Thermistors are temperature-dependent resistors used in circuits that require management of inrush current or temperature sensing. They are also used for a wide variety of applications including: replacement for fuses, automotive cabin heaters, temperature controlled oscillators, 3D printer head heaters and lithium-ion battery protection circuits. Thermistors are designed to change resistance in an approximately linear fashion over a given temperature range. They are available in a wide range of ‘zero power’ ambient temperature resistance, tolerance, maximum power dissipation, operating temperatures and mounting styles.
There are two basic types of thermistors: positive temperature coefficient (PTC) and negative temperature coefficient (NTC). In PTC thermistors, the resistance increases as temperature increases and conversely with NTC thermistors the resistance decreases as temperature increases. NTC thermistors are designed to be most sensitive to temperature changes in the lower (colder) end of the range. They are most commonly used to sense temperature. PTC thermistors are often used in place of fuses, providing a mechanism to limit current as the temperature of a circuit increases.
Manufacturers will typically provide ‘resistance versus temperature’ graphs to represent device characteristics under zero current conditions. They will also provide coefficients for use in the ‘Steinhart-Hart Equation’ (in the case of NTCs) to derive a reasonably accurate relationship between resistance and temperature. A typical thermistor-based temperature monitoring circuit uses an analog to digital converter (ADC) to sense the voltage across a thermistor. A small fixed current that has negligible effect on the impedance due to power dissipation causes this voltage. The voltage potential across the thermistor will be directly proportional to the resistance, which in turn is proportional to the temperature.
Inrush current is controlled by the I2R power dissipation in the thermistor heating the device and dynamically increasing series resistance. This is referred to as a self-heating effect. The power in current spikes caused by effects like capacitor inrush currents will create a voltage drop across the thermistor, dissipating the bulk of the energy and protecting the downstream load. Thermistors used in this way have a minimum time to recover before they can be used again related to the time it takes for heat to dissipate from the device into the PCB and surrounding environment.
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