Measuring vrms

I have an emonPi connected to the official 9v AC power supply and this show vrms as 240-248 volts (we are in the middle of nowhere so mains voltage often tends to be high). I also have an arduino UNO supporting the emonTX shield. If I remove the power supply plug from the emonPi and connect to the shield it registers the same voltage (or thereabouts). However, I have seen a post that says one power supply should be able to suport several devices (if only being used to provide a reference). So I soldered in a splitter into the output of the reference power supply and i only get a vrms on the shield of 144v, compared to the 244v approx shown on the emonPi. Checked with a voltmeter and I am seeing the full 11v or so on the connection.

Any ideas as to what is going on? Is it related to the supply voltage on the shield (5v) vs the Pi, a lousy soldered connection or what. Adding a second power supply seems cray so may have to resort to the novolts firmware on the shield!

What is the power source for the two devices? Is it common?

You need to look at the respective circuit diagrams to determine whether it is possible to use the same a.c. voltage - it’s OK with identical devices, but not necessarily possible for different devices.

The barrel outer is biased to 2.5 V d.c. for the Shield, the 9 or 11 V being on the inner conductor; whereas for the emonPi, the barrel outer is grounded.

If your emonPi and Arduino are both using the same 5 V d.c. supply, there’s your problem - you have removed the bias on the input a.c. by pulling it down to 0 V, so it’s effectively half-wave rectified the wave. The easy solution will be a large capacitor, not less than 1 µF (so that there’s minimal phase shift) in series with the a.c. to the Shield (+ end to the Shield input inner), and leave the barrel outer disconnected.

Wow! This system has enough gotchas to drive you nuts, but fortunately it also has helpful experts to help you out. Both systems were being tun from a USB mains socket (two feeds) but obvuously they share a common earth. Powered the arduino and shield from a seperate USB power supply plugged into the mains (obviously no shared earth there!!) and we are reading 240.3 on emonPi and 243.2 on TX shield which is within the accuracy of the devices without further calibration I guess. Will dig into the component box and find a decent capacitor and try that later today but that is one problem solved. Next to dig into the files and find out about calibration procedures and then set up more feeds to monitor the Tesla Powerwall and Air Source Hear Pump consumption.

HuGE thanks to Robert Wall for not only solving my problem but actually explaining WHY it was happening, which is just as important on this voyage of discovery!!

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