240VAC to 12VAC Transformers

Looking at options for AC-AC transformers for real power measurement.

Could you please outline requirements or what makes a suitable transformer for purpose?

Based in Australia so looking 240VAC to 12VAC.

Could it possibly be as simple as a AC Halogen/LED Driver transformer?


Love the project and thanks to Bob and OEM for their hard work - job well done!

Daniel Jansen

Just a basic transformer AC-AC. Output 9Vac - 12Vac is fine. Not sure what’s inside any particular Halogen or LED transformer. Those online described as such don’t look promising. It should have waveform that is proportional to that of the line voltage.

Even if there is a suitable transformer with a AU plug, you will still need to calibrate to it and will only be guessing at the phase shift. My suggestion would be to get the UK or EU 240V transformer and spend your time looking for a socket or travel adapter to use it down-under.

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There are some scope pics here: AC - AC Adapter and Arduino for Measuring AC Voltage - #46 by dBC that show why the electronic type aren’t suitable.

I’ve not tried one of these these so don’t take it as a recommendation, but something like this: 9VAC 1A Unregulated Power Supply 7DC Plugs | Jaycar Electronics might do the job if you don’t want to go the travel adaptor route.

I’ve been meaning to pick one up and stick it on the scope and see how faithfully it reproduces the AC signal, and how constant its phase shift is across the varying input voltage. Those are the main two criteria to look out for, and the hardest to judge just from the specs. Where in Aus are you? Here in SEQ my mains varies from about 245V to 260V so you’re looking for something that has a fairly constant phase shift across the range it’s exposed to. (I can’t recommend a local solution because I don’t use a VT in my setup).

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Could you use one of the adaptors from the shop with a travel adaptor?

I am using the one from the shop with an aus to uk adapter
But here is my grid voltage as reported by my ABB inverter, which I monitor with the Raspberry pi and Solar123

As you can see its up and down a fair bit, and i wonder what sort of probs i will see with accuracy with my IotaWatt

I use one of these on my emonTX running the 3 phase sketch but don’t have access to a two channel scope to check the actual phase difference between the 240V and 9V sides.
If someone can suggest a method of comparing primary and secondary sides with a single channel scope I can certainly give it a go and report the results.

I don’t anticipate any problems with accuracy with that voltage profile. Percentagewise, it’s not far off from what I see in my US 120V service. Regardless, that’s what the VT is for, we measure the voltage at every sample and compute the power as a function of that, just like your meter.

I know there has been a lot of talk about phase shift varying with voltage, but I see that as a second or third order consideration. The IoTaWatt uses a single phase shift number for an adapter. I’ve tested the UK adapter at 2deg phase lead using 240V 60Hz. While I don’t doubt that a different frequency or voltage would yield different results, I can’t buy into it being more than a few minutes of difference. That’s not going to be significant in the big picture.

btw/ I don’t measure phase shift with an oscilloscope. What I’ve found is that the phase shift at zero crossing is not the same as the phase shift of the entire wave. (VT or CT). There is an algorithm for determining the phase difference between two sinusoidal waves. It’s essentially the same as the power factor calculation. I use that to compare the VT (or CT) output to the live wall voltage using a galvanic isolated IoTaWatt.

Have you seen the Phase Error Vs Voltage graphs in this report: Learn | OpenEnergyMonitor

Once I started doing per-breaker monitoring, I found I was no longer looking at the big picture but rather lots of little pictures, and that’s where I discovered individual loads with very poor power factors. Introduced phase errors there can make a significant difference to the result.

Yes I have. All I can say is that I don’t have one of those adapters, so I’ll give it the benefit of the doubt.

While it’s not explicitely stated how the phase difference was measured there, it implies by the diagrams that it was done by measuring the time difference at zero crossing. My experience is that is not accurate. When I look at those plots, there is a clear phase difference at zero and along the slopes either side, but when you look at the peaks, there is virtually no difference. I believe that’s why my method yields different results. It weights that part of the curve much more than than the zero voltage part. That’s also what is happening in power factor calculations.

I get about 2 degrees for the ideal UK adapter. The crappiest 120V adapter that I’ve tested is in the 5 degree range, but most of the adapters with reasonable weight cores are around 2. The Ideal 120V adapter is 1.55. I don’t believe it changes by two or three degrees with voltage variation.

I use the same technique to measure shift in the CTs. When I measure the net phase difference between a VT and a CT, it is very close (.1-.2) to the difference between the phase leads that I measure for the individual transformers. So it all works for me, and the accuracy of the device seems to be right on as a result.

When doing these calculations, I use a technique where one of the signals is numerically shifted by 30-40 degree, the difference calculated, and then the artificial shift subtracted. That puts the calculation into a cosine range with greater slope and so better resolution.

But the proof of the pudding is in the eating. Look at some of your low PF loads with IoTaWatt and see how the phase correction compares to some standard.

Where does it say that? It is a totally unjustified assumption of yours, and it is wrong.

Hey Robert - easy going tiger

Overeasy is doing a fantastic job supporting and discusion IotaWatt,

And if he’d learn some electrical engineering, and not state facts that aren’t (which he’s done many times - so I’d urge anyone reading his opinions to bear that in mind), then I could have some respect for his software.