Oscilloscope to view mains power

[RR]

Hi,

I've purchased a 20MHz Dual Trace Oscilloscope (see previous thread called
"Opinions on Oscilloscopes") and it's been quite fruitful so far. Thanks to
all who helped me make a decision.

I've been working on low voltage circuits, and building simple rectifiers.
It's great to see the actual wavefrom change from AC to half-wave AC and so
on.

Now, I'm not stupid, and I know not to play around with mains. But I do
want to see mains in action.

I am interested in alternative power sources (solar to battery to inverter
etc.)

I want to see the "modified sine wave" output of my little 150W inverter and
when I buy my 1500W inverter I want to see that too.

I also have 3 phase power to my house (from the electricity company), and
I'd really like to see the 3 sine waves on my scope (with the dual trace, I
guess I can look at two at a time).

Why? Seeing is believing....it really helps me to actually see what's
there. It's just the scientist in me, I guess.

So, a very simple question....

My scope has 1MOhm probes with a 10X switch. I believe this switch makes the
probe 10MOhm.

It seems to me that this is safe to apply to a mains socket (with all other
sensible safety precautions in place - e.g. DON'T TOUCH the metal tip of the
probe or its ground clip!).

Am I wrong?

If you say it's OK, all risk is on me. My widow will not come and find you
and sue you.

BTW, My house (and workshop) has a Residual Current device. I know not to
rely on it, and I also know it doesn't protect against putting yourself
between active and neutral.

tia,
RR

 

[DaveM]

Hi,

I've purchased a 20MHz Dual Trace Oscilloscope (see previous thread called
"Opinions on Oscilloscopes") and it's been quite fruitful so far. Thanks
to
all who helped me make a decision.

I've been working on low voltage circuits, and building simple rectifiers.
It's great to see the actual wavefrom change from AC to half-wave AC and
so
on.

Now, I'm not stupid, and I know not to play around with mains. But I do
want to see mains in action.

I am interested in alternative power sources (solar to battery to inverter
etc.)

I want to see the "modified sine wave" output of my little 150W inverter
and
when I buy my 1500W inverter I want to see that too.

I also have 3 phase power to my house (from the electricity company), and
I'd really like to see the 3 sine waves on my scope (with the dual trace,
I
guess I can look at two at a time).

Why? Seeing is believing....it really helps me to actually see what's
there. It's just the scientist in me, I guess.

So, a very simple question....

My scope has 1MOhm probes with a 10X switch. I believe this switch makes
the
probe 10MOhm.

It seems to me that this is safe to apply to a mains socket (with all
other
sensible safety precautions in place - e.g. DON'T TOUCH the metal tip of
the
probe or its ground clip!).

Am I wrong?

If you say it's OK, all risk is on me. My widow will not come and find
you
and sue you.

BTW, My house (and workshop) has a Residual Current device. I know not to
rely on it, and I also know it doesn't protect against putting yourself
between active and neutral.

tia,
RR

You can safely view mains power lines with a bit of common sense. Never
connect the ground (shield) return of the scope to the hot side of the
mains. Make sure that the neutral and ground wires are correctly connected.
If they are properly wired, then it's safe to connect the scope probe's
shield connector to either neutral (preferred) or to the ground. Then,
connect your probe's tip to the hot wire and safely view the mains waveform.
For the 3-phase power, make sure that you have identified the neutral and/or
ground connections. Scope shield/ground to power neutral/ground, scope
probe tip to the phases.
Use the same strategy on your inverters. Connect the scope ground to the
ground or neutral out of the inverter, and the probe tip to the hot line.
In all inverters that I have seen, the inverter's neutral and ground are
connected together inside the inverter.

NEVER EVER use a 2-blade adapter to isolate the scope. If you get the
scope's case (ground/shield) connected to the mains hot line, or high
voltage inside a piece of equipment, you could easily cause your wife to
become a widow. I'm sure you want to avoid that if at all possible.

Cheers!!!!
--
Dave M
MasonDG44 at comcast dot net (Just substitute the appropriate characters in
the address)

Never take a laxative and a sleeping pill at the same time!!

 

[James T. White]

First, be very careful if you connect anything to the AC mains. The voltage it
operates at and the current it can deliver can make your mistakes lethal. One
simple way of reducing this risk would be to use step-down transformers with the
primary connected to the mains and doing your measurements on the secondary side
at a reduced voltage. Those cheap, plug-in AC power supplies would be good for
this.

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.

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.

Be safe......

 

[RR]

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

 

[mike]

any AC



Thanks for that.



instead of



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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[Rich Webb]

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?

If I wanted to scope mains, I'd not just poke the probes into a handy
receptacle.

If you do everything correctly, you'll just see some rather boring,
noisy sine waves; if you do something wrong then there's the potential
(no pun intended) that, if you survived, you would be wearing the
remains of the o'scope.

Build a test rig with a properly polarized mains plug, in-line fuses,
current limiting resistors, and with shielded and polarized test points.
If it's for a semi-permanent test rig (e.g., an inverter) then build the
fuses, resistors, etc. into it.

BTW, your premise wiring is most likely anti-phase rather than 3-phase
and comes from a single, center-tapped transformer that itself is across
one of the three phases coming out of your substation.

