A user has sent samples of the Talema AZ series CTs and I’ll be adding these to the IoTaWatt table for the next release. They are very small solid core CTs available from a variety of sources both here in the US (Digikey) and in Europe. I recently measured the AZ-0500 and AZ-1000 for linearity and phase-shift. These are extremely tiny little CTs. The hole is about 7mm. Their turns ratio matches IoTaWatt’s 50ma input range, so all that is needed is to add a lead with a 3.5mm jack and optionally a TVS diode.
Their performance, however, is remarkable only in that they exhibit much more phase shift than any of the previously measured CTs.
The AZ_0500 is rated 25A with a ratio of 500:1. They seem to handle that with no signs of distortion and good linearity with IoTaWatt’s 20 ohm burden. The measured ratio of the two samples tested were 509 & 508 at 60Hz and 511 & 509 at 50Hz. So about 2% off.
The AZ-1000 is rated 50A with a 1000:1 ration. Like the AZ-0500 they showed no signs of distortion. Measured ratio was 1005 for both at 60Hz and 1009 for both at 50Hz.
In my experience with VTs, phase-shift in inversely proportional to weight of the transformer. I’m starting to believe there is a similar relationship with these small, very light CTs. While not a lot different from some decent split-core CTs, these exhibited much greater phase-shift than other solid-cores that I have tested. It should be noted that CTs typically exhibit less shift as the burden is reduced. Many of these small CTs are designed to run with a 10 Ohm or less burden, and here they are tested with the IoTaWatt’s 20 Ohm burden.
By comparison, here is the HWCT-004
Not only is the HWCT-004 shift very low, it is fairly flat across the entire range. Shift doesn’t actually cause error, it only introduces it when it’s not sufficiently corrected, and in the case where the VT has a higher shift profile that matches the CT, it can be better to have that higher shift. But what is concerning here is that folks would use the 25A AZ-0500 for small loads because it’s rated to 25A, when actually, the AZ-1000 performs better at low currents, and of course the HWCT-004 (50A CT) performs even better.
I haven’t tested the AZ-0750 yet, but it has the same physical characteristics and my expectation is that it will come in between these two.
Price for these CTs on DigiKey is around $5-$6 in modest quantities. The HWCT-004 on AliExpress is about $2 and $4 from newegg in the US. It’s not much larger and better suited. Here’s a lineup of the HWCT-004, AZ-1000 and Echun ECOL-09 that I used to carry (they were $7 bought with an IoTaWatt including cord, plug, TVS diode and UL/CE listing).