I built a power analyser and I don't know what it's telling me

Mark R

Diamond Member
Oct 9, 1999
Being bored last weekend I built a power quality analyser to monitor my mains supply.

However, it's shown me a couple of things that I wasn't expecting, and I wonder if anyone can explain.

Typical output: powerqual.png

In the early hours, there's a sudden drop in voltage by a couple of volts - this is instantaneous, and maintained until it pops up again, also instantaneously a bit later. I can't imagine that there's some heavy load on my transformer that's switching on at those times. (Hot water and electric heating pilot supplies are switched on much earlier). So do these transformers have tap changers on them? Or is this more likely to be tap-changing on the high voltage network?

What's up with the voltage distortion? Ignoring the 3rd harmonic, which I still think may be an artifact of converting a wall-wart into a potential transformer, what's going on in the morning, particulary with the 5th harmonic? Something, or lots of something appear to be switching on, and causing lots of 5th distortion (but not 3rd). Only catch is that I can't think of any equipment that will cause 5th harmonic distortion but not 3rd (I'm thinking of mains rectifiers and SMPSs - but these are legally mandated to be PFC, so I'm not sure if that's it).

Please try and shed some light on these mysteries.


Elite Member
Jul 26, 2000
What's the voltage on the output of your wall wart? How are you bringing that down to line level?