a conductor and an insulator.   Being used as a resistance, it is a passive element, and is represented by following symbols.

Resistance of thermistor may either go down or up with an increase in temperature. This depends on the type of material used for its construction. The change in resistance is predictable and measurable.

Effect of temperature on thermistor is defined by the temperature coefficient of resistance (TCR). This can be defined as the percentage change in the value of the resistance for a given change in temperatures. Thermistors are divided into two main categories: Negative Temperature Coefficient (NTC), where resistance drops as temperature rises, and Positive Temperature Coefficient (PTC), where resistance climbs as temperature rises.

Operational Characteristics

• Resistance-Temperature (R-T) Relationship: The R-T profile of an NTC thermistor is non-linear and decays exponentially.
• High Sensitivity: They possess a temperature coefficient 10 to 100 times greater than base metals and can register micro-changes of even small fraction of a degree of Celsius.
• Voltage-Current (V-I) Profile: At low current levels, a thermistor behaves like a normal resistor because the current is too low to cause self-heating. As the current increases, self-heating occurs. For an NTC, this drops its resistance, causing the voltage across it to peak and then decline even as current rises.
• Thermal Time Constant (τ): It takes time for a thermistor to come to equilibrium with surrounding. Thermal time constant represents the duration required for a thermistor to register 63.2% of a sudden step-change in ambient temperature under zero-power conditions. Bead designs offer the fastest response times.

Dissipation Factor (D): This denotes the amount of power required to raise thermistor internal temperature 1°C above its surrounding environment (mW/°C). Currents in thermistor are kept low to prevent measurement errors by self-heating.

Comparison: NTC vs. PTC Thermistors

Property	NTC (Negative Temperature Coefficient)	PTC (Positive Temperature Coefficient)
Temperature Response	Resistance decreases as temperature rises.	Resistance increases as temperature rises.
Material Composition	Sintered metal oxides (such as manganese, nickel, cobalt).	Polycrystalline ceramic materials (like barium titanate) or polymers.
Primary Function	Precision temperature sensing, control, and inrush current limiting.	Overcurrent protection (resettable fuses) and self-regulating heaters.

Classification by Temperature Coefficient

1. NTC (Negative Temperature Coefficient): Resistance of NTC drops sharply as temperature increases. These are made from sintered metal oxides like nickel, manganese, and cobalt, and are used for precise temperature measurement, monitoring, and inrush current suppression. A standard NTC thermistor may have a negative thermal resistance temperature coefficient of about 0.0045 per degree Kelvin.
2. PTC (Positive Temperature Coefficient): Resistance of a PTC thermistor increases sharply as temperature rises. These are made from polycrystalline ceramic materials (like barium titanate) or doped polymers, and are used for overcurrent protection (resettable fuses) and self-regulating heating elements.

PTC thermistors are not as popular as NTC thermistors. These thermistors are used in circuit protection. When current passes through the PTC thermistor, it causes heating. This heating will cause an increase in resistance, therefore limiting the current. Thus, the PTC thermistor can be used as a current limiting device.

Classification of thermistors by Shape / Form

Physical shape of a thermistor determines its response time, ruggedness, and how it is mounted in a circuit.  

1. Bead Thermistors

• Small spheres of semiconductor material enclosed in glass or epoxy.
• Features platinum alloy lead wires.
• Offers incredibly fast thermal response times.
• Provides high stability for precision medical and scientific instruments.

2. Disk and Washer Thermistors

• Fabricated by pressing semiconductor paste into flat, circular shapes.
• Features metallized surface contacts on both faces.
• Offers higher power dissipation ratings due to a larger surface area.
• Used primarily for temperature control and circuit compensation.

3. Rod Thermistors

• Extruded into long, cylindrical shapes with leads attached to each end.
• Provides high resistance values and high power handling capability.

Offers slower response times due to their physical bulk.

4. Surface Mount Device (SMD) / Chip Thermistors

• Compact, rectangular blocks with metal end-terminals.
• Designed for automated, high-density printed circuit board (PCB) assembly.
• Widely used in modern consumer electronics like smartphones, laptops, and battery packs.

5. Glass-Encapsulated Thermistors

• Standard thermistor elements sealed inside a rugged glass envelope.
• Protects the semiconductor from moisture, harsh chemicals, and physical wear.
• Capable of operating at much higher temperatures (up to 300°C) than epoxy-coated models.

Thermistors are generally available in nominal values of 1K, 2K, 10K, 20K, 100K, etc. The resistance value of the thermistor is mentioned at a temperature of 25°C.