Powering a microcontroller inductively from mains wire

Question

I am in the process of making a mains power monitor using a SCT-013-000 CT clamp and an arduino mini, running on batteries. Even though I should be able to get many months of operation from two small AA batteries, would it be possible to use the clamp to harvest power to run the unit forever?

If I'm doing the maths correctly, two 10F 2.7v capacitors in series would keep a unit drawing an average of ~0.1mA, 5.4v->2.7v running for a couple of days even if there is no power usage on the mains wire?

Answer 1

If you want to monitor mains power precisely, you have to measure not only mains current but also mains voltage. When you measure mains voltage you can use a small transformer to power the microcontroller inductively because you already have a connection to both wires of mains. No energy harvesting necessary.

Answer 2

Using your current clamp for both power and signal would result in a significant degradation in the accuracy of your measurements. The high turn-count of the secondary on the CT requires thin wire, producing a relatively high output resistance. Applying the dynamic load of the Arduino would change the load on the CT output, causing inaccuracy. While inductive energy harvesting is definitely possible, you wouldn't want it to be from your primary sensing device.

As for using electrolytic supercapacitors in your application, be aware that safely charging/discharging them is best done with a dedicated IC for low-power backup applications. Such controllers include the LTC3225, the LTC4425, and the LTC3226. However, these controllers would require having a low-voltage supply available normally such as a 5V USB power brick/"wall wart". For higher voltage inputs, you could use a Li-Ion battery and the LTC4079.

Implementing inductive energy harvesting could be done with the LTC3588-1 and a second CT. You will need to ensure that the CT output voltage will be appropriate to drive the energy harvester at both low and high measured currents. I am uncertain how the combined energy harvesting and supercapacitor implementations would interact and they might not be easily combined.

Answer 3

Uwe already pointed out the obvious, I am posting this answer for something else.

Please do some research before jumping on in this project and later on end up scratching your head why this power meter doesn't work.

I have implemented an energy meter using BL0921 IC. There are various metering ICs available in the market and there is a good reason you should prefer those ICs over your arduino.

First, you are going for ac power measurement which is not as straight forward as P = V x I. So don't assume you can do analog reads every few milliseconds and multiply it to get result.

You will have to take those measurements at a very high frequency and keep on integrating over time. After that power factor will come in to ruin your day. Those metering ICs take care of all that stuff for you and give you a nice output. They also give you simple methods for calibration of your energy meter. You can use arduino to read the output and use it the way you like. You can use an ESP instead of arduino to directly send it to a server over wifi. Possibilities are endless.