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Ever thought about ... ?

gkenyon
I was asked a series of interesting questions this week about fault protection and overload protection for a particular application. Some of these really make you think, and the physics doesn't always lead you where you think you'd go.


Dropping out of all this, was me pointing out something interesting which I wonder whether it's ever crossed the minds of contributors to this Forum ... so here goes.


Ever thought about what, in typical UK installations, protects the electronics in a plug-in [to a standard BS 1363-2 socket-outlet] phone charger / wall-wart type power converter against:

(a) Fault current (consider both cases of L-N and L-PE); and

(b) Overload current ?




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    AJJewsbury:
    I'm still slightly lost as to what the actual question is. A single USB charger shouldn't draw more than about 15W (0.065A) under any circumstances. But you seem to be asking what will protect against some massive power surge, under some unspecified conditions.

    It'll draw an awful lot more than 0.065A if it suffers from an internal short circuit. No power surge required.

       - Andy.


    I think it's inherent in the design of a wall-wart that you can never get a dead short between the live and neutral pins.


    The worst case that I can think of is where a diode in the bridge rectifier fails shorted.  In that case, you might get another diode in the rectifier and the EMI filter, suddenly seeing the mains voltage across them.  One or both would go "pop" rather violently.


    So long as that "pop" is contained by the case, it's fine.  They aren't designed to be repairable, so if that happened, you'd throw it away and buy a new one.


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  • 0
    AJJewsbury:
    I'm still slightly lost as to what the actual question is. A single USB charger shouldn't draw more than about 15W (0.065A) under any circumstances. But you seem to be asking what will protect against some massive power surge, under some unspecified conditions.

    It'll draw an awful lot more than 0.065A if it suffers from an internal short circuit. No power surge required.

       - Andy.


    I think it's inherent in the design of a wall-wart that you can never get a dead short between the live and neutral pins.


    The worst case that I can think of is where a diode in the bridge rectifier fails shorted.  In that case, you might get another diode in the rectifier and the EMI filter, suddenly seeing the mains voltage across them.  One or both would go "pop" rather violently.


    So long as that "pop" is contained by the case, it's fine.  They aren't designed to be repairable, so if that happened, you'd throw it away and buy a new one.


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