DC on an AC Mains Power Line

From time to time on some of the DIY Audio forums, people write about DC on their AC power line causing the power transformers in their units to buzz or even saturate! In a residential neighborhood how does DC get on the AC power line? I could understand 3rd harmonic distortion from SMPS's being read as DC on a multi-meter, but where would enough DC to saturate a power transformer come from?

Kevin (remove the Zeppelin to reply)

Reply to
Speedskater
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=46rom loads that rectify, you can get a bit of asymmetry in the output current from the (power-pole) transformer, which combines with the wiring resistance to give a small DC value.

I've seen fluorescent lamps cause their ballast transformers to buzz, and switching a tube end-for-end has stopped the buzz. It's possible for the two ends of the lamp to develop different electron emissivities, because pointy bits on an electrode form with aging.

Except for gas-discharge tubes, good design (and applicable regulations) will result in zero DC draw on the AC source. A more likely scenario is that some secondary on the transformer of an audio equipment is delivering DC power, and it's the secondary winding, not the primary (connected to AC mains) that is causing some saturation effect.

Reply to
whit3rd

"Speedskater"

** The problem is not a common one and when it does exist, it is only toroidal power transformers that react badly - as they have high sensitivity to a DC component.

Since toroidal are a favourite of DIY amplifier builders - that is why you see it their forums.

** I have a 1600 watt hot air gun with a " hi " and " lo " (800 watt) setting on the switch. In the "lo" position, it connects a diode in series with the heating element.

When this is being used, one AC voltage peak is some 5 volts lower than the other. Result - any toroidal tranny on the same power circuit goes into heavy saturation and makes a growling/ humming noise.

........ Phil

Reply to
Phil Allison

a couple of ways I can think of. #1 corroded connections creating a diode effect (bad).

#2 Something connected on the line has a heavy half wave load on it.

I'm sure there are other things.

--
http://webpages.charter.net/jamie_5"
Reply to
Jamie

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You can make a transformer more tolerant of DC by increasing the winding resistances. Of course, this reduces efficiency and regulation. However, it can be a good tradeoff. Paul Mathews

Reply to
Paul Mathews

"Paul Mathews"

" You can make a transformer more tolerant of DC by increasing the winding resistances. Of course, this reduces efficiency and regulation. However, it can be a good tradeoff. "

** That idea is completely idiotic !!!

The solution to DC offsets on the AC supply is simple and VERY well known.

It involves a few diodes and a couple of electros in the AC feed to the primary.

....... Phil

Reply to
Phil Allison

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