Using Efergy CT Sensor with EmonTx V3


I had a Efergy CT sensor from the efergy E2 sensor ( and also an emonTx V3 node I bought from the store here. Can I use the sensor from Efergy E2? I did some reading on this site and it looks like because of the CTTurnRatio is different between the efergy CT sensor and YHDC SCT-013-000 CT sensor, therefore the burden resistor should different? The efergy is already hooked up on my main line so I'd like to make it work if it is possible.


Anyone can advise?




Robert Wall's picture

Re: Using Efergy CT Sensor with EmonTx V3

Yes. Don't use the Efergy CT. The Efergy system appears to work on totally different principles to ours and there are components inside the CT that will distort the waveform and give you totally wrong readings. If you wish, it's fairly easy to modify and rectify the problem, but you'll then need to make measurements yourself to determine the correct burden resistor value and calibration constant. I might be able to help if your CT is the same as the one I measured and tested (a dark grey case with a white lead and 3.5 mm 'stereo' jack - I believe there are at least 2 versions).
This thread shows the type I measured and the internal components.

amailtoaben's picture

Re: Using Efergy CT Sensor with EmonTx V3

Hi Robert,

Thanks for the reply. I believe mine is a different one from what you tested. See this link from efergy:

I am fairly new to the energy measurement and might be not quite familiar with the hardware as you do. So forgive me if I asked stupid questions: is the burden resistor on the emonTx board? (At least that's what I read from one of the article here) If that's the case, if I need to adjust the burden resistor, then I have to modify the emonTx board?

Robert Wall's picture

Re: Using Efergy CT Sensor with EmonTx V3

Your link is 404, so I don't know what you have. If you have the white one with the 2.5 mm plug, then from the photo on the UK site it looks as if there is a "saddle" holding the core in place, and when you open it, you'll be able to push a couple of clips in on each side, the saddle will slide out and the core and bobbin with a small pcb attached will follow, with a gentle push on the cable (all non-destructively!). You'll then be able to see what is in there.

The burden resistor is indeed on the emonTx pcb. It's a surface mount component, but there are adjacent holes for a wire-ended resistor, so it would be a case of removing the SMT one and substituting a wire-ended one of the correct value. If you look at the photo here, the burden is the large resistor immediately next to the jack sockets.

If you're unsure, then if our standard 100 A YHDC ct will fit over your fatter US cables (I assume you're in the US as you linked to a US site) you probably should avoid the uncertainty and get one or two of those. If it won't fit, I recommend a ct from the Magnelab range, derated to 33% (get the "current output" version). The 600 A  SCT-1250-000  (ratio 1:7500 is good for 200 A @ 1 V output + overload to 120%, using a burden of 37.5 Ohms).

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