I came across this DIY shield based on openenergymonitor, it has a on-board AC-AC transformer as well. Looks great so I wanted to share the find
Interesting. Someone on here investigated those miniature transformers a little while ago, and found their performance lacking. And has anyone checked the a.c. waveform that's being monitored for the voltage input? They might find it isn't what they expect. It all depends of course on how it is being used, but if the software has copied our VI sketches and multiplies instantaneous voltage and current to get power - er, umm, not sure how to say this: the answer isn't power.
(Hint: they've made the classic mistake of tying the transformer to the d.c. side - you can't do that. Run a Spice analysis on it if you don't believe me).
Robert, here is another similar project, looks interesting and has the transformer issue addressed
There are three issues with the transformer - two depend on the transformer design and one on the circuit design. Dealing with the last one first, the instructables "shield" design that you spotted shorts out one of the bridge rectifier diodes with the bias decoupling capacitor, so irrespective of the transformer properties, the voltage sample looks nothing like the mains waveform. The first and second issues - the two distortions due to the iron and the rectifier load - can be addressed but the sad fact is the transformers we generally use are designed for isolated power supplies, not for measuring, and they are indeed adequate for their intended purpose. But not for ours.
I have this afternoon measured a number of transformers. One was a very old one of unknown make. The core looks to be about 10 VA, but the secondaries are rated at just 1.7 VA. The secondary voltage was an exact replica of the primary: no visible distortion, and only the minutest phase shift at 250 V. If you give the iron an easy time, it will reward you. If you put more copper in as well, you'll have lower resistance windings which will improve the regulation. If you economise on size and weight and drive the iron hard, then you introduce distortions and phase shifts and drop voltage. If you skimp on copper, you drop volts under load and regulation suffers as a result of that too.
But the big fear that I share with Glyn and Trystan is having mains voltages in the hands of amateur constructors. Yes, most of us are careful and well aware of the dangers, but a lack of knowledge in someone who is not aware of the dangers, or a moment's inattention by someone who is, could have tragic consequences. That's why there's a strong desire to keep mains voltages contained within a proprietary unit.
I want to backup what you said, both the technical and the security/safety aspect.
Although it is possible to have your transformer purpose made for measuring (we have done this since years for mains monitoring, and these things are not very expensive), using a part like this still means that amateurs have to deal with the mains voltage on the primary side (which is simply not 100% legal, at least here in germany).
Once you decide that handling mains voltages is not an issue, then you are open for a lot of other solutions like the ones specified in the above posts.
But I strongly recommend to stay with the original solution of using a wall plug transformer. The accuracy that can be achieved with these is 'good enough' and they are completely safe to use. Why play with your live?
Open-source tools for energy monitoring and analysis. This project uses the GNU General Public Licence