Re: Question for O-scope people out there…

Typical scopes can certainly handle the voltage of 110/220 VAC (aka mains
power), but as always when dealing with AC power you have to bear in mind
that the standard scope probes are not "isolated" meaning that the probe
ground leads are common to the scope chassis and so are also common to the
AC mains Earth ground (which ultimately is tied to Neutral at some point in
the system).

So to answer your question, you can certainly measure 110VAC with a scope
but you have to be careful that you never connect the scope probe ground
lead to the Line because that's a short circuit. Also, the short path is
through the scope probe lead so it might in fact "light up" quite literally
unless something else inside the scope blows out first.

You can measure DC voltage on one channel and AC voltage on the other
channel, but again, keep in mind that the ground (or 0V point) of each
channel must already be common or you will short them together as soon as
you connect the scope probe ground leads to the 0V point of each circuit
that you are trying to measure.

This is the same situation as using two probes to measure two different DC
voltages. The two voltage sources must have a common ground. The ground of
the two sources gets connected by the scope probe ground leads to the Earth
ground of the mains AC system but as long as the supply of the DC system is
isolated from the AC mains then there's no problem. This said, some DC
systems must remain isolated from the AC mains. For instance, most, if not
all, raster monitors have internal circuitry that must remain isolated from
AC mains. Connecting a scope (that's plugged directly into the wall) probe
ground to the video input connector ground will likely blow the monitor (for
example the HOT horizontal output transistor may instantly die). A solution
is to plug the scope into its own isolation transformer. This is what I
always do.

Whenever you want to measure the voltage across something like a "current
sensing resistor", you should use two probes connected one on each side of
the resistor and both of the probe grounds to the circuit ground. The
difference between the two signals provides the desired waveform. If you
feel that you must connect a single probe directly across the sense
resistor, you can under certain conditions. For instance, if one side of
the sense resistor is already connected at the 0V point (common ground) then
you can use the 2'nd probe to measure something elsewhere. Bear in mind
that id neither side of the sense resistor is at 0V (ground) then when you
connect a probe directly across it, you cannot use the other probe for
anything because it's ground lead would short one side of the sense resistor
to whatever point to which you connect the 2'nd probe ground lead.

The best solutions:
1/ Think carefully about what you are doing before connecting anything.
2/ Use isolated probes (cost more and must be purchased separately)
3/ Use an isolation transformer where appropriate
4/ Use a battery powered scope so that it remains isolated from Earth
ground.

DO NOT cut the ground lead off of the scope power cord. Doing this will not
isolate it. Remember that the output ground terminal is connected to the
Neutral terminal back at the breaker box. Keep in mind that the Ground
return is a redundancy/backup path for the Neutral path.

William Boucher
http://www.biltronix.com

----- Original Message -----
From: "Altan (GAPAS)" <pinball@aaarfamily.com>
To: <vectorlist@vectorlist.org>
Sent: Tuesday, September 11, 2012 8:34 PM
Subject: VECTOR: Question for O-scope people out there…

Is it safe to use an entry level o-scope (like the Rigol DS1102E or Owon
xxxx) with AC voltage? I suspect the answer is an obvious "Yes", but
wanted to verify. It doesn't seem to be specifically stated (again,
probably because it's obvious).

Being able to look at AC as well as DC would be helpful when trying to
trigger on AC voltage drops and viewing results on regulated DC.

The follow up: is one able to have an AC source on CH1 and a DC source on
CH2?

Thanks!

… Altan
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Received on Tue Sep 11 21:06:29 2012