Saturday, September 25, 2010

Wattching Power Consumption

A few photos of the little rig I cooked up to monitor "Vampire" power usage in my apartment. Unfortunately, the CT (current transformer) I have can only measure currents down about 1A, so it doesn't look like this will give me the information I'm after...

How it Works:
Current transformers measure the magnetic field generated by an alternating current passing through a wire and output a proportional DC voltage. The chuck of plywood has an electrical box on it with a plain-old outlet, and a cord from an old microwave. The hot wire from the cord takes a detour out of the electrical box, through the CT, and then back into the box to the outlet. The two red and black wires carry the 0-5VDC output signal from the CT to my DMM (Fluke 89).

The Neilsen-Kuljian A100-1 CT that I have has three scales, and when set to the 0-10A scale outputs a DC voltage from 0-5V (i.e. 5A AC = 2.5VDC).

Knowing the current drawn by a particular appliance, we can then take this reading (for the toaster oven ~9A) and multiply it by the AC line voltage (I measured it to be 122VAC RMS), and this will give us the power consumption in Watts.

Toaster Oven Math:

Voltage Reading from the CT: 4.53V DC

Which works out to: 4.83V / 5.00V (range of the CT) = 0.966 = 96.6% of 10A = 9.66A

Being ELEN students, we know that P = IV, so therefore:

9.66A * 122VAC = 11078.52W

So there you have it folks; if you ever cared how much power I use to cook my toast in the morning there's your answer.

***CORRECTION: As Brad pointed out, it was in fact not George Ohm who came up with the relationship P = IV. I have not looked into this formula's history too deeply, however I would assume that James Watt came up with some variant of it that was then adapted. My apologies for misleading anyone on this. So in summary: The science is right, the names are wrong.

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