How Much DC is OK on a Power Line?

It is my understanding you cannot measure DC on the AC mains just by connecting your digital meter, set on DC voltage, across the mains.

Not trying to start a flame but

http://www.diyaudio.com/forums/solid-state/2080-dc-filter.html

Eva
.

Not sure what a center-tap on the primary would have to do with DC voltage on the mains.

Nelson Pass
The one and only

If you are experiencing mechanical hum from your
transformer, it is often caused by the presence of
DC on the line. Usually this comes from some appliance
using current asymmetrically, such as a lamp dimmer.

The hum comes usually from toroidal transformers, which
saturate easily with DC, and when they recover, they
draw an extra pulse of current, causing the noise.

You can put a pair of back-to-back electrolytics in series
with the AC power line to block this, and it works fine.
Makes sure the current rating of the electrolytics is
high enough, and the they are joined at a like polarity,
such as + to +.
Nelson Pass

http://www.diyaudio.com/forums/solid-state/2080-dc-filter.html

>>>>>>>

I've done some measurements that show what's really hapening when you connect a transformer to mains

Using a lowpass filter [100k + 47uF] I have measured about 50mV average over time of DC on my mains supply

I also have an old electric heater that in half-power mode uses a diode in series with the heat element to pass only half of the mains waveform. When I plug this heater in half-power mode I get an additional 1V of offset on mains supply

To test the need and the efficiency of DC filtering, I've done some measuremens of the current through the primary of a 750VA toroidal transformer

This oscillogram shows what happens when I connect the transformer to mains and let it deal with the 50mV DC offset
Eva
diyAudio Member

http://www.diyaudio.com/forums/solid-state/2080-dc-filter-6.html
#57

Eva gives actual hands on test data....
.
Jim

What does one AC line conductor, (neutral conductor), of the electrical service intentionally connected to earth have to do with it.

The main reasons for connecting the service neutral conductor to earth is for lightning protection and to establish one common ground plane.

Distortion, OTOH, is the sort of thing that both Nelson and Eva were describing.

How does the DC blocker filter block out the distortion?

Eva has many follow-up posts throughout the running of the thread. Like I said in my earlier post she ran hands on experiments and tests.

If I get a chance tomorrow I will visit the archives on AA. I remember John Curl posting about DC voltage on the mains.
.
Jim

Ralph,

I am not dismissing what you are saying about odd harmonics. They can and do play havoc on AC power systems.

They can cause the secondaries of power distribution transformers to run hotter than normal. And yes be mechanically noisier than normal.

Harmonics.

http://www.fluke.com/fluke/twen/solutions/pq/troubleshooting-power-harmonics.htm

http://www.psihq.com/iread/harmonic.htm

Is DC offset the product of harmonics?
.