Hi All,
I’m sourcing my own VT and trying to go smallest possible.
What is the current requirement of the VT output? I’m looking at a 90ma toroidal to fit in a DIN rail enclosure.
Will this work?
The power required is less than 1mA. So virtually any transformer that does not distort the AC signal will work to give an accurate Vrms reading.
What’s at issue is phase shift. Every transformer will provide an output that is not in phase with the line voltage. This happens with the current transformers as well. To calculate real-power, it’s necessary to compensate for this phase shift such that the signals received from the transformers have the same relationship as the voltage and current being measured. Ideally both the VT and CT would exhibit the same shift, but that’s not a realistic approach. The 14 channels of IoTaWatt can each have a CT with a different phase-shift. Moreover, it’s a moving target. In particular, the phase-shift of a CT is a function of the current being measured.
Looking at the IoTaWatt device tables used to configure these transformers, you can see that most of the data is phase shift data, at both 50Hz and 60Hz.
In general, heavier transformers have less shift. Coincidentally, higher capacity transformers are usually heavier. You can see where this is going…
You can find exceptions, and I haven’t tested any toroidal transformers, but in general I would not recommend anything less than 1000mA based on this experience.
Transformers are also rated at an “efficiency level”, which is an attempt to limit the standby power required. Without getting into the merits or lack thereof in this application, the current standard in the US is level VI. As it happens, big surprise, it seems heavier transformers are needed to achieve this efficiency. Both of the wall-wart models available at level VI are 1500mA and have minimal phase shift of < 0.2°. The old 500mA units are usually 3° or more.
You can get a pretty small 1000mA wall wart for less than 10 bucks. I have measured the shift on many and they are in the tables. I have a standing offer that I will measure and add to the tables any VT if you send me two samples (which I will retain).
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