### AC-AC - why is this needed?

I ordered an emonPi Solar kit, which includes 2 CTs (one for grid <-> fusebox, and one for fusebox <-> solar) and an AC-AC for a reference signal.

It's all working well, but I'm curious to know, what value does the AC-AC adapter add? I'm not sure what additional information it gives us. The direction of current flow between grid <-> fusebox is inferred by polarity of the CT on that cable. (isn't it?)

Does it just give a more accurate power reading, as current can be multiplied by Vrms (say, 248V) rather than 230V assumed?

### Re: AC-AC - why is this needed?

There's a slight problem with your thinking on the direction of current flow - it is alternating current, meaning it flows in opposite directions on each half cycle. So, knowing that, tell me what the direction is. You can't, without having a reference, and that's where the voltage input comes in. I think you need to read the introduction to ac power in Building Blocks.

So there's that and yes, it does indeed give you a more accurate power value because you know the exact value of the voltage, and means you have a value for real power (W) rather than guessed value for apparent power (VA).

### Re: AC-AC - why is this needed?

Right, phase difference. Of course. Thanks for clarifying.

I knew it was going to be a daft question! I'll stick to the software forum in future :-)

Cheers!

### Re: AC-AC - why is this needed?

Can a suitably insulated voltage divider be used in place of the AC-AC adapter to supply an exact value of the voltage?  e.g. 100k and 4k

### Re: AC-AC - why is this needed?

You mean isolated, not insulated.

If you have a galvanic connection to the mains electricity supply, then you must enclose the unit in either a double-insulated plastic or an earthed metal box, and you cannot have any connection to the outside world unless that connection provides isolation to at least 500 V (and the actual test voltage it must withstand is considerably higher at 2120 V).

Why must you isolate? Consider what happens when your 4 kΩ resistor fails, or the neutral connection falls off. (Those resistor values are quite wrong, but that doesn't affect the argument.) The whole circuit is then live to the full mains voltage, and enough current will pass through the 10 kΩ resistor to at best destroy anything connected, and at worst to deliver a fatal electric shock. We'd prefer you didn't kill yourself or members of your family.

If you want something better than an isolation transformer, or you want to read about the precautions necessary to do what you ask safely, see http://openenergymonitor.org/emon/node/1987

### Re: AC-AC - why is this needed?

Cool, thnx.  So in theory it would work, but in practice it's a death trap.