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NTC Thermistors

NTC Thermistors
2010-11-16

Negative Temperature Coefficient (NTC) thermistors are thermally sensitive semiconductor resistors which exhibit a decrease in resistance as absolute temperature increases. Change in the resistance of the NTC thermistor can be brought about either by a change in the ambient temperature or internally by self-heating resulting from current flowing through the device. Most of the practical applications of NTC thermistors are based on these material characteristics.

Inrush Current Limiting Devices

RTI manufactures Exsense inrush current limiting devices using specially formulated metal oxide ceramic materials. These devices are capable of suppressing high inrush current surges. They are especially useful in power supplies where the low impedance of the charging capacitor exposes the diode bridge rectifier to an excessively high current surge at turn-on.

Thermistor Terminology for Inrush Current Limiting Devices

  • IMAX - The maximum steady state RMS AC or DC current.

  • IOP - The actual operating current.

  • RIMAX - The approximate resistance under maximum steady state current conditions.

  • MAX Operating Temperature - RTI's recommended maximum ambient temperature is 65°C without de-rating. (Ref. Fig. C for de-rating information)

  • Recovery Time - Exsense devices require time to return to their ambient resistance state in order to provide adequate inrush current limiting at each power turn-on. This time varies with each device, the mounting configuration and the ambient operating temperature. RTI recommends a minimum of 60 seconds. The selection of a capacitor bleeder resistor can reduce the required cool down time requirement.

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