Problem with Offline Full Bridge DSO Probing...

[Mechatrommer]

i was probing as topic. refer to picture below. if i hook ch1 only, i get nice half positive rectified. the same if i hook ch2 only, except negative signal (all so far as expected), but... when i hook both ch1 and ch2 to v+ and v- respectively at the same time, i got distorted signal, why and how?

meditating of it, its like the DSO ground (or the rectifier's, not sure which) is brought up to the middle and both signal get superimposed, probably by 1Mohm resistor to earth ground at both DSO channel/probe. but how DSO ground (earth) brought up, coz even though the offline circuit is unearthed, from what i've learnt, neutral wire already tied to earth ground in someway. so how can dso ground (earth) is elevated to middle potential compared to offline neutral (also an earth potential)? any explanation? (the dark side is shrouding the force, i have a headscratch)

[TerminalJack505]

I'm going to guess that you have the probe's ground clips connected to two different points.

[Mechatrommer]

then you guess wrong. ground clips are disconnected.

[w2aew]

There is always 344V between V+ and V-, just as you would expect from a full wave bridge.  The common mode change of V+ and V- likely due to the floating ground connection and the resulting reaction of the scope's ground and your circuit. 

[Mechatrommer]

There is always 344V between V+ and V-, just as you would expect from a full wave bridge.  The common mode change of V+ and V- likely due to the floating ground connection and the resulting reaction of the scope's ground and your circuit. 

yes, the differential between v+ and v- is always 330-344V, but the hard ground is missing likely due to the bridge topology itself, where i'm suspecting the common mode/ground can shift up and down if high impedance to ground is provided. my scope is hard grounded to earth (ds1052e), iirc last time, connecting ground probe directly to anywhere in the bridge (or subsequent circuit) will trip the elcb, but not if high value resistor is placed in between, which in effect exhibit this peculiar behaviour. the same effect when connected to +ve probe of dso, which has 1Mohm impedance to "hard ground", which what was done above. i believe the mystery lies in the bridge topology, i need to trace the current flow closely when i have time.

[ejeffrey]

then you guess wrong. ground clips are disconnected.

Well that is even worse.  don't do that.

In this case, I guess that your scope's ground isn't connected.  Either that or your circuit is floating.  This means that you have no return current path for the signal current except for capacitive coupling.  Fix your grounds and it will be fine.

[Mechatrommer]

ground "clips" are disconnected... dso is "hard" grounded to earth from its power cord... +ve probe have 1Mohm impedance to the "hard" ground. i'm probing directly at mains/offline. is there anything wrong with that? sure if the neutral line is floating, then... the circuit/bridge is floating, but afaik, its not. i cannot connect my gnd probe to anywhere in the circuit, elcb will trip. what i'm trying to figure out why the bridge seems to be floating, which connected directly to neutral line, which afaik is not floating (connected to earth in someway outside my home).

ps: i know (and just read again) probing mains is a "dont do" act, but well...