Need to register proper values on a 3 phase 240V system - AC/AC issue

I have multiple 3 phase systems I need proper values for. I am using emonTX with 500amp CT sensors.

In order to register properly, I need to sample the voltage of my 1st phase.

My issue is, for the life of me, I cannot find a 240v 60Hz AC 9V 1a output adapter… There is no 120v in these particular systems.

In order to see if all works, I plugged in a 120V to 9v ac/ac adapter to another electrical system that obviously has 120V. I get values, but they are obviously not correct at all as im sourcing a 120V circuit and monitoring a completely different circuit of 416/240v.

What is one to do if 240v 60Hz is not avail at all.?? I really need to get this up as its part of a billing system…


Three weeks ago, you said you have two systems. I take it you’re working on the 240 Line-Neutral, 415 V Line-Line system? And it’s 60 Hz.

What can you get?

You don’t need an “adapter” as such, and you don’t need 1 A. We use the “Ideal” one because it’s certified and finger-proof.

Can you obtain an open-frame 240 V : 9 V isolating transformer and put it in a suitable enclosure, with a fuse in the primary side? One like this:
The no-load voltage wants to be about 11 V at your nominal voltage. (We specify transformers like that at full load - but powering the emonTx is almost no load!). The VA rating is not very important, 6 VA is probably as small as you want to go, bigger is physically bigger and more costly, to no real advantage.

If the output is less than that, then it will still measure just fine, but there might not be enough to power the emonTx, then you’d need a 5 V d.c. USB adapter to power the emonTx.

Obviously, we won’t be able to tell you the calibration details for something we haven’t seen before, but we can tell you the measurements to make and how to calibrate the system.

HVAC Control Transformers with 240 VAC primaries and a variety of secondary voltages ranging
from 2.5 to 12 VAC are readily available.

Here’s a link to some examples:

Not suggesting that one be bought from Flea-Bay, just showing a few examples of what’s available.

You are using an emonTX for billing?
While I believe an emonTX system is great for getting an idea of what the usage looks like, I am pretty sure no one would say it is suitable for use as the source of billing. It does not have the necessary accuracy for either party to be happy.

But if you still want to proceed with this, just get one with a 240V 50Hz primary. It will work fine, probably better.
Going the other way is not a good idea if there is significant loading on the transformer. If there is very light load on the transformer it won’t get hot. There is, of course, some debate on how this will affect the accuracy and phase shift. There will be some. Is it good enough to use for billing, I would say not, but it depends on the accuracy that the parties need and if anyone else has a say in what is used for billing. Some Authorities Having Jurisdiction have specifications for accuracy for billing and I doubt that any of these types of systems would be able to meeting them. However if the billing was for internal company department charges it is probably fine.

I think most of us would agree there, but I would remind you that at least one user in your part of the world obtained a substantial refund from his supplier, assisted at least in part by evidence from his Arduino Uno and emonLib.

That is good news for that person. Clearly possible for utilities to make mistakes in billing, usually in their favor :wink:. Not sure what the original poster is doing, but possible he could be on the giving end of the refund in a future similar event without the proper care in selection of the system used for billing.

Nothing is perfectly accurate. It all comes down to how much accuracy do you need/want/can afford.

All noted and thx. We already have the numbers the devices should be drawing as we can see this in real time individually. If I can get the numbers from the monitors close to the real deal then compare, Ill know the system is off by what and adjust accordingly. As long as I get a decent ballpark figure all should be good.