NTC Thermistors
About NTC Resistance?
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What is an NTC resistor?
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What types and types are there?
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NTC Resistors Purchase Criteria – What's it all about?
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Our practical tip: Handling and processing of NTC resistors
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FAQ – Frequently Asked Questions about NTC Resistors
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Conclusion: How to buy the right NTC resistor
What is an NTC resistor?
NTC resistors are temperature-dependant resistor components , also known as thermistors or thermistors. They change their electrical resistance in the opposite way as the related PTC resistors: Their resistance decreases as the temperature increases, so they have a negative temperature coefficient.
NTC resistors are made of different metal oxides, their base body represents a volume resistance. Artificially installed "defects" in the material, the so-called doping, increase the self-conductivity with increasing temperature.
NTC thermistors convert the physical properties of metals to resist electrical current as temperature increases. Technically, this is used in a variety of ways. NTC resistors are found in many areas of electronics and electrical engineering. The reason for this is the relatively low production costs, compact designs and the large range of available nominal resistance values.
The higher the temperature, the lower the resistance.
What types and types are there?
NTC resistors are available for different applications. Their specific properties are optimized for the respective application. Thermistor for measuring purposes and for temperature compensation have a nearly linear and relatively flat temperature resistance characteristic . This often eliminates the need for complex linearization by means of additional electronic circuits including microcontrollers, at least if there are no very high demands on measuring accuracy. However, if inrush currents are to be limited or an on-delay is to be achieved, a steep characteristic curve is desirable. Linearity plays either a subordinate role or an exponentially increasing course is desired.
NTC resistors are available as wired components in disc or drop design , as well as cast in aluminum cups for fixing by screwing . Either an external thread or a metal eye can be present. SMD designs are also used.
NTC Resistors Purchase Criteria – What's it all about?
In addition to the desired characteristic of the resistance temperature characteristic, the nominal resistance at nominal temperature (RN or TR), which is usually specified for 25 °C, is important when selecting the suitable thermistor. NTC resistors with different tolerances are available. For applications with increased accuracy requirements, it is recommended to favor types with low aging and long-term stability. When used under harsh environmental conditions, the resistance of the housing or the casing must be checked; special designs are offered for liquids. For measuring tasks and for temperature compensation, the self-heating of the NTC thermistor should be as low as possible in order not to falsify the measuring results. If the required measuring current and thus the resulting power loss at the NTC resistor cannot be kept sufficiently low, then we recommend choosing a larger, more resilient design with good heat dissipation. The allowable working area of the thermistor specified in the data sheet must at least correspond to the expected temperatures in use. Also calculate the self-heating by the resulting power loss.
Our practical tip: Handling and processing of NTC resistors
Even thermistors can only tolerate certain maximum temperatures . When soldering, observe the permissible soldering temperature and duration according to the data sheet to avoid damage to the protective sheath or irreversible deviations in resistance. If higher currents flow, the thermistor can heat up considerably. Therefore, avoid installation on the circuit board near temperature-sensitive components.
FAQ – Frequently Asked Questions about NTC Resistors
Why is there no voltage specification in the data sheet?
As with many "normal" resistors, this information is partially missing. In this case, the voltage load capacity is determined by the product by the voltage drop at the thermistor and the flowing current, i.e. the maximum permissible power loss.
What is derating?
The permissible power loss in mW or W is usually specified for an ambient temperature of 25 °C in the data sheet. Deviating temperatures or poor heat dissipation will lead to a reduction in nominal power loss.
Conclusion: How to buy the right NTC resistor
Thermistors with flat and linear characteristics are suitable for measuring tasks and for temperature compensation. When it comes to inrush current limiting and delay circuits, types with the steepest possible characteristic and a sufficiently high current carrying capacity are the right choice. If temperatures of liquids or vapors are to be detected, hermetically sealed versions are available for this purpose, which can be screwed and sealed into the pipe or container wall.