How Much DC is OK on a Power Line?

^^^Give me your address and i will send you a royality fee..sounds like you already own the market for being an a-hole.

6550,

That is exactly the type of response I would have expected from you. Sad, just very sad.

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.