Separate emonTx for current clamp and AC-AC adapter


My current OpenEnergyMonitor setup just uses the CT clamp, as it's outside in the electric cupboard and there is no mains sockets.

I'd quite like to add on an AC-AC adapter to improve the accuracy, so i've got 2x choices - drill through the back of the electricity cabinet to get a cable to inside, to plug into the mains, or have a 2nd emonTx transmitting the AC-AC data separately to the one sending the readings from the CT clamp.

Is the latter possible?

Also, if you do have an AC-AC adapter connected, do you get to see both readings - both the current ones direct from the clamp, and the improved reading with the AC-AC adapter as separate feeds?




TrystanLea's picture

Re: Separate emonTx for current clamp and AC-AC adapter

Hello Ian, unfortunately its not that easy to transmit the voltage signal independently, the way the more accurate real power calculation works is that its sampling at high frequency (52 samples per 50Hz sinus) and multiplying the instantaneous voltage and current reading together for every sample. Some ideas have been discussed for transmitting the phase timing information between two emontx's but it gets quite complicated.

You do get both readings, the first is realpower and the second apparent power, there's more on this in the AC power theory section of building blocks here:

Robert Wall's picture

Re: Separate emonTx for current clamp and AC-AC adapter

What Trystan is saying is that you can measure voltage in one emonTx and current separately in another, do the rms calculation on each and multiply the two rms values together to get apparent power (VA).

In order to measure real power, you need to measure both voltage and current at the same instant (or very nearly), multiply the two together and then take the average. There's no practical method to synchronise two emonTx's and to transmit the instantaneous readings from one to the other in order to do the real power calculation. Note that real power = V x I x cos(Φ) only works for pure sine waves,  the average of instantaneous V x instantaneous I works for a distorted wave too.

ichilton's picture

Re: Separate emonTx for current clamp and AC-AC adapter

Thanks for the explanations - that makes sense!

Sounds like drilling through the wall might be the best option!




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