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I've had a lot of joy with energy ICs, although I've not used the one you referenced, but they all seem to follow the same basic principles.  The big advantages they give you is they have dedicated high speed DSPs, and very high resolution AtoDs.  The former means all the processing burden is removed from your cpu.... you just need to fetch the results from the IC at your leisure.  The latter gives you great dynamic range on energy measurements, typically in the order of 1000:1.

I'm not sure what you mean by "instead of the AC-AC converter".  The main purpose of the AC-AC converter in the emontx world is to isolate you from the mains, and to reduce the mains voltage to a safe level.  A resistor divider is then used to further reduce the voltages to levels safe for your cpu.  The alternative is to use a resistor divider connected directly to the mains, but unless you know what you're doing that is very dangerous.  And even if you do know what you're doing, it doesn't provide any isolation, so the entire circuit has to be considered potentially live and lethal right up to the isolation barrier you'd need to design in.

Most energy ICs expect a maximum swing on their V and I inputs of +/-500mV (peak), which incidentally makes them nicely compatible with all those CTs that produce 333mV (RMS) at maximum primary current.  The typical way to reduce V down to the required 500mV limits with energy ICs is with a resistor divider connected directly to the mains.  That means the entire IC (including the screw terminals for the CT) should be considered live.  You'd then need to isolate that side of the board from the cpu side of things, typically at the serial bus.  It also means anything on that side of the board will need its own isolated 3.3V supply... it can't use the same Vcc as the safe side of the board.

So whether you're using the emontx design, or an energy IC, an AC-AC converter up front will give you isolation.  The alternative should only be done by trained professionals familiar with all the safety standards.

Thank you for that very informative read. To be honest, i'm really new to energy monitoring and would like to have a start using this modules. I saw the AC-AC converter sold in the shop that it was rated for 230v-240v 50Hz. The power lines in my country are 220v and 60Hz, so I'm not sure if it will work. On the other, I was able to buy a power analyzer board (consisting of the Cirrus Logic IC and other components along with optocouplers for isolation) with serial output so it can be easily interfaced to my arduino. The board measures Real power, apparent power, reactive power and other parameters. Am I correct to assume that the AC-AC converter is used by emonTx  to measure those parameters as well? If so, what I intended to is to buy an emonTx shield V2 and the current sensors and pair it with my power analyzer board instead of using the AC-AC converter. Will that be alright? :D 

I would imagine your "power analyzer board" would completely replace the entire emonTx shield.  If your board includes isolation and provides you with a safe serial interface to the IC, then you should be able to do all your energy monitoring via that.  Unless there is some other feature of the emonTx shield that you feel you need?

Your board is most likely configured to expect a 333mV type CT.  I'm not that familiar with the CTs sold with emonTx and whether or not, with appropriate burden resistors, they can be configured that way.  Hopefully someone else will chime in.   What's the maximum current you anticipate flowing through the cable you plan  clamping?

If you post the specs to your "power analyser board" we may be able to comment further.

If you need details of how to do the voltage monitor safely without using a transformer, read up on GIVMon. It is not a trivial task.

I've taken a quick look at the data sheet for your evaluation board and nowhere does it mention that there is any galvanic isolation between the inputs and the USB output. You need to be very careful to check that the necessary safety isolation is in place before you use this on live mains. If you cannot prove that it is safe to use, you must assume that it is not safe. It is after all made available for "evaluation" purposes.

The AC-AC converter is only a transformer, safely enclosed. It does absolutely no signal analysis or processing of any description. The sketch running in your Arduino takes instantaneous measurements of voltage and current - about 40 per cycle at your frequency - and computes rms voltage and current, real and apparent power from that. Which, as dBC writes, sounds exactly like what is going on inside your "power analyser" board. For that reason, I think you would have problems pairing - or splitting the functionality between - the AC adapter/Shield/Arduino and the power analyser board.

I would expect the shop AC-AC converter to work happily at 220 V, 60 Hz although you might be happier using a locally sourced equivalent. The voltage (9 V) is normally specified at the rated load, it should give about 11 V ac on no load - any other voltage and you'll need to change a resistor on the shield and change the calibration in the sketch, neither of which is a major problem.

CTs have a VA rating like any other transformer. (It's just that most don't mention it in the data sheets.)  If you reduce the burden resistor value to lower the output voltage, you are actually reducing the power the ct needs to transfer, so making life easier for it. You should get less distortion and possibly less phase error that way.

(You'll still need the shield if you want the RFM12B radio module.)

Thank you! It was stupid of me to realize this, about the power analyzer already having the functionality of the emonTx. I'm still purchasing the shield though, since I already have a couple of SCT-013 CT's and it provides a very nice interface along with the RFM12B modules. I appreciate the help, thank you very much! :)

p.s. for clarification purposes, here's the the link to the power analyzer http://www.e-gizmo.com/KIT/pwranlyzermkii.html

Ad dBC pointed out, you MUST use the optically isolated port and you must regard the whole unit as live (and therefore potentially lethal) while it has any connection at all to the mains electricity supply.

I can't see a CE mark on the picture so you need to satisfy yourself that the unit is safe to use (if you're in the EU).

Your power analyser board uses a shunt for measuring current, so no CTs are required.  The current supplying your load has to pass thru' your power analyser board, and is limited to 8A, so you'll only be able to measure up to 1760W  (assuming 220V).