>-----Original Message-----
>From: Howard Winter [mailto:HDRW@H2ORG.DEMON.CO.UK]
>Sent: 22 June 2004 10:47
>To: PICLIST@MITVMA.MIT.EDU
>Subject: Re: [EE:] USB powered coffee warmer
>
>
>Michael,
>
>On Tue, 22 Jun 2004 09:16:52 +0100, Michael Rigby-Jones
>wrote:
>
>>...<
>> I suspect they use polyfuses for protection.  As these
>operate thermaly,
>> rather than by directly sensing current, I suspect
>they make take quite some
>> time to trip if the current overload is marginal. That
>could explain what
>> you saw on your hub?
>
>Hmmm... I'm not familiar with hos polyfuses work - do
>they have to have power removed to reset?  I sort-of
>thought they'd reset when the fault current was removed.
>

They are basicaly a type of PTC, they have a low resistance normaly, but
under fault current conditions they heat up and the resitance increases
greatly.  Simply removing whatever is drawing too much current will allow
them to reset.

Very handy devices for this type of application; cheap and resettable with
no user intervention.

Regards

Mike




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