Forrest W Christian ha scritto:
> I have an application where I need to monitor voltage and current of 
> three phase power in datacenters .... typically in the US, we're talking 
> 120/208 Wye (120 each leg to ground , 208 leg to leg).  
> 
> The current is easy - three current transformers wired to an appropriate 
> circuit to rectify, filter, and measure...  pretty standard 
> engineering.   Plus, current transformers are available pretty cheaply.
> 
> Measuring voltage is a bit tougher in my mind.   I'd really like to keep 
> the AC voltage isolated from my circuits.   I have lots of ideas on how 
> to do this (build a second circuit, with a PIC which is interconnected 
> with the line voltage, and sends a serial datastream out a optocoupled 
> line, etc.), but all of them involve building something which attaches 
> directly to the AC line (which means the possible need for UL listing, 
> and the added dangers/liabilities of having a product which does so).
> 
> I guess I'm looking for a couple of things... first of all if there is 
> something commerically available that I've missed that will measure line 
> voltage and output it as some sort of isolated low voltage - similar to 
> a current transformer.   Cost is definitely an object.


Hmmm, take a look at ADE7754 or 58 Energy-Meter, and/or Crystal (CIRRUS) 
ones as well. In their design I can see one or two 470K-1MOhm resistors 
placed in series with 220/380VAC to the differential input of their 
chips. And this is what I've done in some cases too.

I also noticed that, if the PIC is powered by a switching power supply, 
then its GND will measure kind of 0 with respect to the mains Neutral - 
thus only one resistor from the mains to the EnergyMeter may be needed, 
and the other input can be grounded. Of course, adjustments will be made 
in software.

Dario
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