PTC Thermistors
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About PTC resistors
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What was a PTC resistor?
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What types and types of PTC resistors are available?
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Purchase criteria for PTC resistors – what is it all about?
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Our practical tip: Use as overcurrent protection
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FAQ – Frequently Asked Questions about PTC Resistors
What was a PTC resistor?
PTC resistors are also called PTC thermistors. They have a positive temperature coefficient (PTC is the abbreviation for positive temperature coefficient). This means that the resistors are functionally the exact opposite of NTC resistors (NTC for negative temperature coefficient), which are called thermistors. PTC resistors conduct the electrical current better in cold conditions than in hot conditions, their resistance increases as the temperature increases . The resistors, also known as PTC thermistors, are manufactured from ceramic materials, such as barium titanate or, in the case of self-resetting fuses, polymers.
PTC PTC thermistors consist of very small crystal elements that form blocking layers at their limits. If the defined Curie temperature is exceeded, the resistance increases and the PTC thermistor becomes increasingly high-resistance.
What types and types of PTC resistors are available?
PTC resistors are available in different versions for different applications. Types in ceramic thick film technology with almost linear temperature gradients are used as sensors mainly for temperature measurement and monitoring purposes . Ceramic-based PTCs and non-linear resistance characteristics are used, for example, as overcurrent protection, for inrush current control, as overheating protection and as self-regulating heating elements.
Polymer-based resistors serve as self-resetting fuses. They have a very steep temperature coefficient and become almost suddenly high-resistance after exceeding the rated temperature due to over-current self-heating .
PTC resistors usually have a drop or disc shape. Types with screw-on eyelet for thermal coupling to components or housings are also used.
Purchase criteria for PTC resistors – what is it all about?
The PTC thermistor must be fitted using the appropriate design and the appropriate mechanical dimensions. It is also important that the electrical properties match the application. For example, a PTC thermistor that is optimized for temperature measurement is neither suitable as overload protection nor is it possible to use it as a self-resetting fuse. Conversely, the same applies.
Each thermistor has a specific work area in which it has the desired properties. This information is provided in the respective data sheet, which contains the so-called resistance temperature characteristic. The graphic display allows you to easily select the appropriate electrical resistance. The assignment of temperature and resistance value is thus immediately recognizable and does not require long computational exercises on your part.
Characteristic of PTC thermistors is that their resistance depends not only on the temperature but also on the applied voltage and in a relatively hohem on the operating frequency. Please also refer to the product datasheet.
Our practical tip: Use as overcurrent protection
PTC resistors, which act as an overcurrent fuse, can heat up relatively strongly. It is therefore important that you position them on the circuit board with sufficient distance to heat-sensitive components.
FAQ – Frequently Asked Questions about PTC Resistors
For which operating voltages are PTC resistors suitable?
Depending on the version, PTC thermistors are suitable for voltages in the lower two-digit range up to several hundred volts for mains voltage applications.
What was the meaning of the term nominal resistance coefficient?
The nominal resistance coefficient (RR) is the resistance value in ohms of the unloaded PTC thermistor at a defined temperature . A temperature value of 25° C is often specified for this purpose, and can then be recognized, for example, by the designation R25.
Why is the resistance value also voltage-dependant?
At higher operating voltages, the field strength increases on the crystalline elements of the component. This results in resistance reduction.
Conclusion: So you buy the right resistor
Make sure that the PTC thermistor has sufficient voltage and current capacity. It should also be noted that each PTC resistor heats up with increasing current flow. This must be taken into account from the outset. If the PTC thermistor is to be used to detect temperatures, i.e. as a sensor, versions with screw-on options are available. This enables good thermal coupling when temperatures of heat sinks, motor housings, etc. are to be recorded. PTC resistors in a compact drop form are suitable for direct temperature monitoring of motor or transformer windings because they can be accommodated in the winding in a space-saving manner.
If the PTC resistor is to be used in line-voltage-operated devices or safety-relevant applications, make sure that the necessary test marks – e.g. VDE – are available.