Line Voltage Has Offset

"D from BC"

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

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.

"Greg Neill"

Go away.

.... Phil

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

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.

"D from BC" "Phil Allison"

BTW

You are far from alone in making this particular mistake.

...... Phil

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

Yet!

Now it wasn't me who said that!

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

... 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?