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.
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?
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.
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:
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.
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.
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:
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?