multi-CT (6) MQTT Publisher shield

Submitted by Elwell on Tue, 10/09/2013 - 02:57

Hi Folks,

Rather than using RF to send the usage telemetry from our (metal) meter box, I'm going to build a multi-CT shield to add to a PoE equipped etherten or Arduino Ethernet (both use the whiznet 5100 chip, so none of the mqtt issues associated with the nanode ENC chipset.

I plan to use all 6 analogue inputs and bring them out for the seedstudio 30A CT clamps - the board itself will be V basic, none of the voltage measurement stufff (thats already picked up by other kit such as the UPS (via apcupsd) and the PV inverter (during the daytime))

sketch will basically just loop over all inputs and publish to a broker on the LAN.

I was going to stripboard a version for me, but if there's sufficient interest I'll see about producing a 'real' PCB and sharing.

Anyone interested?

Andrew

Re: multi-CT (6) MQTT Publisher shield

Submitted by Robert Wall on Tue, 10/09/2013 - 10:16.

One small caveat: unless you bring the voltage measurement to the same place as the current, you'll only be able to calculate apparent power. The calculation of real power implies multiplying each voltage and current sample and then averaging. If you send individual samples of voltage and current from different places, you will have big problems with timing delays.

Re: multi-CT (6) MQTT Publisher shield

Submitted by Elwell on Tue, 10/09/2013 - 12:03.

Thanks,

> you will have big problems with timing delays.

realistically for a home install, is this going to be that much of an issue? I know my AC voltage is typically in the range 248-258v -- so even with a slow voltage sampling It won't be out *that* much (will it?)

Even if I'm only calculating apparent power, having 2 channels for the sources (grid and PV) I can still work out the relative usage by the other 4 circuits (ie, where our consumption is going). It may even be possible to "recalibrate" on the fly given the logging direct from the solar inverter -- ie, if that says I'm producing 1000W at the output stage then the CT on that cable in the meter box should be close to 1000W (hopefully!)

My plan for timing is to throw everything into the messsage broker every 6 seconds

Thanks for the suggestions - anything glaring I've missed so far?

Andrew

Re: multi-CT (6) MQTT Publisher shield

Submitted by Robert Wall on Tue, 10/09/2013 - 12:37.

I don't think you understand the difference between apparent power and real power - you need to do some reading: http://openenergymonitor.org/emon/buildingblocks/ac-power-introduction

They are only the same for purely resistive loads. Some small loads can have appalling power factors and you may have 20 VA of apparent power and maybe 3 W of real power.

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