I’m wondering if it is possible to get more compact CTs? With the EmonTX V4 having 6 inputs. the ability to monitor multiple circuits is more attractive. However, the limiting factor is likely to be the space in the consumer unit (yes aware of the dangers of fiddling inside the CU).
Is it possible? Only need to be a max 16A for most domestic circuits.
Yes, we are planning to stock a wider range of CT sensors. The ECS06 here is particularly small https://www.echun-elc.com/Products.asp?BigClassID=81 there used to be an ECS1050 50A CT on there but I cant seem to find it now. Or for a solid core option the Talema is a nice compact option https://www.bec.co.uk/wp-content/uploads/2020/05/AQ-current-transformer-range-RS.pdf
Left to right: 20A 0.1%, 25A 0.5%, 50A 0.5%, 100A 3%
Sorry I realise it’s the ECS10-50 https://www.echun-elc.com/productsShow.asp?id=412 you can order the ECS10 in quite a few configurations.
If nothing else, the AZ series gives you yet another option - i.e a 40 Amp CT.
Is the one second from left (white) a clip on?
Yes it is. I’m looking at it on Trystan’s desk.
It is an Echun ECS0625 - 6mm opening.
I currently have an IotaWatt, and am looking at alternatives, now that production of the IotaWatt has stopped.
I found this project, and it looks really awesome =).
However, my DB is quite cramped, and it will be difficult to get the larger clamp-style CTs around the wires for each circuit.
For IotaWatt, I previously wired up some CTs using the Talema AZ-0500, along with a TVS diode P6KE6.8CA (datasheet)
I checked the OpenEnergyMonitor shop, and there’s still only fairly large split-ring CTs for sale there. However, I did see that @TrystanLea mentioned above they might stock a wider range at some point.
Does anybody know if I could use the Talema AZ-0500 that I have, with a emonTX4 or emonPi V2 box? (Still figuring out which one I should get to replace the IotaWatt).
Is the wiring also the same as well? I assume it still needs the TVS diode etc?
Anything else I need to bear in mind, or is it plug-and-play with those CTs?
Hello @victorhooi and welcome!
The emonTx4 and future emonPi2 are both designed as standard to use 333mV voltage output CT sensors. There’s no burden resistor on the board as that’s built in to the CT sensor with voltage output CT’s.
There is however a footprint on the PCB for burden resistors that you are welcome to solder on yourself. It looks like the Talema AZ-0500 is rated for 25A and a 500:1 ratio, so 50mA at 25A. That suggests perhaps a 6 ohm burden resistor would be best, though if the circuit is only 20A, 8 ohm would give you better use of the ADC range.
The emonTx4 has the option for throughhole as well as surface mount resistors (size 1206). It’s best to get higher precision resistors e.g 0.1% tolerance.
TVS diodes are already included on the board.
Thanks for the useful info!@
I obviously missed the part about how emon is now using voltage-output CT sensors, rather than current-output ones like the Talema.
I’m still getting up to speed on some of the theory here.
However, it seems like I can use my existing Talema CTs (and I don’t need to wire in the TVS diode either) - I just need to connect them to a 3.5mm audio plug.
And I’ll need to add the burden resisters as you mentioned - I’m looking at the PCB photos here:
https://docs.openenergymonitor.org/emontx4/hardware.html
Do you know where
Or would it be possible to put the resistor inline with the CT itself, on the cable?
Or alternatively, Magnelab make the UCT-0300, which is a voltage-output CT:
This is available in say, 10A variants (which I assume would work well with a 10A circuit).
And it’s solid core and fairly compact (datasheet). The opening is 7.62mm, which is similar to the Talema, however, the overall diameter is 42mm, versus 21.5mm for the Talema.
Would that be a better out of the box solution for using with the emonTX v4?
Or is there any chance that the OpenEnergyMonitor project would sell either smaller voltage-output CTs and/or solid-core CTs in the store anytime soon?
The obvious place is inside the plug - this way, you don’t risk open-circuiting the c.t. when you unplug it. I’d say this is preferable to having the burden on the p.c.b. But if you do, the pads for the SMT resistor or the holes for the wire-ended one are obvious - adjacent to their respective sockets.
There’s no phase error information on the Talema AZ-0500 datasheet that I found, which could well be highly significant, so it’s hard to say which is the better, that or Magnelab, though my knowledge of Magnelab (along with the larger size and full data being published) suggests Magnelab would be the higher specification device.
This is for @TrystanLea to answer.
Are you asking if it’s OK to put the burden in series with one of the CT leads like this?
(R represents the burden. L is the CT, in both drawings)
Physically inline with the cable, as Robert described is OK.
But, electrically, it needs to be across the leads. i.e. in parallel with them, like this:
I thought this was too obvious - but maybe not.
A true current transformer - the name is a clue - is a current source. To understand it, turn all your thinking about voltage sources (the ubiquitous type most people think about, like a battery or the mains supply) on its head: It wants to generate whatever voltage is necessary to drive its output current into your burden, so it’s always safe to short-circuit a c.t. in the same way as it’s always safe to open-circuit the mains or a battery.
A current source with a burden behaves just like a voltage source that has some internal impedance. (Look up Norton’s and Thevenin’s Theorems.)
Therefore, the value of the burden is defined by the c.t’s output current and the voltage you want to feed into the voltage-sensing c.t. input of your emonTx4, which is 0.333 V rms nominal maximum.
So I ordered some of the Magnelab voltage-output CTs.
Unfortunately, I probably should have checked the dimensional diagrams carefully before ordering…they are absolutely huge! (The dimensions they gave are also in inches, not millimetres…which I must have missed - reminder to always check units…ugh).
Here’s a photograph of the 10A Magnelab next to the Talema AZ-0500:
I don’t think the Magnelab’s are going to fit in my DB (below photo is with the cover removed - I’ll need to put that back):
So it seems maybe the Talema’s are a good option - I’ll just need to wire them up to a 3.5mm plug, and then also put the burden resistor across the two leads of the Talema, right?
@TrystanLea Do you know if there are any compact CTs (solid core or otherwise) that you would be offering in the store anytime soon?
Magnelab seems to have a new 333mV output CT - the UCT-0350-050 (Datasheet).
From what I can see, it’s significantly physically smaller than the UCT-0300-10 I purchased before.
26.8mm versus 42.2mm outer diameter.
Does this look like it’d be suitable for using with the emonTX4?
I just need to wire it up a 3.5mm audio jack, right? (No need for a TVS diode).
(I also saw from another post that apparently there’s a bit of a complexity with the emonTX4 support- and it’s been replaced with a emonTX5 and/or emonPi2? I’ll need to read through all the threads - but it seems I might need to wait for some more hardware?)
@TrystanLea do you, or anybody else from the OpenEnergyMonitor team know if there are still plans for smaller CT’s to be available? (I assume with 3.5mm plugs already on there - for those of us who are horrid at soldering…aha). Would the Magnelab UCT-0350 I found above be suitable, or did you have other ones in mind?
One of the reasons for adopting the 0.333 V input for the emonTx4 and all subsequent variants (including the emonPi) was the wide availability and choice of current transformers, both ring-core and split-core. Any 0.333 V output c.t. will be suitable, only the reading needs to be multiplied somewhere in the digital processing to make the calibration correct if the default happens to be wrong. For example, if you’re using a 50 A c.t. on an input with a default calibration of 20 A and you can’t or don’t want to change it, you multiply the current/power/accumulated energy it reads by 50/20 (2.5).
Correct. The c.t’s burden, which is what limits the voltage the c.t. generates, is inside the casing, so there’s no possibility of it becoming disconnected.
