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  1. #1
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    too hot if irrush current without NTC

    In my circuit using just NTC (norelay) to limit irrush current into a toroid transformer, the issue is NTC is too hot 150 degree. Is it OK for long term use? Or I must use a by pass relay.

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    Re: too hot if irrush current without NTC

    Hi,

    An NTC needs to become hot to properly work as inrush current limiter.
    Whether 150°C is OK or not you should find in it´s datasheet.

    For my taste: I don´t like to have a part in my developed devices that gets that hot at normal condition.

    Klaus
    Please don´t contact me via PM, because there is no time to respond to them. No friend requests. Thank you.



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  3. #3
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    Re: too hot if irrush current without NTC

    Ideally - have the inrush NTC on long leads so that the solder joints do not degrade over time from over temp - this is quite a common fault on many psu's ...


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    Re: too hot if irrush current without NTC

    Also, if you give up on the NTC...(as you note, it doesnt really work when hot)...then you may not want to use a relay to switch out a inrush resistor...instead, you can use a cheap 1W DCDC module, and get it to switch back to back IGBTs which short out the inrush resistor....this saves having to use relays which have a limited lifetime....

    use back to back igbt's if you have the inrush res in the AC part...but just use single fet if its post rectifier bridge



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    Re: too hot if irrush current without NTC

    Once you have a bypass relay I like a PTC. Epcos makes a line that’s joule rated for inrush applications.

    The advantage is that’s its self protecting and fail safe in case the relay fails or the unit is repeatedly turned on/off.



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  6. #6
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    Re: too hot if irrush current without NTC

    the issue is NTC is too hot 150 degree...
    150C is not really "too hot"- yes it is too hot to touch but so are many power devices. Most power devices have a junction temp (max) rating >150 and a max case temp of 150C or so. By the way, NTC devices are more robust in their design compared to many power transistors and they do take lots of abuse.



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