Understanding DC offset in AC mains

[adauphin]

Being a beginner for the most part, going to dive into the reaches of the 1054Z, I would like to find a good article or video on DC voltage in the AC mains.

This has likely been discussed in some depth multiple times, I'm just trying to get a grasp on why there is, or can be DC voltage on the mains. I've watched many videos on AC and DC triggering and coupling, I have a slight understanding on how a small amount of DC can raise the amplitude of the signal, just looking to know how and why DC can be bad or good.

One reason is I have an occasional hum in my torridial transformer in one of my amps. I would like to narrow down the source and one culprit is my microwave. This happens when the oven is idle, also happens when the plasma tv is on.

This hobby is becoming increasingly fascinating and all the videos have helped tremendously. If I'm going to try to track down the cause, I'm very curious as to how and why DC is present in any form on the mains.

I'm not going to blindly go probing the mains..so no worries there. I have other projects involving an older preamp and possible leaking caps I need to investigate as well.

Thank you for all the great info and help here... 

[coppice]

Mains is universally supplied by a transformer, which is incapable of transferring a DC voltage to the load. However, small local DC components can occur in the voltage waveforms to individual loads, due to the combination of half wave (or other asymmetric) loads creating a big DC component in the current waveform, and the impedance of the wiring.

Power supplied from a UPS is different. A UPS should not produce a DC component in its output, but what happens in practice varies somewhat.

[adauphin]

Ok...I see what you're saying.

I guess I just need to track down the culprit within my house. I also want check a few older components for any bad caps that may introduce some DC, but for the most part I want to speculate the microwave is one.

I can hear the hum from the torodial 5 feet away when the microwave is running, the torodial goes quiet when it shuts off. Occasionally the torodial will hum without the microwave being on, then just opening the breaker then closing it again will quiet the transformer.

I thought maybe the Fios entering the house could be a problem also. I checked the inside wiring and outside ground, but still doesn't rule it out.

[tecman]

DC on the mains can create headaches for the utility.  When I was in University many years ago, one of the seniors made a big inductor.  He used a couple of car batteries to pump DC into the power line, eventually causing a pole transformer to fail spectacularly with oil spewing out.  Seems, by design, he got the transformer to saturate on each half cycle causing it to thermally fail in a relatively short time.

paul