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From: Hammy on 25 Jul 2010 07:34
On Sun, 25 Jul 2010 15:39:36 +1000, Grant <omg(a)grrr.id.au> wrote:


>
>In particular, note how the Y cap on left goes from neutral to
>earth, which is also the DC output negative line.

Are you saying that safety earth ground is used for secondary
refrence---> return currents?

Strange I didnt think any current other then small leakage or fault
current was allowed into earth.

I know its not uncommon to have earth ground on the secondary but its
for fault detection. Primary to secondary insulation failure not for
secondary return currents.

If that is the case wouldnt this cause your earth ground which is
ideally at 0V to bounce all over the place? Given transformer DCR and
trace R and the high peak currents bounceing around the secondary.
..

From: Grant on 25 Jul 2010 18:43
On Sun, 25 Jul 2010 07:34:25 -0400, Hammy <spam(a)spam.com> wrote:

>On Sun, 25 Jul 2010 15:39:36 +1000, Grant <omg(a)grrr.id.au> wrote:
>
>
>>
>>In particular, note how the Y cap on left goes from neutral to
>>earth, which is also the DC output negative line.
>
>Are you saying that safety earth ground is used for secondary
>refrence---> return currents?

Yup, madness in there :(
>
>Strange I didnt think any current other then small leakage or fault
>current was allowed into earth.

Same here.
>
>I know its not uncommon to have earth ground on the secondary but its
>for fault detection. Primary to secondary insulation failure not for
>secondary return currents.

What I have seen is earthed screens (electrostatic screen?) in
power transformers, single turns of foil (non-shorting) between
primaries and secondaries, connected to mains (safety) earth.
>
>If that is the case wouldnt this cause your earth ground which is
>ideally at 0V to bounce all over the place? Given transformer DCR and
>trace R and the high peak currents bounceing around the secondary.

That's exactly what I think is happening, seeing high injected
current to earth, which is also DC output negative. There's no
earth to the transformer indicating shielded windings.

The RCD is tripping at about 12mA rms, as reported by the Tek
TDS3034, dunno if that is true rms, ought to be, for an instrument
with AU$13k replacement value ;) But there may be some vagueness
in the measurement as I don't have a diff amp input module for the
DPO, and I'm wary of isolating the mains ground to it. After all,
we got safety earth fitted for a reason.


In general, the product seems a new design (over-designed, too many
internal connectors for a tight production design), by somebody
who's not read the book on modern mains rules or layout techniques
very well (example, the ref. des. -> part value, not a part
reference).


Contrast this power supply with others I often see (at similar
power levels) that has multiple CM chokes, several X caps,
usual Y caps, TVS + PTC components, and an obvious divide between
mains and output sides. Might add some examples of what I consider
decent layouts for the 250W power level. Most I see are switching
battery chargers, 6A to 10A into 12 cell SLA or VRLA batteries.


The problem power supply is sold by a .au & .nz nationwide
electronics chain (Jaycar), you'd think they'd follow the rules?

I dunno which authority over here to ask if this does meet the
C-tick mark it claims. All stuff sold here is supposed to have
one, on the one hand, but is ignored by some store chains on the
other, seemingly because they can claim to be simple importers
and sellers? But I doubt Jaycar can claim that.


It doesn't meet the rules as I remember them from decades ago,
particularly the physical isolation barrier between mains and
secondary.

But then, I once return a PC PSU under warranty when its active
PFC board went Bang! --> tracks too close together for 240V.
Oddly enough, the warranty replacement was a different model by
same manufacturer, indicating a design flaw?

Grant.
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