measure AC mains with oscilloscope

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.

Try it!

John

"1jam"

** 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.

.... Phil

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.

--
My liberal friends think I'm a conservative kook.
My conservative friends think I'm a liberal kook.
Why am I not happy that they have found common ground?

Tim Wescott, Communications, Control, Circuits & Software
http://www.wescottdesign.com

p t

That doesn't add up to me.

In my ignorance I would expect 'scopes to have a very input impedance so we can connect them to tiny little circuits....

"fungus"

That doesn't add up to me.

** 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.

... Phil

ion

t ?

I don't expect to be able to measure it but I don't see why it should destroy the 'scope.

Where do you buy such low grade crap?

--
You can't have a sense of humor, if you have no sense.

"Michael A. Terrell"

** Even this bargain price probe is rated at 600V.

... Phil

That thing with the two stars...is it because you're writing footnotes? I don't get it.

"fungus" "Phil Allison"

That thing with the two stars...is it because you're writing footnotes? I don't get it.

** You don't get much - do you ?

The insults thing? You're right I don't get that at all.

Maybe it's normal in the less 'mainstream' newsgroups but in sci.electronics.basics...? Maybe you should seek professional help.

OTOH I *do* understand:

a) You don't know how to use your newsreader software (you'd just filter out people you don't like...)

and

b) That you're incapable of understanding why people inventing weird new quoting styles is a bad thing in a public forum read by thousands of people.

Phil's been pretty good in my opinion. (What he's said has been 'spot on'. No?) He's almost like a door guard here at SEB.

George H.

I'm not questioning his electronics knowledge, but this is 'basics' so people should be allowed to ask questions, no?

And the insults part. fungus=3DXXXX

Was that 'spot on'?

You seem to know him. Maybe you can explain where I went wrong... what causes doggy to bite.

Oh, Phil A. insults and bites everyone. You either get use to it, or post questions somewhere else.

George H.

Cerberus?

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.

--
"For a successful technology, reality must take precedence 
over public relations, for nature cannot be fooled."
                                       (Richard Feynman)

Don't worry about Phil. I would bet if his better half even glanced at him slightly, he would heed to the subservient level of obedience.

Generally people like Phil need a place to unload after being around incompetence all day. It does not mean you are one of them.

And remember, every one has a different scale of where the line is drawn, this is my opinion only.

I am sure Phil will be blasting away at me after this one!

Jamie

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.

--

Reply in group, but if emailing add one more
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.

--
You can't have a sense of humor, if you have no sense.