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WHAT IS A THERMISTOR?

What is a thermistor?

Thermally sensitive resistor is a resistor that has a great change in resistance when the temperature changes.

Thermistor = A resistor that is sensitive to temperature. It is a small electronic device that responds to changes in temperature (including ambient temperature) by varying the resistance value.

There are two main types of thermistors: NTC and PTC. these two types are made of different materials, but both materials respond to temperature changes in a specific way.

The resistance value of a negative temperature coefficient thermistor (NTC) decreases with increasing temperature. Selecting a negative temperature coefficient thermistor with the proper rating for the circuit makes it an ideal inline inrush current limiter. The thermistor starts out with a high resistance value, which causes the temperature to spike when the inrush current comes in, and then drops to maintain a low power consumption level once the inrush current reaches its peak. Then, when the normal circuit current passes through the thermistor, it will remain at a high enough temperature level to maintain a low power consumption level. Therefore, it is more desirable than a fixed-value resistor that cannot reduce power consumption. In addition, this type of thermistor is often used as a temperature sensing device in circuits.

Positive temperature coefficient thermistors (PTCs) have a resistance that increases with temperature. These thermistors are often used as inline self-recovery fuses. Once the Curie or switching temperature is reached, the resistance spikes rapidly, making them ideal for overcurrent situations. However, PTC thermistors are unique in that they operate similar to NTC thermistors in that the resistance decreases as the temperature increases until the Curie or switching temperature is reached.

The parameters to focus on before selecting a thermistor include the total constant resistance (usually measured at "room temperature", i.e. 25°C), how high the resistance needs to be or how fast it needs to be, and various other information such as operating temperature and resistance tolerance (accuracy).





What is an NTC thermistor?
The property that resistance decreases as the temperature rises is called negative temperature characteristic, and the thermistor used to represent negative temperature characteristic is called NTC thermistor (Negative Temperature Coefficient Thermal sensitivity resistor).

What is the B constant?
It is a constant that indicates the change in resistance calculated by the following formula using the resistance value at the specified 2-point ambient temperature.

B=ln (R/R0) / (1/T-1/T0)

R: Resistance value at ambient temperature T (K), R0: Resistance value at ambient temperature T0 (K)

What is the zero load resistance value?
The DC resistance value of a thermistor measured at a specified temperature, also known as the no-load resistance value.
It is usually specified as the value at the standard temperature of 25°C.
Measurements should be made at a power low enough to ignore the resistance change caused by its own heating.

What is withstand voltage?
It is the voltage that can be withstood even if applied for 3 minutes at 25°C in still air. This is done by slowly increasing the applied voltage from 0V and making it reach the withstand voltage.


What is the thermal radiation coefficient?
The amount of heat lost per unit of time when there is a 1°C temperature difference between the heating element and the ambient temperature.



Common coatings for thermistors include
Epoxy coatings, suitable for lower temperature applications [typically -50 to 150°C (-58 to 316°F)
Glass coatings for higher temperature applications [typically -50 to 300°C (-58 to 572°F)
These coatings provide mechanical protection for thermistor solder beads and wire connections, while providing some protection against moisture and/or corrosion.
Thermistors typically come with very small diameter (#32AW or 0.008" diameter) solid copper or copper alloy wire. These wires are often tinned for ease of soldering.

Thermistor Stability
Finished thermistors are chemically very stable and are not easily affected by aging.

How are NTC thermistors measured?
After immersing them in silicone oil at 25°C, 50°C, and 85°C, and leaving them for 10 minutes to stabilize, they are measured using a multimeter.
At this point, select the range that will not heat up due to the applied power and perform the measurement.

What kind of applications are NTC thermistors used for?
NTC thermistors are used in a wide range of circuits such as sensors for temperature detection and for preventing surge currents in power supply circuits.

Automobile Intake air temperature sensor, exhaust air temperature sensor, cooling water temperature sensor, lubricating oil temperature sensor
Air conditioners, heating and cooling Oil heaters, hot air heaters, room coolers, solar systems, air conditioners
Office automation equipment, Photocopiers, fax machines
Medical equipment Skin surface temperature measurement, Linguistic solution temperature adjustment, incubator
Cooking apparatus Various stoves, steamers, rice cookers, induction cookers, toasters
Health and beauty apparatus Electronic thermometer, warm water flushing toilet seat, hairdressing apparatus
Various home appliances Refrigerators, water heaters, irons, instantaneous water dispensers, electric kettles, coffee makers, washing machines, televisions, video cameras, stereos, radios
Information and communication Mobile phones, rechargeable battery packs, computers


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