Need some advice

I am building a system using Arduino Yuns and emonTx shields to monitor the power supplied to our customers at our Internet data center in Tempe, Arizona.  The sketch I use uploads the emon measurements through the Bridge library using an HTTP get to a PHP script I have running on one of our web servers.

I have no trouble with my system monitoring 120V circuits.  My problem is that I have one 480V, 3 phase customer drop that I would like to monitor.  I realize that to monitor this properly, I would need to sample the voltage on each phase but for this I am only going to sample the voltage on the first leg.  Line to neutral is 277V.  Can anyone provide information or advice on how to connect the 9V input on the emonTx shield to a 277V circuit?

All advice is appreciated.

Robert Wall's picture

Re: Need some advice

You need a single phase transformer with a 277 V or greater primary and a 9 V secondary - ideally measurement grade that is not working near saturation and therefore has little phase shift and waveform distortion. The VA rating you require is next to nothing, as it is used essentially on no-load. The emonTx shield is designed for a nominal 9 V transformer that gives about 11 V on no-load. However, the secondary voltage is not critical; provided it is greater than a couple of volts and less than about 30 V, you can change resistors on the emonTx shield to accommodate the actual secondary voltage that you have out of your transformer.

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