Line Voltage Has Offset

[D from BC]

I did an oscilloscope diff. test on a wall outlet.
The sine was offset??

|<----scope--->|
Outlet ---N------x10 Probe tip 1------Ch 1
|
Outlet----G-------------------------- Earth
|
Outlet----L-----x10 Probe tip 2-------Ch 2 Inv

I confirmed with a DMM on DC setting.
Approx. 10VDC with respect to L.

Just a bit curious how the line gets offset.


D from BC
British Columbia
Canada.

 

[Phil Allison]

"D from BC"

I did an oscilloscope diff. test on a wall outlet.
The sine was offset??

|<----scope--->|
Outlet ---N------x10 Probe tip 1------Ch 1
|
Outlet----G-------------------------- Earth
|
Outlet----L-----x10 Probe tip 2-------Ch 2 Inv

I confirmed with a DMM on DC setting.
Approx. 10VDC with respect to L.

Just a bit curious how the line gets offset.

** Totally WRONG test procedure.

To measure a small DC offset in the presence of a large AC voltage with a
DMM - you must first remove most of the AC voltage with a low pass filter.

Try using a 1 megohm resistor in series with a 1 uF film cap wired across
the AC supply - then measure the DC component across the cap with your
DMM. Allow for the effect of the resistor if you like, it reduces the DC
reading by about 10%.

Bet the new result is way under 1 volt.



......... Phil

 

[Greg Neill]

I did an oscilloscope diff. test on a wall outlet.
The sine was offset??

|<----scope--->|
Outlet ---N------x10 Probe tip 1------Ch 1
|
Outlet----G-------------------------- Earth
|
Outlet----L-----x10 Probe tip 2-------Ch 2 Inv

I confirmed with a DMM on DC setting.
Approx. 10VDC with respect to L.

Just a bit curious how the line gets offset.

Could be that some load is creating what is called
harmonic distortion of the power "signal". This
can result in slightly non-symmetrical upper versus
lower halves of the sinewave (or more complicated
relationships) and a non-zero DC component of the
waveform.

It might be interesting to perform a spectrum
analysis of the waveform.

 

[Phil Allison]

"Greg Neill"


** You are a gullible damn fool.

Go away.



..... Phil

 

[D from BC]

"D from BC"


** Totally WRONG test procedure.

To measure a small DC offset in the presence of a large AC voltage with a
DMM - you must first remove most of the AC voltage with a low pass filter.

Try using a 1 megohm resistor in series with a 1 uF film cap wired across
the AC supply - then measure the DC component across the cap with your
DMM. Allow for the effect of the resistor if you like, it reduces the DC
reading by about 10%.

Bet the new result is way under 1 volt.



........ Phil

Bummer...
I think that was probably a classic DMM gotcha.
Too much faith in my DMM.
I just fired myself...

Oh!
Just got rehired.

I did the RC filter circuit as suggested ..
Line offset voltage = 0.0001VDC (zilch..)
That's more like it..

I dunno wtf I did wrong with the scope set up..
I repeated the set up and the scope shows no offset.
Must have goofed a vertical setting.
Double fooled..

Embarrassed


D from BC
British Columbia
Canada.

 

[Phil Allison]

"D from BC"
"Phil Allison"

Bummer...
I think that was probably a classic DMM gotcha.
Too much faith in my DMM.
I just fired myself...

Oh!
Just got rehired.

I did the RC filter circuit as suggested ..
Line offset voltage = 0.0001VDC (zilch..)
That's more like it..

I dunno wtf I did wrong with the scope set up..
I repeated the set up and the scope shows no offset.
Must have goofed a vertical setting.
Double fooled..

Embarrassed

** However, you DO get major brownie points for TAKING the advice,
acting on it and then admitting your error.

BTW

You are far from alone in making this particular mistake.



....... Phil

 

[Joerg]

Bummer...
I think that was probably a classic DMM gotcha.
Too much faith in my DMM.
I just fired myself...

Oh!
Just got rehired.

I did the RC filter circuit as suggested ..
Line offset voltage = 0.0001VDC (zilch..)
That's more like it..

I dunno wtf I did wrong with the scope set up..
I repeated the set up and the scope shows no offset.
Must have goofed a vertical setting.
Double fooled..

Embarrassed

Look at the bright side: Nothing exploded, your scope still works and
the house hasn't burned down.

 

[Jamie]

Look at the bright side: Nothing exploded, your scope still works and
the house hasn't burned down.

Yet!


--
"I'd rather have a bottle in front of me than a frontal lobotomy"

"Daily Thought:

SOME PEOPLE ARE LIKE SLINKIES. NOT REALLY GOOD FOR ANYTHING BUT
THEY BRING A SMILE TO YOUR FACE WHEN PUSHED DOWN THE STAIRS.
http://webpages.charter.net/jamie_5"

 

[Joerg]

Yet!

Now it wasn't me who said that!

 

[Michael A. Terrell]

"Daily Thought:

SOME PEOPLE ARE LIKE SLINKIES. NOT REALLY GOOD FOR ANYTHING BUT
THEY BRING A SMILE TO YOUR FACE WHEN PUSHED DOWN THE STAIRS.

Yet you contiune to climb back up the stairs,just so you can be
pushed right back down.

--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

 

[John Smith]

...
Yet you contiune to climb back up the stairs,just so you can be
pushed right back down.

It didn't hurt your head though, did it? I mean you have always been
like "that", I can tell from past posts ...

JS

 

[MooseFET]

Bummer...
I think that was probably a classic DMM gotcha.
Too much faith in my DMM.
I just fired myself...

Oh!
Just got rehired.

I did the RC filter circuit as suggested ..
Line offset voltage = 0.0001VDC (zilch..)
That's more like it..

I dunno wtf I did wrong with the scope set up..
I repeated the set up and the scope shows no offset.
Must have goofed a vertical setting.
Double fooled..

.... or there was a DC offset and now there isn't.
Was the neighbor doing some sort of "science experiment" involving
passing large DC currents into a "ground rod"?

Do you live in the SanFransisco area near a BART station?