# Sanity check before I order more emontx's

Now that I have my setup running and a single EmonTXv3 with the AC->AC adapter, presumably I have decent vrms coming in via that. When I order two more EmonTXv3’s, do I need more AC->AC adapters or can I just safely power them by USB. Everything will be connected to different sub panels of the same feed.

Or is local-vrms somehow used to help calculate the ADC’s for the CT’s?

The a.c. adapter is used by the emonTx to calculate real power. For that you need the voltage, the current and crucially the phase relationship between them. If your emonTx doesn’t have an a.c. adapter, it can only guess the power based on assumptions - those being that the actual voltage is the same as the nominal voltage set in the sketch and the combined power factor of the load on the circuit you’re measuring is unity. Both those assumptions will almost certainly be wrong, by how much determines the amount of error in your readings.

What’s your electricity supply system - you haven’t given the country where you live but your IP address is Canada, so would I be right in guessing you have a single phase split supply (120 - 0 - 120 V)?

If you wanted, you could power all three emonTx’s from the 5 V d.c. USB supply (removing JP2 on all of them), and give all three the same output from the a.c. adapter to measure the voltage and to get a phase reference from. (But you must make sure you get all the polarities right.)

If you want to be really accurate (but it’s probably overkill), have two a.c. adapters, one on each leg, and try to arrange your c.t’s so that as best you can, one is dedicated to one leg and one to the other. You’d need to compromise with the 3rd emonTx and with the 240 V circuits.

Yes, Canada so yes, split-phase (120-0-120).

Quick description:

I have a meter on a pole (A)

I have a panel in an outbuilding (B) with an EmonTX.
I have a panel in a house C with an EmonTX + AC adapter.
I have a sub-panel in the house (D) with an EmonTX.

It looks like this:

``````      A
/     \
B       C
\
D
``````

If I have a 9v AC adapter on an EmonTX in panel C, do I need a AC adapters on each of the EmonTXs that are in Panel B and Panel D or is the vRMS supplied by the EmonTX in Panel-C sufficient.

I think what you’re saying is that the vRMS is used locally within each EmonTX as opposed to globally within EmonCMS. So I need one AC adapter on each EmonTX.

Yes, if the emonTx’s aren’t co-located (more or less) so that you can’t (or it’s too hard) to share the 9 V a.c, then each must have its own. And you need something to supply power to the emonTx anyway, so these not a lot of difference in the cost. The power calculation is done inside the emonTx, it’s not possible to calculate the real power accurately inside emonCMS.

Thanks. That’s just what I needed to know.

Sure could use an 8-CT EmonTX or something.