ac waveform clipping

[made2hack]

Hello all,
I was wondering if this was normal or what I am looking at.

It seems to me that the peaks are being clipped on my AC waveform and wondering if this was indeed the case. I am probing the secondary on a transformer (12V secondary).

Thanks,

[drussell]

If you're looking at the AC coming off your power line then, yes indeed, they are usually quite distorted and clipped.  That actually looks pretty good compared to around here normally. 

It is mostly caused by the typical capacitor-input rectifier circuit found in so many electronics.  Remember that such a supply doesn't draw any current until the input wave rises above the current capacitor charge level, then it blasts current, almost like a short.  Regular, old-school battery chargers with a transformer -> rectifier -> battery are also very bad for creating that kind of clipped wave for the same reason.

This is why various regulations have begun to be brought in requiring power factor correction on inputs to DC supplies, etc. in many cases.  Gone are the days of nice resistive heating and lighting loads with the odd motor here and there as "average" loads.  Virtually everything now is nowhere near a pure resistance and causes one kind of distortion or another to the mains power input.

[made2hack]

So i tried unplugging everything in that room and turning off the lights. Not much change. Of course i cant account for neighbours and whatnot.

How far do you think the inerferance runs?

[drussell]

So i tried unplugging everything in that room and turning off the lights. Not much change. Of course i cant account for neighbours and whatnot.

How far do you think the inerferance runs?

It affects the entire grid.  It may be somewhat better or worse at various points on the grid at different times and the power companies have huge banks of capacitors, etc to try to be able to switch in different reactances at various times to compensate as much as they can but it is still always a mess everywhere. 

[made2hack]

Ok, so I guess I shouldn't worry about it? THe waveform I'm getting looks ok?

[drussell]

Ok, so I guess I shouldn't worry about it? THe waveform I'm getting looks ok?

Yeah...  That is about as clean as you're going to get looking at the AC line in most areas.

For testing equipment designs here I use a local solid state AC regenerator that can create any power signal I wish...  Anywhere from fraction-of-a-percent distortion clean sine wave to any arbitrary waveform, any frequency, any distortion level I choose...  With that and some thermal / environmental control ability available it makes testing power supply designs, etc. a breeze...

[Audioguru]

You are not looking directly at the electricity waveform. instead you arte looking at the output of your transformer that might have a little core saturation at the voltage peaks or a small overload.
Try a high quality name-brand transformer.

[Damianos]

You are not looking directly at the electricity waveform. instead you arte looking at the output of your transformer that might have a little core saturation at the voltage peaks or a small overload.
Try a high quality name-brand transformer.

I think that the reason of "clipping" is this.
Probably the transformer is of "impedance protected" type. They are used in most external power supplies, desk lamps and other devices, that don't use any other protection circuitry (fuses, thermistors etc).

To monitor the shape of the line voltage it is needed to use a "normal" transformer with a little load.