emontx newbie to measure subsctions


I'm planning to use a emontx with 4 ct.

One ct is for mains to measure global home energy.

I have a panel with switches isolators for each room in the house.

Can I use the other 3 ct to measure energy of some room like laundry o kitchen?

thank you

Robert Wall's picture

Re: emontx newbie to measure subsctions

Yes. But which version of the emonTx do you intend to use? The 3 main inputs are set for 100 A maximum, the fourth (only available on the V3) to 18 A, so if your maximum current per room is significantly less than that, you will lose accuracy. It's possible to change the scale of an input by removing a resistor from the PCB and replacing it with a higher-valued one, but you should only do this if you have some experience with electronics.

cecco's picture

Re: emontx newbie to measure subsctions

Thank you for your reply!

I was planning to use v3.

My mains is 220V 6KW max.

I want to meter some appliances like washing machine, dryer, fridge

I can replace resistor on the pcb but i need some guidance of which to change and higher value of the new ones.

Where i can find instructions?

Thank you

Robert Wall's picture

Re: emontx newbie to measure subsctions

If you have Eagle, get the schematic and PCB layout files from Github - not essential but it will make it easier to know what you are doing. The resistors you need to change are the CT burden resistors that connect straight across the CT, these are the closest to the back of the CT jack sockets, and there's a pair of plated-through holes at each end. They are 22 Ω (marked 22R0) for the main inputs and 120 Ω (marked 1200) for input 4, they are R7, R8, R9 & R18. You need the closest value that will give you about 1.1 v rms at the CT primary current you want to measure, bearing in mind the CT secondary current is 50 mA per 100 A of primary current if you're using the shop YHDC SCT-013-000. So for 6 kW @ 220 V, that's 27.27 A primary current, 13.636 mA secondary current, so 80.66 Ω is the actual value you want. I'd use a 75 Ω 1% 0.2 W (or so - dissipation is negligible) metal film wire-ended resistor. You might be OK with 82 Ω but you are eating into the safety margin for component tolerances unless you're prepared to measure a bit less than your 6 kW. Unsolder the SMT resistor and use the holes to solder in the new wire-ended one.

All the theory is in Building Blocks, as are the calculations for your new current calibration constant(s). (And you'll need a programmer because you'll have to edit the sketch and load the new version.)

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