On Tue, 23 Oct 2018 at 04:07, AB Pearce - UKRI STFC <
alan.b.pearce@stfc.ac.uk> wrote:

> The answer to this is obvious.
>
> 1. The transformer has no shield between the primary and secondary
> windings.
> 2. One side of the  output is grounded to earth.
>
> This produces a situation where there will be capacitive coupling between
> the transformer windings which while of high impedance at mains frequency
> is still low enough to allow a small current to flow. This is then
> rectified by the diodes on the transformer secondary and charges the
> capacitor.
>
> I was going to suggest that as a likely reason, but with less confidence
:-).

______________________

Another nice "trap" is to have two "Y" capacitors on the input side from
each AC line to chassis ground, and have output ground also connected to
chassis ground, BUT no external ground connected to chassis ground. The
chassis is now at (about) VAC/2 and various other couplings may then allow
various magic to work.

With typical Y cap values you can typically feel the 1/2 AC chassis voltage
as a not-nice sharp but not too too painful bite on the backs of your
fingers. Touching the chassis with a firm finger contact MAY not cauise any
shock sensation.

Connecting the equipment to another item via a signal lead that is chassis
grounded at both ends (eg PC to printer with only one of the two items
having a mains ground) can result in destruction of the port at one and/or
other end and possibly more complete destruction of either item. Long long
ago I had a new Centronics-cable-connected printer destroyed on first being
plugged into a PC's printer-port because only one of the two had an earth
wire in its cord :-(. (That's one way to learn such lessons).



Russell
--=20
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