A thermistor, or thermal resistor, is defined as a passive component whose resistance fluctuates when the temperature in a system changes. For this reason, they are an inexpensive, accurate, and dynamic method for measuring temperature. It is worth noting that a thermistor’s resistance is determined by the material used to produce it.
Since controlling temperature is a critical part of many manufacturing processes, thermistors often determine the success or failure of an application. Essentially, thermistors are utilized to monitor the temperature surrounding a device and temperature change in the device as well. As they are popularly used for temperature sensing, they are found in a wide range of circuits, equipment, and devices.
Different Types of Thermistors
Thermistors can be broadly categorized into two categories: negative temperature coefficient (NTC) and positive temperature coefficient (PTC) thermistors.
Negative Temperature Coefficient (NTC) Thermistors
NTC thermistors are made of a semiconductor material with conductivity that falls between that of electrical and non-electrical conductors. With NTC thermistors, resistance decreases as the temperature increases. As the temperature increases, a thermistor moves electricity quickly and efficiently. Furthermore, the behavior of an NTC thermistor is determined by the components it is composed of. For instance, manufacturers can change the mixing ratios of oxides or doping material to meet desired requirements. Additionally, oxygen content also has a major effect on firing and variations in the cooling rate.
These thermistors are usually made in discs, rods, plates, beads, or chips using a sintered metal oxide. Metallic oxide NTC thermistors are produced using a fine powder that is compressed and sintered. Common materials for the thermistor itself are manganese, nickel copper, iron, titanium, silicon, or germanium crystals. For higher temperatures, thermistors can be made from aluminum oxide, Beryllium oxide, Zirconium dioxide, Yttrium oxide, or Dysprosium oxide.
A common type of NTC thermistor is the glass encapsulated kind which is completely sealed to eliminate the potential for reading errors, meaning that they are ideal for severe environmental conditions.
Positive Temperature Coefficient (PTC) Thermistors
PTC thermistors, on the other hand, work in the opposite direction from their NTC counterparts. With PTC thermistors, as temperature increases, the thermistor’s resistance increases. There are two major types of PTC thermistors: switching and silistor variations.
Switching PTC thermistors are non-linear, and the resistance has an initial drop, alongside an increase in temperature. Once the resistance reaches a certain point, it increases rapidly, making it perfect for protective use. In contrast, silistor PTC thermistors are linear and utilize a semiconductor as their base. They are often found in a numerous temperature sensing equipment, and they are made from doped silicon.
Due to their low price points and accuracy, industrial settings and commercial environments benefit from thermistors by allowing users to determine, control, and monitor temperature. In fact, a common use for thermistors includes being a part of an HVAC system, a critical part of thermal support and air flow. In this example, a thermostat is used which is a highly sensitive thermistor.
Another example of a commonly used thermistor is found in automobiles for measuring the temperature of oil and coolant. Vehicle thermistors, as they are called, inform drivers when the vehicle is overheating. They are directly connected to the dashboard instruments and acquire the necessary information about how the vehicle is operating.
Other common types of thermistors:
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