How Much DC is OK on a Power Line?

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

Hmm. So, with the AC line having one side at ground, there's supposed to be a DC level? The idea that a hair dryer can shift the ground potential to a DC level seem far fetched- it would require that the neutral side of the AC line be not connected to ground.

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

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.
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Jim

Well Jim, I'm certainly open to finding out I've been wrong about things. Its how we learn :)

As far as the earth connection, what I am talking about here is that the hot side of the line is measured with respect to the neutral (ground). So if there is a DC component present, the AC voltage will be shifted with respect to the neutral. *That*, since the neutral is tied to the earth connection, will take some current and will likely heat the wiring up in the process.

Now if you have as an example Eva's hair dryer, running on half power so it its only conducting when the diode conducts, I can see that that might be interpreted as DC. But, it can also be interpreted as harmonic distortion. I'm not arguing, IOW, that the phenomena does not exist, just the explanation. One could as easily argue that a 2nd harmonic in the output of an amplifier is a DC phenomena as well.

I would like to see Eva's scope readings, but the images don't load off of DIYaudio.

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.