Re: Shock from USB printer cable

From: petrus bitbyter (p.kralt_at_reducespamforchello.nl)
Date: 09/14/04 

Date: Tue, 14 Sep 2004 23:23:40 GMT

"Keith Weisshar" <keith.weisshar@verizon.net> schreef in bericht
news:juo1d.13904$z_3.10759@trndny07...
> "petrus bitbyter" <p.kralt@reducespamforchello.nl> wrote in message
> news:jTe1d.86129$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
> >
> >
> >
> > ---
> > Outgoing mail is certified Virus Free.
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> >
>
> > 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.
>

It nevertheless contains a transformer to separate the mains from the
printer. These type of power supplies use to run on >25kHz and needs much
smaller a transformer then the ones that run on 50/60Hz. The transformer
core is a ferrite instead of iron. But it is a transformer. There even may
be a second, much smaller, transformer used for the feedback although most
of the times a opto coupler is used for it these days.

petrus bitbyter 

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