EmonTX Non reactive CT measurements

I have bought the home energy bundle kit (433Mhz). Emontx has the CT123 firmware installed. When I monitor the serial out on the board with no CT's connected I get the following:

41 3 33 3293
39 3 32 3293
41 2 26 3293
38 4 32 3293
41 3 32 3293
41 3 31 3293
41 3 32 3293
40 0 32 3293
42 4 32 3293
40 3 33 3293

When I connect the CT to port 1, and compare the output from it compared to a clamp on ammeter I can see 40 from emontx and 18.1 amps on my ammeter. If I turn off the oven, my ammeter shows 4.5amps and the emontx reports 42.

When I connect the CT to port two I shows anywhere between 0 and 6, the ammeter shows 15+amps (with the oven on and washing machine running).

Connected to port 3 output from emontx is 700 with an ammeter reading of 4.5amps and a reading of 725 at 14amps on teh ammeter.

Obviously the outputs from the emotx are not representative of the real world power usage. And have considerable differences when disconnected. I have the ammeter clamped just above the CT clamp on the main tail coming into the consumer unit (with the CT label facing the unit).

Really don't know how to fault find this. Can anyone help?




Robert Wall's picture

Re: EmonTX Non reactive CT measurements

Those numbers are 'presumed power' (i.e. rms current x nominal rms voltage) in watts for the 3 inputs, then the supply voltage to the processor in mV.

40 W is about what we expect from noise and other pick-up, so in effect you're not seeing the current.

1. You have got your c.t. clipped on only one conductor of course - like your clamp ammeter. That's the usual reason people read nothing - they clip onto a 3-conductor extension lead!

2. Have you plugged the c.t. in? Quite often, the socket when new is very stiff and unless the plug has gone all the way in, you get nothing. If you can see any metal of the sleeve, it's not all the way in.

3. Are you able to read mA of a.c. current? If you are, if you put your multimeter on the mA range directly across the c.t. tip & sleeve of the plug, you should read 7.5 mA with 15 A flowing. Connect your meter before you clip the ct onto the tail - take care not to open-circuit the c.t. whilst current is flowing.

4. Is the c.t. good? If you can read about 100 Ω between tip and sleeve, that's OK.

martinl's picture

Re: EmonTX Non reactive CT measurements

Robert thanks. Turns out it was Number 2 - I hadn't plugged it in far enough (muppet). Thanks for your help - much appreciated.

Robert Wall's picture

Re: EmonTX Non reactive CT measurements

"I hadn't plugged it in far enough" That's where my money was. You're not the first, and quite likely not the last either ;-)

I take it you've got sensible numbers and have adjusted the voltage to suit your local average mains voltage:

emontx.power1 = ct1.calcIrms(1480) * 240.0;

(240 V is the nominal UK centre voltage - yours might be higher or lower by a few percent, the limits are 230 V ± 10% confusingly, due to history!)

martinl's picture

Re: EmonTX Non reactive CT measurements

Yes I now have sensible numbers. Luckily I do have 240v RMS here because I hadn't spotted that adjustment. Thanks for the heads up. 

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