I have a small UPS that I am using to power the 5V PSU to the emonPi, but I have left the 12V AC adapter directly connected to the mains supply because I am not confidant that the UPS is going to give a good representation of the mains supply for accurate measurements.
My question is: Is it a good idea to have the emonPi powered with 5V DC but not the 12V AC during a power outage?
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The 12 V (nominal 9 v) a.c. does not power the emonPi, it only measures the mains voltage. All the power comes from the 5 V d.c. supply. It’s not good practice, but it shouldn’t do any harm to your Pi, as the current available is very small and easily handled by the protection diodes.
I would expect the UPS will not give a good measure of the mains voltage during normal operation, but of course your power and energy readings will fall to zero during a mains outage, because your’re monitoring a zero voltage. To get around that, you’d need to feed the a.c. adapter via a changeover relay maintained by the mains (and that needs to be done carefully so that there’s no chance of a back-feed into the mains from your UPS - like a double pole changeover with a 3mm air gap between the contacts).
Thanks Robert. I suspected it was OK but thought I’d ask. I’m not bothered what it logs during an outage since, for me, outages are generally few and far between. I just felt that I ought to have a UPS there to prevent data corruption caused by sudden death to the Pi’s power supply.
Edit: actually, thinking about it, having the reading drop to zero during an outage sounds like a good idea as I can then easily look back at the logs and see when and how long the outage was (although I can’t image I’d care that much to bother doing it 
).
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