Re: Shock from USB printer cable

From: Keith Weisshar (keith.weisshar_at_verizon.net)
Date: 09/13/04 

Date: Mon, 13 Sep 2004 21:41:35 GMT

It's not a linear power supply. It's a switch-mode power supply in a
rectangular box mounted to the bottom of the printer with a screw. The
label on the black box says AC adapter and has 120-220VAC input and a 24VDC
output.

"petrus bitbyter" <p.kralt@reducespamforchello.nl> wrote in message
news:jTe1d.86129$C7.64538@amsnews05.chello.com...
>
> "Keith Weisshar" <keith.weisshar@verizon.net> schreef in bericht
> news:ryb1d.6770$sX2.4401@trndny09...
> > I don't have an oscilloscope available.
> > "Michael A. Covington" <look@ai.uga.edu.for.address> wrote in message
> > news:4144c3a9$1@mustang.speedfactory.net...
> > > Anything that will light a neon bulb is MORE than enough to destroy
> > digital
> > > circuits, and probably enough to endanger a human being.
> > >
> > > Do you have an oscilloscope available?
> > >
> > > "Keith Weisshar" <keith.weisshar@verizon.net> wrote in message
> > > news:KgR0d.237$W73.48@trndny03...
> > > > The movement of the voltmeter isn't fast enough to measure short
> pulses.
> > > > The neon tester glew dimly when placed between USB connector and PC
> > ground
> > > > but there isn't enough current to feel it except for the short
pulses.
> > > > The
> > > > neon tester goes out with even a very light touch between the USB
> > > > connector
> > > > and the PC ground. The slight shock is felt only if I touch the USB
> > > > connector and PC case firmly, not if I touch it lightly. There
isn't
> > > > enough
> > > > current to cause a spark when connecting the USB connector to the
PC.
> > > >
> > > > "Michael A. Covington" <look@ai.uga.edu.for.address> wrote in
message
> > > > news:4143c829$1@mustang.speedfactory.net...
> > > >>
> > > >> "Keith Weisshar" <keith.weisshar@verizon.net> wrote in message
> > > >> news:xmO0d.565$VV2.219@trndny06...
> > > >> > The USB cable sends periodic pulses to the PC even when off and
> this
> > is
> > > >> > what
> > > >> > is actually felt. It's not a standard 60hz shock like from a
wall
> > > > outlet
> > > >> > which would be much worse than a shock from the USB connection.
Do
> > you
> > > >> > have
> > > >> > any USB printer and have you gotten the same shock from the USB
> > > >> > connection?
> > > >>
> > > >> To feel a shock you need about 25 volts. Something seems to be
> wrong.
> > > >> My
> > > >> next move would be to use a voltmeter to measure the pulses. I
think
> > > >> there's some kind of leakage in the power supply of either the
> printer
> > or
> > > >> the computer.
> > > >>
> > > >>
> > > >
> > > >
> > >
> > >
> >
> >
>
>
> Keith,
>
> You need to kow:
> - To feel a shock requires some tens of volts and some milli amperes of
> current albeit for a very short time.
> - To ignite a common neon bulb requires about 90V and it extinguishes
below
> some 60V. Current in the micro ampere range.
> - To measure voltages you need an electronic instrument with a high
> resistance (>10MOhm). This is because you're dealing with high impedances
> (otherwise you'd been dead already.) Check for AC *and* DC voltages.
> - Equipment with a two pin mains connector are - or should be - "double
> insulated". That's to say none of the metalic parts you can touch has a
> galvanic connection with the mains or the ground. They're floating.
>
> Assuming your mains installation is OK and following the discussion so far
> the only thing I can imagine the cause of your shocking experiences is a
> leaky power supply of the printer. That leak can be capacitive i.e. the
> parasetic capacity of the transformer and/or a deliberately placed
capacitor
> meant for decoupling. They are not necessarily defective but I can't see
> that on this distance. It is also possible that these - or other
components
> lost a part of their insulation resistance and became leaky.
>
> It's hard to proceed without some electronic skills and equipment. You can
> try another power supply or printer of the same type and look for
> differences. I'd try an insulating transformer to do the same. A good one
is
> shielded inside and has very low parasetic capacity to the mains. Even a
> less professional one may solve your problem... if you can find one.
>
> petrus bitbyter
>
>
>
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