I've heard 'no never do that' and 'it'l blow your fuse / you will die' etc... just trying to understand.
As long as you are careful to connect your probe ground to neutral and the probe signal to hot, it should be ok right? Assuming the scope can handle ~350V peak.
No i'm not going to try it. I have a little AC walwart to use as isolation.
There are ways of doing it but if you're asking this question, just *don't*. The chances of smoking your scope are significant and the chances of injury or death, nonzero.
Most are isolated, but it's certainly worth checking.
I wouldn't clip the probe ground to neutral. Neutral typically has some voltage relative to ground, and that can cause complications.
Assuming the scope can handle
There's no problem with touching a 10:1 probe to the high side of the AC line. I do that all the time. If you scope is grounded, and neutral is close to ground, that should get you close to the actual waveform.
You can use two probes, and set your scope to A-B mode, and see the actual line-neutral or line-line voltage. Just connect one probe to line, the other to neutral or to the other line, ground the scope, ignore the probe ground clips. If you do this, just don't crank up the volts/div knob too much; if you overload the input of either channel, the subtraction won't be right.
** NO scope I have come across will accept 350 volts at the input without severe internal overload. Usually scopes have protection against damage up to some voltage like this, but don't count on it. Prolonged overvoltage at the input will at least smoke a resistor an maybe kill a JFET or two.
Anyone with a need to view high voltages uses a suitable probe with either a
10:1 or 100:1 division ratio - the latter are often rated up to several thousand volts.
** Transformer type AC wall warts are very safe - but be aware that the output voltage will be higher than labelled under no load and the wave becomes very distorted if loaded by a rectifier and filter cap. Even with no load, the wave is often distorted by the effect of magnetising current in the primary - typically the point in time where the AC voltage crosses zero will be shifted.
Neutral is called "Neutral" instead of "the other ground" because it isn't always right at ground. Sometimes the plug is wired backward and it isn't close to ground at all.
Do like J.L. said -- use 10:1 probes and connect to hot. If you need the accuracy and your scope supports it, connect the other to neutral and subtract.
If it kills you right off the bat then I must not have been a very good kid, because I've been dead for years now, and while it's nice here it sure ain't heaven.
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** With a max setting for the vertical attenuator of 10V or 20V per division and 8 divisions on the screen - what voltage limit does that suggest ?
In my ignorance I would expect 'scopes to have a very input impedance so we can connect them to tiny little circuits....
** The input protection circuit has to work on all vertical ranges, down to
5mV or less per division - cos a user might just probe say 240 VAC with the scope set to max sensitivity.
This is a non trivial problem.
Have a look at some scope schematics to see how it is done.
I'd agree with that, George. Phil can be (mostly is) objectionable and rude, but his pronouncements on technical matters are usually correct and to the point.
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over public relations, for nature cannot be fooled."
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When last I did that with an analog scope, I saw a near-sine that was composed of a series of straight lines. Each line was fairly long, so it took only about 10 to make a full wave. It looked like a vector drawing.
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zero, and remove the last word.
Try looking at the ground connection some time, to see the noise imposed on the neutral. It's really nasty on three phase, with the harmonics adding in the neutral.
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