I am trying to develop a non-contact whole-house power usage monitor.
I have reviewed jasonT's project, which uses AC clamps to measure current usage.
However, from reading this post on how meters measure power, and this post on apparent vs. real power, it seems I need to be able to measure the instantaneous voltage as well as the instantaneous current, in order to calculate an accurate "real" power usage. From the first referenced post:
Real power is calculated by multiplying the voltage and current at any instant (one pair of samples). Apparent power can be calculated by measuring the amplitudes of the voltage and current over the course of one or more cycles and multiplying the two results together.
(I'm wondering if they got it backward! Real power = requires measurements over multiple cycles, but Apparent power = one instantaneous VA measurement?)
My question is this: how can I measure the instantaneous voltage in a non-contact manner? I understand that current can be measured inductively with a clamp, which is just a simple transformer which outputs a voltage proportional to the current flowing through the conductor.
If I'm not mistaken, it seems that using a capacitive measurement technique would be correct (like in an electrician's voltage detector probe), but I can't fully wrap my head around what such a circuit would look like. Would it be just a simple wire placed near the mains conductor, at a fixed distance, with only one end connected to an amplifier (for example, similar to this circuit)? If so, how would such a circuit be tuned to the correct proportional output?
If anyone could point me to any resources or suggestions I would be grateful.
Best Answer
You don't want contact with the mains. OK, so is your project going to be battery powered?
If you buy a low voltage, AC output wall-wart, with UL, CE, DIN and every other safety marking you can think of, you can regard its output as at least as safe as any other appliance in your house. Then, in the low voltage output, you have a reasonable facsimile of the mains voltage waveform. The accuracy will degrade if you also rectify that output to power your project. If the last 1% accuracy is important, then why not buy two, one for the reference, and one to power the project, or a DC output one to power the project without further messing about. Your current measurement is already transformer coupled, why not have the voltage measurement transformer coupled as well?
To confirm real and apparent power calculations ...
for real power, repeatedly compute the instantaneous power as the product of the instantaneous voltage and the instantaneous current many times a cycle, then average the power
for apparent power, compute the mean voltage by averaging, and seperately the mean current by averaging, then take the product of these
... and here an average means either a) average over a long time, the more cycles the more accurate or b) sum synchronously over an integer number of whole cycles