[sdiy] scsi shock

[cyborgzero at comcast.net]

One word: REFERENCE. After having worked on 3000 volt systems, I can
honestly say that referencing is *everything*... I watched 3 mos of
work go up because of a leaky cap.. It was just bad enough to allow a
plasma ball the size of a basketball rise and jump around destroying
every piece of work we had for the test fixture.

I have also seen a phone get hooked up to a phone line with no
referencing, and *boom* a big flash shot out of the phone... No
referencing at all....

Bad/damaged/leaky caps can do this sort of thing, esp if they are used
to reference an external system....

Rob

----- Original Message -----
From: "Tim Ressel" <madhun2001 at yahoo.com>
To: "Dave Krooshof" <synthos at xs4all.nl>; <synth-diy at dropmix.xs4all.nl>
Sent: Tuesday, May 07, 2002 5:38 PM
Subject: Re: [sdiy] scsi shock


> The answer, in a word, is leakage. All transformers
> have a certain amount of leakage. Because of the high
> impedance bewteen the primary and secondary, this
> leakage can build to quite a large fraction of the
> input voltage. Usually the secondary side is
> referenced to ground in some meaningfull way, and this
> tends to mitigate the problem.
>
> This can be a wierd one to solve. Sometimes when I am
> hooking up stereo and/or video gear, I'll get a hum
> from a ground loop or leakage. I'll solve it by
> identifying any plugs that are not polarized (2-prong
> US plugs), and turn them around one at a time unti the
> hum goes away. It's strange, but it works.
>
> In your case, i think what happened is this: Leakage
> from one or more pieces of gear is causing a large
> voltage differential. this is a high impedance thing,
> so the minute you attach the cable, the voltage gets
> shorted out. You probably got across the circuit while
> it was open and charged.
>
> One solution is to get a power strip with a switch (if
> they have such things where you live), and plug it all
> into the power strip. One flick of the switch and no
> more shocks.
>
> --TR
>
>
> --- Dave Krooshof <synthos at xs4all.nl> wrote:
> > Hi list,
> >
> > I email the list as we had some nice discussions on
> > grounding and shocks.
> > Here's another story to share, and questions to
> > ask...
> >
> > I'm typing with a sore hand, as I just got shocked
> > by my
> > image scanner (apple color onescanner) while
> > patching it.
> > The SCSI chain is: Mac G3 -> CD Toaster -> Scanner.
> > All worked fine this very evening. Nothing got moved
> > a lot.
> >
> > I just patched the cd toaster back into the chain,
> > with both the computer and the scanner switched on.
> > (yes.)
> > I was holding the 'outgoing' SCSI cable from the
> > toaster
> > and touched the connector on the scanner with that
> > same hand.
> > > Bzzzap! <
> > Took a voltmeter, found 126 volts AC.
> > The wall socket is 239 volts at the moment (good
> > weather today).
> > It's a zero/phase/ground wall socket. All equipment
> > shares 1 socket.
> > The scanner doesn't seem to have any short circuits,
> > as far
> > as I can see now.
> >
> > questions are:
> > - Why didn't my computer crash, or smoked the SCSI
> > controller
> > or simply die?
> > - Why didn't the safety ground switch flip? (It does
> > work normally)
> > - If a short circuit is there, why isn't it 5, 12
> > dc, or 230 volts AC?
> > (can't test the voltage on the lamp, as it can't be
> > switched on while
> > not connected to the mac. No trace of 126 volts
> > anywhere inside).
> >
> > Any hints on finding the error are welcome.
> >
> > I'll give my hand a rest, now.
> >
> > Thanks,
> >
> > Dave
> > --
> > ____________________________________________________
> > And I will make thy windows of agates, and thy gates
> > of carbuncles,
> > and all thy borders of pleasant stones. (Isaiah
> > 54:12)
> >
> >         <Please> no html-mail! html does not belong
> > in email. </please>
> >
> > http://www.xs4all.nl/~krooshof
>
>
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