On Mon, 29 Feb 2016 21:47:38 +0100, Dimitrij Klingbeil wrote:

The probe tip is not the issue, but the scope itself, hanging from the
probe ground, that is.

P.S.

There is a simple though unwritten rule about power supply testing:

"Never connect the ground (common, chassis etc.) of any test equipment
to the switching node (power transistor collector, drain or power IC
output pin and its associated signals) of a switching power supply!"

It is valid for all types, no matter if flyback, forward or resonant.

The reason for this rule is that a "switching node" usually drives a
square wave with high voltages (some 500 to 600 V in a flyback, may
happen to be as much as 800 or 1000 V in a resonant one), and that a
significant amperage is readily "available" at that node too, due to the
output transistor's low impedance. Neither is the supply designed to
safely drive that into "RF ground" nor is the test equipment made for
being "muscled around" at that sort of voltages and dV/dt rise times.

Grounding the test equipment would mean that the whole power supply
(plus any safety isolation transformer) is being swung around and
letting the test equipment "float" would mean to also swing around the
test equipment. Apart from the obvious safety hazard, this can also
damage the test equipment and even compromise the test equipment's
electrical safety by frying the "Y" capacitors between mains and
secondary or stressing the isolation barrier in the test equipment's
power supply and / or mains transformer, possibly beyond the level of
stress that it was rated for.

So, whenever you troubleshoot some switcher, take heed of this rule.

It's simple to remember, and it can save lives, test equipment,
and some power supplies under test too

I'm grateful for that expansion, to be honest. I was kind of struggling
to get my head around what you were getting at in your earlier postings;
didn't make much sense to me on the first read through and although after
a second read I was beginning to sense your meaning, it still wasn't 100%
clear.
At least now I think I can finally see where you're coming from.
Naturally I read up on safety precautions when dealing with switchers
from books I have and all sorts of diverse sources on the net, but I can
honestly say that what you have outlined above has NOT been covered by
anything I've seen or read up until now. This would seem to be a glaring
omission on the part of those who we rely on to prime us up on the hidden
dangers and pitfalls of troubleshooting such equipment.
Another good reason for me to avoid dealing with switchers in future if
at all possible!!