XPost: alt.binaries.chatter   
   From: no@spam.com   
      
   On Wed, 01 May 2013 22:38:58 +0100, Ian Field    
   wrote:   
      
   >   
   >   
   > "Major Scott"  wrote in message   
   > news:op.wwevayr22eh2io@red.lan...   
   >> On Wed, 01 May 2013 15:59:33 +0100, David Moorman    
   >> wrote:   
   >>   
   >>> In article , "Major Scott"    
   >>> wrote:   
   >>>   
   >>>> I have a USB hard disk enclosure, and I'm running it without it's case   
   >>>> as I'm   
   >>>> testing another drive with it.  When I touched the hard disk with my   
   >>>> hand and   
   >>>> the case of the computer (which is earthed) with my arm, I felt a   
   >>>> tickle.  So   
   >>>> I connected a multimeter to it, and the hard disk is at 90 volts AC with   
   >>>> a   
   >>>> short circuit current of 0.136 mA.  Is this normal?  Is this a good idea   
   >>>> when   
   >>>> working with delicate computer electronics?  I suppose the ground of the   
   >>>> USB   
   >>>> lead will get rid of it but it seems a bit odd.  The USB disk unit is   
   >>>> powered   
   >>>> by a cheap wall wart that came with it, rated 12V DC 1.5A.   
   >>>   
   >>> Something is effed up.   
   >>   
   >> It works though.   
   >>   
   >> I've had a similar voltage out of the composite output of a VCR.  That's   
   >> the trouble with these double insulated things that aren't earthed.   
   >   
   > Any double insulated SMPSU has filter caps from each mains pole to the   
   > equipment 'ground' - they form a capacitive voltage divider (Xc + Xc).   
      
   That sounds rather strange to me.  Especially where delicate electronics are   
   involved.  Imagine I connected the USB hard disk, while it was powered up, to   
   the computer, and the USB plug's grounded chassis managed to touch one of the   
   signal contacts.   
      
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