In past articles we have covered monitoring DC current for solar and wind based applications.
Monitoring AC (grid or grid tie solar) is a bit trickier as voltage flows bidirectionally, and it’s harder to insert a shunt inline. We use what is called a Current Transformer, or CT. It clips over your existing wires into your breaker box, so no disrupting the existing lines is necessary. In order to monitor AC current with a CT, you need to put a resistor across its output. This is called a burden resistor. To determine the correct resistance of this resistor, we need a few pieces of information.
As you can see in the picture, the maximum current rating, and the current output at that rating is displayed on the CT. Its rated for 120 amps, and outputs 40ma at that draw. If we divide the 120a by the .040 amp (40ma) output, we get 3000, which is the number of turns on the secondary.
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Now, we need to know the Primary Peak Current (PPC), so we Multiply the RMS Current (120A) by the Square Root of 2 (1.414). This gives us a PPC of 169.68 Amps. We can calculate the Secondary Peak Current (SPC) by dividing the PPC by the number of turns (3000), so the SPC = .05656 amps.
Now, to determine the Ideal Burden Resistance. We need to know the AREF voltage of our microcontroller, which in the case of the Arduino UNO, is 5v. We divide the AREF by 2, and that by the SPC, so our calculation looks like this:
(5/2) / .05656 = 44.2 Ohms
Therefore, 44.2 Ohms (a standard 1% value) is the ideal resistor to put across the output of the CT to measure current. A complete measuring circuit and Arduino code to follow shortly.