Forrest W Christian wrote:
> 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.   

Use the same method for measuring voltage that you propose for measuring 
current: a transformer.  With a UL rated device, you will get the same 
safety isolation as the current transformer gives you.  Rectify/filter 
the output to, as you say, keep the AC voltage isolated from your 
circuits.  Use a step-down turns ratio to get the output down to a safe 
level and then use a resistor divider to scale it for your a/d 
converter.  Plan on designing clamp circuitry for surges, spikes, etc.
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