[sdiy] scsi shock/on the subject of shocks

[Magnus Danielson]

From: Dave Krooshof <synthos at xs4all.nl>
Subject: Re: [sdiy] scsi shock/on the subject of shocks
Date: Fri, 10 May 2002 00:25:26 +0200

Hi folks,

Been too distracted to be able to answer this up till now. There is
certainly the issue of the transformer leakage. It's real and I've
been bitten hard time with it... (wasn't my fault/design, but I had to
analyse it). You also have the Line-filter of most new devices which
sits there even when the power-switch is off, also adds to the leakage.

There are two things necessary to "heal" transformer leakage:

1) Protective ground connection
2) Signal ground to Protective ground bonding

You need the protective ground connection since this is most of the
times the only source of a "Neutral" voltage, i.e. a voltage which
should not shock either you or any equipment, that you can trust
(naturally things can fluke, but you should allways design cables etc
to ensure the protective ground is the last thing you looses
connection too).

The bonding of Signal Ground (actually, the secondary side of the
transformer) to the Protective Ground should be done through a
resistor or possibly a resistor shunted with a suitable capacitor.
Suitable values in this case is to ensure leakage does not generate
any excess voltages. Taking the voltages below 1 V should be the
minimum effort.

Cheers,
Magnus - seen DSPs reprogrammed into filters dues to transformer
leakage - fixed with a 250k resistor!