Electronics > Beginners
Oscilloscope: Square wave view while in FFT mode?
rstofer:
I would expect the mains to have both odd and even harmonics. I would expect spikes all over the place.
If I absolutely HAD to measure the mains, I would probably use a step-down transformer to reduce the voltage and remove the earth ground reference. A little 120-6.3V filament transformer would be ideal. There would be no ground reference in the output so I could connect my scope without worry.
The transformer itself will introduce and eliminate harmonics. Anything that distorts a pure sine wave will do something to the harmonics. But it might not be significant and certainly there is value in the added safety.
Did I mention that I don't probe mains? Of course I did! There's a reason I don't like jamming my scope probe in wall outlets.
rstofer:
--- Quote from: Wimberleytech on January 29, 2020, 06:31:53 pm ---
--- Quote from: rstofer on January 29, 2020, 06:28:08 pm ---
I look at signals on PCBs and breadboards and they are usually powered by a wall wart or floating power supply. The voltages are low and I like it that way. My idea of high voltage these days is 12V. No more of the high voltage, high current, industrial systems. I'm retired!
--- End quote ---
AMEN and ditto (but I splurge from time to time and go to 24 volts--automatic gates are sometimes 24 volt systems).
--- End quote ---
OK, just this once!
The problem with a forum reply is that it won't discuss ALL of the ways this kind of thing can go sideways. I'm perfectly will to say "I don't probe mains" if it discourages the practice. I can't be there to watch the process so I just try to discourage it. Have I done it? Sure! But I'm not going to talk about it.
james_s:
There's an easy and safe way to probe mains if you really want to do that, use a HV differential probe. I have a Tektronix P5205 that I use when I need to measure something like the primary side of a switchmode power supply. I've never bothered to look for harmonics on the AC line though, as others have said it doesn't really matter what's there because you can't do anything about it anyway. Just measure the output of the power supply, if you see noise you can work on figuring out the source but in most cases I would expect that it's internally generated by the power supply. It's pretty hard for noise on the line to get through a switchmode power supply, those tend to generate quite a lot of their own noise.
PA0PBZ:
--- Quote from: tinkerbotstl on January 29, 2020, 06:11:52 pm ---Thanks, I'm primarly trying see the harmonics on a AC line. Then I want to see if that passes on to a computer power supply, and ultimately to the USB ouput.
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First thing a computer power supply does is rectifying (turning into DC) the incoming AC, then it gets smoothed by one or more capacitors, then it creates a new AC voltage of much higher frequency, this goes into a transformer which outputs different voltages, they get rectified again, smoothed and finally you have your output voltages of +12, -12, +5, -5 and 3.3V
Will you be able to see the incoming AC voltage harmonics on the 5V line? Nope! :)
rstofer:
These replies get back to my point:
1) Where do you expect to probe?
2) What do you expect to see?
3) What can you do about it if you do see it?
4) Is seeing something you can't change worth the risk of probing the circuit?
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