I've been wondering where the GND path could come in.

But well, it could exist. The only path: The capacitor (-) GND goes to
a PWM driver board, the GND of which goes to a DSP board, the GND of
which goes through the USB to the PC.

I just wonder whether the PC GND (0V) =3D Earth. If that's true, yes the
capacitor (-) would be at Earth potential. Wonder what the PC SMPS
would be doing ..

Thanks for the explanation.

Cheers,
Manu


On Mon, Oct 22, 2018 at 8:38 PM 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 windin=
gs.
>
> 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 rectifie=
d by the diodes on the transformer secondary and charges the capacitor.
>
> The way around this is to have an earthed shield between the primary and =
secondary windings. A simple form of this is a single layer winding with on=
ly one end brought out for connection to ground. A properly done shield is =
a layer of copper tape with the two ends insulated from each other so it do=
esn't form a shorted turn, and then a wire is soldered onto it to bring the=
 grounding connection out. The shield always has to be grounded to be effec=
tive, if it is not then the original scenario described below will exist.
>
>
> -----Original Message-----
> From: piclist-bounces@mit.edu <piclist-bounces@mit.edu> On Behalf Of Manu=
 Abraham
> Sent: 21 October 2018 11:03
> To: Microcontroller discussion list - Public. <piclist@mit.edu>
> Subject: [EE] Voltage accumulation
>
> Hi,
>
> I did not forget to put the sq brackets this time, Bob . :-)
>
> Not a query, but tripped on this one, wondered for a while, later on
> alone realization dawned.
> Thought about sharing this one on the list.
>
> Have a 230V/12-0-12 CT transformer, of which 0 &12 connected to a
> Bridge rectifier, the output of which is smoothed out by a 680uF/200V
> capacitor, providing about 16VDC (12 x 1.4142). The power supply
> worked well and good enough.
>
> The transformer was left connected to the mains for a while, in absent
> minded mode of operation. A few days later, just before wanting to
> test a small board measured the voltage on the power supply, saw it
> was about 80V.
>
> Sat in thoughts for about an hour what was wrong, thought maybe the
> transformer windings had sprung a leak. Thought about it for a moment,
> then thought probably the capacitor was getting charged due to no
> load.
>
> Discharged the capacitor with a huge spark. Things pretty much settled do=
wn.
>
> At least thoughts burned up a few hours, where that 80Vdc was coming from=
..
> After that, sat back and laughed at myself.
>
> Cheers,
> Manu
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