[richidoo]

Can I measure the DC offset of my AC power with multimeter set to DC volts?
Thanks
Rich

[JoshK]

Not many DMM have that capability to measure DC on an AC waveform and will give erroneous results. 

[mfsoa]

What level of offset is considered "normal"?


Thanks

-Mike

[richidoo]

OK, thanks Josh. 

[TomS]

If you can borrow a 'scope, that probably work nicely as long as it can handle 120v p-p.  I have a cheapo one for crude work that comes in handy for those little jobs that don't require any sort of real precision.  Tom

[Occam]

Just a slight expansion on Tom's suggestion. What I do on my scope is set my scope to 'AC coupled' (which puts a capacitor is series with the signal) and center the image. Then by pressing the 'DC coupling' button you able to rapidly switch back between the 2 and visually see the amount of dc offset. If you've a scope with multimeter functions, which many PC based USB scopes have, you can simply measure the mean voltage which is the dc offset (assuming your mains is reasonably close to a sine)

PS - the better Fluke multimeters can directly read the dc offset on a AC signal

[richidoo]

Thanks Tom and Paul.
A friend has a scope. We'll give that a try. Just wanna track down a tranny hum.
Rich

[TomS]

Just a slight expansion on Tom's suggestion. What I do on my scope is set my scope to 'AC coupled' (which puts a capacitor is series with the signal) and center the image. Then by pressing the 'DC coupling' button you able to rapidly switch back between the 2 and visually see the amount of dc offset. If you've a scope with multimeter functions, which many PC based USB scopes have, you can simply measure the mean voltage which is the dc offset (assuming your mains is reasonably close to a sine)

PS - the better Fluke multimeters can directly read the dc offset on a AC signal

Thanks Paul.  I was hoping you'd come to my rescue 

[HAL]

If you have access to a Fluke 43B power line analyzer, you can check the harmonic distortion on the AC line.  Had a similar problem with lots of audio gear in the house.  Turned out the the AC line harmonic distortion was running about 6%. 

The local power company did some investigation.  Found out the meter base contacts had corrosion.  After clean up, the AC line distortion runs about 2%.   No more power transformer hum. 

[richidoo]

Wow, HAL, that is cool. That corrosion probably also cost you some dough on the electrical bill. Good discovery. Thanks
Rich