How Much DC is OK on a Power Line?

Say what no DC is best.

nothing that an isolation transformer can't fix.
All my low current stuff goes thru a small, 400va iso transfomer and it and the gear are dead quiet.

I have to take umbrage with the quote above. Unless there is a center-tap on the primary side of the power transformer (and there never is) this DC thing is of no consequence whatsoever as the power transformer will simply see that total voltage across its windings.

It may be a distorted waveform, but there won't be any DC. Keep in mind that one thing transformers get used for all the time is to block DC, and values at considerably higher levels than just a few millivolts.

However, *something* does happen, the distortion I just mentioned. Fluke Instruments published a very nice white paper about 20 years ago that explores this phenomena. It turns out that the primary (no pun intended...) cause of mechanical noise in power transformers is the 5th harmonic (300Hz in the US). The paper also gives a formula to allow one to calculate the distortion on the AC line, if you know the current drawn and the source impedance of the line transformer (perhaps the one on the power pole outside your house) winding.

The 5th harmonic can cause power transformers to become mechanically noisy, power rectifiers to radiate mechanical and electrical noise, and creates forces in hysteresis motors that can make them run backwards.

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.