A thermistor is a resistor whose resistance is susceptible to temperature. Therefore Thermistors, or semiconductor thermistors, are thermal components and sensitive resistors. The main features of the thermistor are high-temperature sensitivity, small thermal inertia, long life, small size, and simple structure. Thermistors come in different shapes and are one of the most widely used sensitive resistors.
Main Parameters of Thermistors
Thermistors have the following main parameters:
(1) Nominal resistance value R1. R1 refers to the resistance value marked on the component, which is the resistance value measured at 25°C, and the resistance value does not change by more than 0.1%, so we often use R25°C to represent R1 (in Ω).
(2) Rated power. Under the specified technical conditions, the power consumed by the thermistor for long-term continuous operation is called the rated power, PE (in W). The rated power value provided by Risunsemi in the parameter table refers to the power value at 25°C. When the temperature exceeds 25°C, the thermistor should be derating.
(3) Temperature coefficient of resistance. The temperature coefficient of resistance is the change in resistance value when the temperature changes by 1°C under zero power conditions, expressed in αT (unit: 1/°C). Suppose the resistance value before the temperature change is R, the transformation of the resistance value after the temperature change is ΔRT. In that case, the temperature change is ΔT, and the temperature coefficient of resistance is αT=(ΔRT/R)ΔT.
(4) Transformation point temperature. The transition point temperature is the inflection point temperature on the resistance-temperature characteristic curve of the thermistor, usually expressed by TC, and the unit is °C or K. The transition point temperature is also called the Curie point temperature.
Classification of Thermistors
There are many types of thermistors. The following is a brief list:
A Temperature coefficient of resistance classification: positive temperature coefficient thermistor (PTC), negative temperature coefficient thermistor (NTC).
B Resistance value changes with temperature classification: slow change type (i.e., linear), sudden change type (i.e., nonlinear).
C Heating classification: direct heating and side heating.
D Working temperature range classification: normal temperature type (-55～315°C), low temperature type (<-55°C) and high temperature type (>315°C)
E Materials classification: ceramic, semiconductor (single crystal), metal film, plastic, silicon carbide (SiC), glass thermistors, etc.
F Structure classification: rod shape, ball shape, washer shape, disc shape, bead shape, wire tube shape, disc, square piece, thin film, thick film, etc.
G Package classification: lead type (suitable for through-hole soldering) and SMD type (suitable for surface assembly)
After that, The figure below shows the appearance of several thermistors.
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| Power Thermistor (NTC) | Temperature Compensated Thermistor (NTC) | Thermistor (NTC) for Electronic Thermometer |
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| Linear Thermistor (PTC) | Motor delay start thermistor (PTC) | Thermal Protection Thermistor (PTC) |
NTC Thermistor and How it works
NTC thermistors are made of polycrystalline metals such as iron, cobalt, nickel, copper, manganese, titanium, vanadium, and other oxide semiconductors, and their resistance value decreases as the temperature increases. They are commonly used in circuits for temperature compensation, temperature detection control, and inrush current limitations. So the resistance-temperature characteristic curve of the NTC thermistor shows in Figure 1.
The resistance-temperature characteristic curves of NTC thermistors of different materials are different, but they all have a negative temperature coefficient in common, and the range is generally -(1～6)×10-2/°C. Limiting inrush current usually connects an NTC thermistor across the input line of the bridge rectifier, as shown in Figure 2.
After the NTC thermistor is connected to the AC circuit, a considerable current will be charged to the filter capacitor. Due to the NTC thermistors (RT) access, its resistance at room temperature is relatively large, which limits the surge current. As the RT temperature increases, the NTC thermistor value decreases sharply, with little effect on the input current.
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| Figure 1 | Figure 2 |
Application of Thermistor
Thermistors are mainly used for the temperature compensation of instrument lines and temperature compensation of cold junctions of thermocouples.
The self-heating characteristic of the NTC thermistor is used to form an RC oscillation stabilization circuit, a delay circuit, and a protection circuit to realize automatic gain control. Because the self-heating temperature of the NTC thermistor is much higher than the ambient temperature, the characteristic of the thermistor is used in the flow meter, flow meter, gas analyzer, and thermal conductivity analysis to make a particular detection element.
PTC thermistors are mainly used for overheating protection of electrical equipment, non-contact relays, constant temperature, and automatic gain control. In addition, they can also be used in the protection of motor starting, time delay, color TV automatic degaussing, fire alarm and temperature compensation, etc.
Thermistors from Risunsemi
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