NTC For Power Supply
NTC Thermistor devices are made of a specially formulated metal oxide ceramic material which is capable of suppressing high current surges. TP type NTC devices, being of relatively high resistance, shall limit the inrush current for 1~2 seconds during which time the device decreases in resistance substantially to a point where its voltage drop is negligible. The devices are especially useful in power supplies (see FigA) because of the extremely low impedance of the capacitor being charged, of which the bridge is usually subjected to an exceedingly high current surge at turn-on point. NTC Thermistor of Inrush Current Limiting High inrush
As shown in Fig. , the current surge can be eliminated by Placing a NTC thermistor in series with a filament string. Yet, if the resistance of one NTC thermistor does not provide sufficient inrush current limiting functions for your application, two or more may be used in series or in separate legs of the supply circuit (Fig.A). Be noticed, the thermistor cannot be used in parallel since one unit will tend to conduct nearly all the current available. Thus, thermistor may be used in the AC (point A1 or A2) or the DC(point D1 or D2) locations in the circuit.(See Fig. A) The resistance of NTC thermistor is designed higher than the total resistance of filaments when the circuit is turned on. As current begins flowing, the thermistor shall immediately self-heat . Then, in 1~2 seconds, its resistance will be reduced to a minimum and become insignificant to the total resistance of a circuit. With the same concept, current surges in electric motors can be held to minimum. Fig. C shows a typical DC motor s turn on surge before and after the application of a TP type thermistor to the circuit.
Details From WMEC Application