Re: Oscilloscope to view mains power

RR wrote:
Make sure that the front end of your scope can handle the peak voltage of 

any AC

voltage you will be measuring, divided by the probe factor if appropriate. 

For

example a 120 VAC RMS line will have a peak voltage of 170 V to 

neutral/ground

(1.414 * RMS Voltage). AC voltmeters read RMS voltage. In this case, 

your

scope would need to have a peak rating greater than 170 VDC if you use 1X 

probes

and greater than 17 VDC if you use 10X probes. 


Thanks for that.


Also, remember that the shield on your scope is connected to ground so be
careful not to connect the ground lead of your probe to the line side


instead of

the neutral.  If you do, you will likely ruin your scope probe.  Using a
step-down transformer will provide isolation and eliminate this problem.



I read elsewhere that if I don't care too much about accurate voltage
measurements, I can just connect the probe tip to neutral or active and not
bother about connecting the probe shield.

The scope will then show the PD between the probe tip and the internal scope
ground (which should be the same as the power points ground since they'll be
on the same house power circuit - except when I'm measuring the inverter).

No circuit between the probe tip and its shield would seem to be safer, on
the face of it.

Does that make sense?

tia,
RR



I second the low voltage transformer suggestion.

Assuming your probes are rated for the voltage to be applied,
put the scope in difference mode (subtract channels).
Don't hook the ground to anything. Probe around using both probes.
If you're interested in high frequency events, hook the two probe
ground leads together...won't make any difference at 60 Hz.

NEVER, EVER hook your probe ground to
ANY PIN on the wall socket.  Yeah, yeah, call me conservative, but
I'm an ALIVE conservative.

It's IMPORTANT that you not make any assumptions about the socket in the wall. I've seen 'em with open grounds, reversed hot/neutral. Measure
with your voltmeter and be sure.

Another problem with measuring line voltages is hooking it up.
I've seen people stick screwdrivers in the socket so they have something
to hook the probe to.
You're messing around, got one more probe to hook up. What the hell, just stick this screwdriver in there temporarily...wanna see my collection of screwdrivers with big black divots in the shaft?
I've also seen people put alligator clips on a line cord.
"Sounded like a good idea at the time."
Try not to do anything stupid.
mike

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