Extending CT cable


Has anyone used a headphone extension cable to lengthen the cable to the CT clamp? I'm wondering if it would have a significant effect on the accuracy of the readings. Unfortunately the tx board is a little too far from the glcd/base in its current location.


MartinR's picture

Re: Extending CT cable

Do You mean it's too far for radio transmission? If so, I had the same problem so I reduced the baud rate on the RFM12s and it improved the range enough for my system to work.

The JeeLib library uses 49.2kb/s, which is totally unnecessary when you're just sending a few bytes every 5 seconds or so. I used 3.918kb/s by adding a line after the RFM12 initialisation as follows:

rf12_initialize(MYNODE, freq,group);
rf12_control(0xC657); // approx 3.918 Kbps, i.e. 10000/29/(1+0x57) Kbps

obviously you'll have to do this on all the nodes in your system.

markp's picture

Re: Extending CT cable

Ah, perfect. The system isn't local to me so changing firmware requires a bit of driving, but I'll give that a go and see how I get on. Sounds like a sensible thing to do anyway, given the payload size isn't huge and as you say the transmission interval is pretty long.

I'll let you know how I get on :)

gapintheclouds's picture

Re: Extending CT cable


I know this an old thread, but has anyone tried the headphone extension cable idea? At the moment, I have no mains power near the meter, so I can't power the emonTx v2 and the AC-AC voltage adapter. I'm currently thinking it might be easier to lay a headphone extension cable through a pre-existing hole to another room with mains power, than laying a mains extension cable to the area near the meter.



Robert Wall's picture

Re: Extending CT cable

I wouldn't buy a headphone extension cable - too costly!

I've done around 10 m, using twin twisted screened pair ("microphone") cable bought off-the-reel, earthing the screen only at the CT end. At the emonTx end there's no screen connection, just the two cores to tip and sleeve of the jack.

I've not done the sums on phase shift due to cable capacitance at around 100 pF/m, but as the system impedance is around 100 Ω, I wasn't going to bother either! Sensitivity doesn't come into consideration (except as a second-order effect) as the CT is a current, not voltage, source.

gapintheclouds's picture

Re: Extending CT cable

Great. Thanks for the info. Good to know that the extra cable doesn't affect the performance.

Do you reckon I could use shielded ethernet cable (or indeed unshielded as I have loads of that)? One day I was hoping to get a hall effect sensor to monitor gas usage as well (http://openenergymonitor.org/emon/buildingblocks/gas-meter-monitoring). So I could just lay the one cable and add the hall effect sensor later for example.

Robert Wall's picture

Re: Extending CT cable

You are looking at 1 V (in round terms) for max current, and the ADC is 10 bits, so it's sensitive to less than 1 mV. The twisted pair would be fine, but I'd prefer to see a screen around it as well - depending of course on what sources of interference you might have nearby. Equally, putting digital signals in the same cable might be OK, but you'd be safer not doing so.

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