Burden resistor selection for 60A Supply, 13A Hot Water and 13A Solar

Submitted by sheppy on Tue, 21/04/2015 - 00:27

I'm awaiting the arrival of my EMONTX V3 and would like to calibrate it to reflect my 60A supply rather than its default 100A I also would like to calibrate separate channels for the 13A Immersion feed, 13A Solar, and 6.5A Spa heater. The Main supply fusebox is very cramped so I want to make the burden resister as low as possible to limit interference from adjacent wires. My gaol is to see standby power in sufficient detail to see if something has been left on. I realise the V3 already has one high sensitivity input and I'd like to make this even higher in sensitivity to reflect the 6.5A spa divert load what its connected to.

Is there an actual sum I can do to work this out? I envisage 3 different results spread across the 4 channels.

Also has anyone had much success extending the CT cable without running into interference?

Re: Burden resistor selection for 60A Supply, 13A Hot Water and 13A Solar

Submitted by Robert Wall on Tue, 21/04/2015 - 11:00.

The sums are all in Building Blocks - see the article on calibration theory. It's only straightforward Ohms Law, but you need to allow a few percent for component tolerances. I aim for not more than 1.1 V rms across the burden at max current.

But you have conflicting requirements. By making the burden resistor small, you are raising the current you require for the maximum input, which decreases the sensitivity. But having said that, because the CT is in effect a search coil and a current source, I can't see the burden value having much influence on pickup from adjacent cables. I've extended the CT cable to about 10 m with no discernible effect, but I used "microphone" cable and there were no interference sources nearby.

Re: Burden resistor selection for 60A Supply, 13A Hot Water and 13A Solar

Submitted by sheppy on Wed, 22/04/2015 - 10:09.

Thanks for the reply, I will read the calibration bit. Did you use the thin microphone cable?

Re: Burden resistor selection for 60A Supply, 13A Hot Water and 13A Solar

Submitted by Robert Wall on Wed, 22/04/2015 - 15:59.

I used the heavier Spiratronics WL2-013 ¼" diameter cable (apparently no longer stocked, but similar cables is widely available elsewhere, e.g. here), but this should do, earthing the screen only at the CT end.

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