NTC in power supply

[alok1982]

Why to use NTC at input of power supply? How to select NTC with a particular rating?

 

[alec_t]

It reduces surge current at power-up, when it is cold and has a high resistance. Current flow then warms it and its resistance drops to a low value. The power rating of the NTC should exceed the power it dissipates when in the low resistance state. The NTC current rating should exceed the current flowing in the low-resistance state.

 

[JimB]

Assuming that you mean a Negative Temperature Coefficient resistor, which can be wired in series with the supply to the PSU.

The NTC resistor will limit the inrush current, that is the current surge which occurs when a power supply is just switched onto the incoming mains supply.

The surge is caused by the initial magnetising current in the transformer, and, any reservoir capacitors on the DC lines after the rectifier.

I have never done calculations for such an application myself, but the things which you must consider are:
The normal running current into the PSU, that must be sufficient to keep the NTC hot enough to give a low resistance to minimise the voltage drop across the NTC.
The resistance when the NTC is in its cold state must be high enough to limit the inrush surge, yet low enough for there to be sufficient current for the PSU to start up and draw current to heat up the NTC to its hot working resistance value.

JimB

 

[crutschow]

Generally the NTC rating is determined by the minimum resistance and current rating needed for the NTC in its hot state. You then must live with the surge limit that it provides in its cold state.