🚀 go-pugleaf

Eeyore wrote:

> john@jjdesigns.fsnet.co.uk wrote:
>
> > Eeyore wrote:
> > > Rich Grise wrote:
> > > > Sylvia Else wrote:
> > > >
> > > > > I've recently bought a D-Link DIR-300 wireless router. While
> > > > > disconnecting its power supply from it, I got a tingling sensation when
> > > > > I touched the plug (to be clear, I'm talking about the 5V DC plug, not
> > > > > the mains plug).
> > > > >
> > > > > An AVO meter shows about 90V AC to earth from either side of the 5V
> > > > > plug. Short circuit current to earth is about 0.1mA.
> > > > >
> > > > > Still, I thought these things were meant to be completely isolated. Has
> > > > > there been a change of approach?
> > > >
> > > > If the wart is 2-prong, turn it around in the socket. If it's 3-prong,
> > > > get an outlet tester.
> > >
> > > Sylvia's experience is 100% normal. I've even modelled it in the past which
> > > is why I reckon the Y cap between the rectified pri and sec sides is 2n2.
> > >
> > > I was myself at the time specifically interested in the value of the leakage
> > > current and its waveform shape. It gives a great 'buzz' with lots of
> > > harmonics in poorly grounded audio systems. The audio groups regularly get a
> > > question why their laptop buzzes on the mains but not on battery power. About
> > > once a month on average.
> > >
> > > Cue EMC discussion ?
> >
> > I reckon whomsoever allowed a capacitor/resistor/whatever to straddle
> > a magnetically isolated, electrical safety barrier, ought to be
> > marched out of their keep-the-public-safe-committee office and shot.
>
> I could be tempted to agree.
>
> Needless to say there's a reason for the method which as I said you'll find in the
> EXCELLLENT app notes at powerint.com. It surprised me too.
>
> The transformer would have coupling capacitance anyway.
>
> This one's a real little runner. It can make life very much more complicated.
>
> BTW, try connecting a modern printer to your PC at night with low illumination.
> Look out for the sparks.
>
> Graham

That truly is an excellent site. I went to have a quick check to check
on this puzzle  and was there an hour. Will be returning.
After running through 20  <10Watts  reference design pdfs, it seems
95% of their wall wart type external supplies -do not- have that mains-
DC straddle capacitor. Go beyond 10Watts and the 'unsafe' capacitor
starts appearing, usually 2n2.
Then I had to start reading the other app' notes to see what you were
specifically pointing at.
Only relevant refs I came across were AN14, AN20, AN15.
AN14 makes a coy coment about the capacitor helping common mode
"emission currents" (WTF!).
AN20 "Transient suppression" has this capacitor wired as part of a 3
capacitor splitter. Seems  a bodge arrangement so that  a 3kV input
transient turns up at the 'safe' DC output as only 1.5kV!.
Only actual technical reference I found was AN15 (emc design).
Specifically Figure 33 and their transformer stray coupling item
marked "CW4". The straddle capacitor being added to "augment" that
particular stray.
So, based on the above notes, I'm reading that this capacitor is
purely there for the manufacturers benefit!. It is fitted to help kill
off the nasty HF noise that naturally couples across via C strays from
the transformer primary to secondary.
Essentially, the manufacturer generates the sodding switched mode
noise in the first place then has no alternative but to find some
method of filtering their rubbish out of the floating DC secondary.
Only means they have is the naff bridging capacitor, which results in
the leaking AC, the tingles and the sparks in the dark.
Ban the damned things, that's what I say :)
Ps. My wall wart that blew some kit up had a Varistor fitted instead
of that cap!.