Cheap AC transformers are no good

The Open Energy Monitor uses an AC transformer as a voltage sensor, but the choice of this transformer can influence the accuracy of your monitor.

Jean Claude from JeeLabs (brilliant engineer BTW) investigated the difference between a bad and a good AC transformers and here are the results in images:

1) Bad quality transformer:

2)Better quality transformer:

You can read his complete post on his blog: JeeLabs Daily Weblog.