Hello,
I have some doubt’s/queries about the oscilloscope probing on a transformer.
All the below that I am writing is what I have understand from personal reading and searching and must not be taken as 100% right.For safety reasons the negative clips of a
bench oscilloscope are connected between (there is the exception of a portable oscilloscope with the
option of isolated channels) them and all together to the PE Ground pin of his power socket. This probe’s PE Ground connection causing some “problems” when it comes the probing of the oscilloscope to a device/PCB.
At figure 1 because Neutral wire is connected through the oscilloscope’s negative clip to the PE ground this will lead to the imbalance currents between Line and Neutral conductors on the RCD breaker and the result will be the RDC triggering.
At figure 2 there is a short circuit between Line and Earth-Neutral (if electrical system has Neutral Grounding).
Figures 3 & 4 are OK and will not cause any short circuit because of the transformer’s galvanic isolation;
from the other hand on high frequencies because of the parasitic capacitances between transformer’s core & windings and in addition of the oscilloscope probe’s characteristics like attenuation and capacitance
the “OK” in respect of the short circuit and safety it depends.
Now this that I can’t understand and be sure is why/if the probing of the oscilloscope on the figures 5 and 6 (7 & 8 - secondary output 24Vrms) are wrong. This probing I have only tried
with portable oscilloscope with isolated channels and with bench oscilloscope
but with differential probe at the transformer’s primary winding.
The oscilloscope or the circuit at the figures 5 and 6 (7 & 8 ) will blow out or just the measurement will be wrong?
I believe I missing something between potentials of PE Ground in respect to transformer’s secondary “virtual” Ground (I don’t know if there is an official term for this Ground), so for this I can’t understand the whole concept. I remember that I read on the internet that only the PE Ground has truly zero potential and the “electronics” Ground (officially Common Gound) is not zero but just a reference point where is considered zero; this is what I can’t understand. If you have any book or link or your opinion to suggest me all are welcomed.