Home power quality problem?

[jeffchrisope]

Howdy -

Apologies in advance if this is a stupid question, but I've been scratching my head about an issue all week and it's driving me nuts.

The quick background:  I am an EE student, and for one my classes (microelectronic devices) we are using some cobbled together step-down transformers in the lab for some of our rectification projects and other stuff.  I decided I wanted to build my own identical transformer so I could do some extra experiments at home, and with the exact parts list in hand I did just that.  Easy enough - it was just a 120V -> 12 V (center tap) transformer and a pile of breakers and fuses. 

I tested my version of the transformer at the school's electronics lab, and was getting the same results as I did with the "official" lab transformers - specifically, the expect voltage between the ends of the secondary winding, or between an end and the center tap, with the DMM.   Using the 'scope at the lab, the output was also unsurprising - a lovely sinusoid came out, with the probe on the post for the secondary winding end, and the ground clip on either the center tap or other secondary end. 

NOW comes the rub.  I brought my transformer home and did the same tests with my home 'scope and DMM.  DMM results were the same, but the scope at home did NOT show a sinusoid.  Instead, I got what the attached image shows - rather than a sinusoid, it looks considerably more like, say, a capacitor charging and discharging (at 60Hz) ...!  I tried the measurement on four different circuits in the house with identical results.

So ... what am I seeing here?  Am I going insane?  Do I have some hideous load on my system that I'm not thinking off, that would affect all the circuits in the house?  Or worst of all (worse than insane, even), do I have some fundamental problem in the wiring of my house - say, something screwy with the ground?

Thanks in advance for any help!
Jeff

p.s.  (and yes, I did bring my home 'scope to the school lab to confirm that it wasn't a scope problem ... no dice)

[SeanB]

Harmonic distortion, very likely because there are a lot of switch mode power supplies and such in your home neighbourhood, all being fed from the same power transformer, and likely also quite a distance from your power company main distribution transformers, which means the distortion as the power supply capacitors charge up on each cycle clamp the voltage peaks quite a bit.

Nothing really wrong with your home supply, though you could ask the power company to do some harmonic analysis and add correction to the network around you. Your school likely has a very large distribution transformer at it, and this has a low enough impedance that the loads do not show as much distortion, but the home side will show it as the cabling to the transformer is a lot longer and thus has more resistance and a lot more impedance.

[MrAl]

Hi,

I agree with the switchmode power supply idea and also add another idea.

The wave does look like what we would see if there was a triac circuit somewhere nearby.  A triac that it being triggered for lower output power will cause very non linear loading of the power line and that would explain that sharp corner.

Power supplies can also do something like this especially if they have a triac or SCR in them used to cut power.

You could try going around the house and turning things off and maybe find out what is causing this.  If it's not in your house though then you'll have to ask the neighbors.

You could also see how bad your line loads with a regular resistive load.  If it does not load much with a heavy load, then the problem could be outside your home.  If it loads easily, then it could be anywhere.
You could also try measuring right at the box where the power comes into the house.  Be very very careful though as there could be 220vac there too.

[floobydust]

You could be seeing core saturation on the transformer, the secondary peaks are compressed.
What is your mains voltage in the lab compared to home?