RTIE RTI Electronics | Surge-Gards Thermistors

RTI Surge-Gards Thermistors
Inrush Current Limiters

RTI Electronics manufactures SURGE-GUARD™ Inrush current limiting devices (thermistors) 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.

NTC Thermistors
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.

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.Thermistor Terminology for Inrush Current Limiting DevicesRTI’s SURGE-GARDs ™ are used in many applications today that require limiting inrush current when power is applied to a system. The most popular application is the inrush protection of the AC current in switching power supplies (SPS). The primary reason for having surge current suppression in a SPS is to protect the diode bridge rectifier as the input or charging capacitor is initially charged. This capacitor draws significant current during the first half AC cycle and can subject the components in line with the capacitor to excessive current. The inherent equivalent series resistance (ESR) of the capacitor provides very little protection for the diode bridge rectifier. Use of the proper SURGE-GARD™ will provide maximum current protection when the power supply is turned on and allow the design engineer to select lower peak current rated diode bridge rectifiers for use in their SPS.

If the resistance of one SURGE-GARD does not provide sufficient inrush current limiting for an existing application, two or more may be used in series or in separate legs of the power supply circuit. SURGE-GARDs™ should not be used in parallel since one unit will tend to conduct nearly all the current available. SURGE-GARDs™ may be used in the AC input side or in the circuit on the DC line between the charging capacitors and the diode bridge rectifier circuit.

The term thermistor originated from the descriptor THERMally Sensitive ResISTOR. The two basic types of thermistors are the Negative Temperature Coefficient (NTC) and Positive Temperature Coefficient
(PTC). The NTC thermistor is best suited for precision temperature measurement. The PTC is best suited for switching applications. The NTC thermistor is used in three different modes of operation which services a variety of applications. One of the modes exploits the resistance-versus-temperature characteristics of the thermistor. The other two
modes take advantage of the voltage-versus-current and current-over-time characteristics of the thermistor.

IBS Electronics is an authorized distributor. We offer a broad range of thermistors from RTI. You’ll find NTC inrush current limiters, surge limiters, probes, and over-current protection PTC thermistors. For all your protection needs (switching power supplies, AC motors, UPS systems, frequency generators).