# measure AC mains with oscilloscope

Started by December 26, 2011
```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.

```
```On Sun, 25 Dec 2011 23:45:52 -0900, 1jam <com@example.net> wrote:

>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.

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.

>No i'm not going to try it. I have a little AC walwart to use as isolation.

Most are isolated, but it's certainly worth checking.
```
```On Sun, 25 Dec 2011 23:45:52 -0900, 1jam <com@example.net> wrote:

>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?

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
>~350V peak.
>
>No i'm not going to try it. I have a little AC walwart to use as isolation.
>

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"
>
> 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 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.

> No i'm not going to try it. I have a little AC walwart to use as
> isolation.

** 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

```
```On Sun, 25 Dec 2011 23:45:52 -0900, 1jam wrote:

> 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.

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
```
```On Dec 27, 12:24=A0am, "Phil Allison" <phi...@tpg.com.au> wrote:
>
> ** NO scope I have come across will accept 350 volts at the input without
> severe internal overload. Usually scopes have protection against damage u=
p
> to some voltage like this, but don't count on it. Prolonged overvoltage a=
t
> the input will at least smoke a resistor an maybe kill a JFET or two.
>

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"
>
>"Phil Allison"
>
> ** 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.
>

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

```
```On Dec 27, 10:43=A0am, "Phil Allison" <phi...@tpg.com.au> wrote:
>
> ** With a max setting for the vertical attenuator of 10V or 20V per divis=
ion
> and 8 divisions on the screen =A0- =A0what voltage limit does that sugges=
t ?
>

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

```
```"fungus=  cunt "
"Phil Allison"
>
> ** 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 ?
>

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

** FFS  -  the scope is not destroyed.

But at some voltage resistors will smoke.

And stop   OVER  SNIPPING   !!!!!!!!!!!!!!!!!!!!

You asinine pig.

.....  Phil

```
```On Dec 27, 11:21=A0am, "Phil Allison" <phi...@tpg.com.au> wrote:
> "fungus=3D =A0cunt "
> =A0"Phil Allison"
>

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

Who wrote that? I think is was me ... but it
the single '>' seems to indicate it was you.

Maybe you ought to rethink your little girly
star-quote thing and start using the normal

> ** FFS =A0- =A0the scope is not destroyed.
>

So why are people saying, eg.:

"The chances of smoking your scope are significant..."

"** NO scope I have come across will accept 350
volts at the input without severe internal overload"

> =A0 =A0But at some voltage resistors will smoke.
>

Of course... but at a voltage which is available
in every household and on every workbench?

```