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Full bridge rectifier shows wrong curve on scope

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StillTrying:
"One probe is on the positive bridge rectifier output (red/orange wire), i.e., scope channel 3 (pout). The negative output (black wire) is connected to scope channel 4 (nout)."

At least the overall shape of those 2 bottom traces look close enough to the bottom 2 on my simulation, and would be even closer if the screen's 0V were in the same place.:)

"and checked it was really going from 1.65V to -1.65 V (approximately)."

Barely enough voltage for a bridge rectifier.

I don't know why the scope's ( CH3-CH4 ) is not working, it looks like it's doing ( Noise-CH4 ).

It looks like you might have one probe on X1 and one on X10, which would help to confuse the MATH.

alsetalokin4017:
Your math is trying its best..... even though "it's cheating".   >:D

But what do you expect the math to look like when one of your signal channels is ten times the amplitude of the other?

I'll put this here again so people don't have to look for it on the first page.  Consider this an exercise in scoposcopy.    :wtf:



(please note that all the numbers have meaning and correspond to what is actually shown by the traces on the graticule)

petert:
Thanks, you were both (@StillTrying, @alsetalokin4017) right regarding the scaling of the probes. I was trying out so many things I didn't notice that anymore haha.

After I had both probes on the same setting, it got a similar result to yours, even if somewhat noisy.

Regarding the "cheating": the reason was that I was trying to find out why the isolation does not work after unplugging the earth lead from both the scope and signal generator, which still puzzles me. (Which was a good example use of the math function, though ;)


Maybe weird house wiring? A UPS seems like the easiest way to isolate the signal generator (if I had one :D). Any other ideas?

StillTrying:
Marvelous, I looked here to see if you'd got there yet. :)

I'm not 100% convinced that your generator's 0V is earth grounded, can't you measure Ohms between the scope's GND clip and the generator's 0V, anything less than about 1k Ohms would mean it is grounded.

"somewhat noisy."

A BR's DC output is often a bit noisy without a few k of load on the DC side to keep the diodes conducting with no non-conducting open circuit time.

Even when the input side IS floating you still can't short the input to output of a BR with 2 scope probe GNDs. To capture this one I saved the input sine on screen and then moved the scope's GND and probe to the DC output side.
https://www.eevblog.com/forum/beginners/fwb-full-wave-bridge-rectify/msg1117268/#msg1117268


Ysjoelfir:
Just a shot in the blue: To isolate the grounds of your sig gen and scope... you didn't happen to use a multi-outlet extension cord, isolated the ground of that, and plugged both devices into the same extension? That would make both devices basically isolated from mains earth/ground, but the grounds of the scope would still be connected to the ground of the signal generator.



This whole "isolation"-Thing was a perfect example that university professors are sometimes dumb as hell, as one of those blew up three brand new scopes before he trusted me when I told him that he is right now shorting the 3 phase mains over the gnd clip... I got a bad grade in the lab report for pointing that out though...

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