 

[Steve]

The best way to do this safely is to put your scope in differential
mode. What you'll be doing is looking at the difference between
the two channels (A-B). In this mode you will NOT connect
the shield of your probes to the mains at all. Simply connect
one probe to one line of the mains, and the other probe to
the other.

Make sure that both input attenuators are set the same.

Steve

 

[RR]

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.

Thanks for smart suggestions.

The probe specifies Maximum Working Input Voltage at X10 (10MOhms setting)
as:
<400VDC+Peak AC.

I'm in a 240VAC country, so I think that's within spec (240V RMS = 340V
peak).

What I don't understand with the above spec is the "+Peak AC". Is it saying
it can handle up to 400V peak?

tia,
RR

 

[RR]

If you do everything correctly, you'll just see some rather boring,
noisy sine waves; if you do something wrong then there's the potential
(no pun intended) that, if you survived, you would be wearing the
remains of the o'scope.

Yes, it's the noise (or lack of it) that I'm particularly interested in.

BTW, I try the simple low voltage transformer and saw a 12VAC sine wave.
Interestingly, there was a little ripple in the curve just after each
postive peak.

I assumed this was some noise added by the transformer. That's one reason
I'd like to see the "raw" output from the socket.

Build a test rig with a properly polarized mains plug, in-line fuses,
current limiting resistors, and with shielded and polarized test points.
If it's for a semi-permanent test rig (e.g., an inverter) then build the
fuses, resistors, etc. into it.

Great idea. I'll do that.

BTW, your premise wiring is most likely anti-phase rather than 3-phase
and comes from a single, center-tapped transformer that itself is across
one of the three phases coming out of your substation.

I'm waiting on the arrival of my "Fundamentals of Power Electronics" book,
so I look forward to properly understanding that last statement.

thanks,
RR

 

[RR]

If you do everything correctly, you'll just see some rather boring,
noisy sine waves; if you do something wrong then there's the potential
(no pun intended) that, if you survived, you would be wearing the
remains of the o'scope.

Yes, it's the noise (or lack of it) that I'm particularly interested in.

BTW, I try the simple low voltage transformer and saw a 12VAC sine wave.
Interestingly, there was a little ripple in the curve just after each
postive peak.

I assumed this was some noise added by the transformer. That's one reason
I'd like to see the "raw" output from the socket.

Build a test rig with a properly polarized mains plug, in-line fuses,
current limiting resistors, and with shielded and polarized test points.
If it's for a semi-permanent test rig (e.g., an inverter) then build the
fuses, resistors, etc. into it.

Great idea. I'll do that.

BTW, your premise wiring is most likely anti-phase rather than 3-phase
and comes from a single, center-tapped transformer that itself is across
one of the three phases coming out of your substation.

I'm waiting on the arrival of my "Fundamentals of Power Electronics" book,
so I look forward to properly understanding that last statement.

thanks,
RR

 

[Walter Harley]

[...]
The probe specifies Maximum Working Input Voltage at X10 (10MOhms setting)
as:
<400VDC+Peak AC.
[...]
What I don't understand with the above spec is the "+Peak AC". Is it
saying
it can handle up to 400V peak?

Yes, that's correct. "400 volts DC and 400 volts peak AC". As opposed to,
say, 400V rms AC.

Peak = 1.4 x RMS, for a perfect sine wave. So they could have said "400VDC
+ 284Vrms AC". But if already know it's a sine wave, there's no point
looking at it on a scope... and anything other than a sine wave will have a
different RMS to peak ratio.

 

[James T. White]

[...]
The probe specifies Maximum Working Input Voltage at X10 (10MOhms setting)
as:
<400VDC+Peak AC.
[...]
What I don't understand with the above spec is the "+Peak AC". Is it
saying
it can handle up to 400V peak?

Yes, that's correct. "400 volts DC and 400 volts peak AC". As opposed to,
say, 400V rms AC.

I would interpret that to be 400 volts DC or 400 volts peak AC. For example
it would exceed the scopes spec if you wanted to measure a 50 V p-p AC
signal riding on top of a 375 VDC bias voltage.

 

[Walter Harley]

[...]
I would interpret that to be 400 volts DC or 400 volts peak AC. For
example
it would exceed the scopes spec if you wanted to measure a 50 V p-p AC
signal riding on top of a 375 VDC bias voltage.

Yes, we're saying the same thing. The bottom line is: the scope doesn't
know the difference between AC or DC or a combination thereof, it just
doesn't want instantaneous voltage to exceed 400V.

 

[mike]

[...]
I would interpret that to be 400 volts DC or 400 volts peak AC. For
example
it would exceed the scopes spec if you wanted to measure a 50 V p-p AC
signal riding on top of a 375 VDC bias voltage.

Yes, we're saying the same thing. The bottom line is: the scope doesn't
know the difference between AC or DC or a combination thereof, it just
doesn't want instantaneous voltage to exceed 400V.

At 60 Hz., you're right. But the AC capabilities of a probe derate
quite significantly as the frequency goes up. Hence the addition
of the DC number plus the derated AC number. Usually a chart in the
probe manual.
mike

--
Wanted, Serial cable for Dell Axim X5 PDA.
Return address is VALID but some sites block emails
with links. Delete this sig when replying.
FS 500MHz Tek DSOscilloscope TDS540 Make Offer
Bunch of stuff For Sale and Wanted at the link below.
MAKE THE OBVIOUS CHANGES TO THE LINK
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