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Joshua Boniface 2018-03-07 22:43:58 -05:00
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@ -29,10 +29,6 @@ P = V * I
A/C power complicates this ideal formula slightly. A/C power works on a fluctuating voltage in a sine wave, which alternates at a specific frequency. In North America, this is 60Hz (cycles per second). What this means it that every 1/60th of a second, the voltage peaks (120V for a standard household circuit as mentioned), and then dips down to 0V before repeating. And as you know from the formula above, when the voltage is 0, the power usage will also be 0, so the effective power usage also looks like a sine wave with a frequency of 60Hz. While you usually don't notice this fluctuation in your daily life, when trying to measure the power of a circuit, it becomes very important to take this into account. The next section goes over how to create a current monitoring circuit for A/C power.
The sine wave power mentioned above also adds another complication: the idea of real power. While the [details are complex](https://en.wikipedia.org/wiki/AC_power#Active.2C_reactive.2C_and_apparent_power), if you're just trying to figure our "how much power am I using", this isn't a huge deal: you can just know you're always using "less" than what you calculate with the simple formula. But devices like UPSes get complicated. Most are rated in Volt-Amps, such as my APC Smart-UPS 2200. It's rated at 2200 VA, but in reality it can't actually power 2200 _Watts_ - the VA value represents the _apparent power_ maximum of the UPS. The real supported Watt value is actually lower, around 1800W for this model. What this means, is, you may see yourself using >1800 "Watts" by simple calculation, and you might still be "fine", because your _real power_ isn't exceeding 1800W. But if your usage spikes, you can very quickly learn that what you thought was "enough", wasn't, even though your "Watts" value is still under 2200. Always size based on the "real" Watts value rather than the rated VA value of a UPS, or alternatively factor in the power factor of your devices into your calculations. For simplicity though, for the rest of this post I'll use the basic definition of "Watt" when describing caltulations and values.
The last thing to touch on for those unfamilliar is the idea of a Kilowatt-Hour, or kWh. One kWh is equal to 1000W (1 kW) of power used continuously for 1 hour. Since this is a time-based measurement, versus the instantaneous measurement of the `P = V * I` formula, it is the most common unit of billing for electric power. As a concrete example, assumming the previous example load of 1A at 120V, working out to 120W, you simply multiply this out by 1 hour to get the usage of 0.120 kWh of energy every hour. Over two hours it would use 0.240 kWh of power, and etcetera for as long as the device is active. When you get your monthly power bill, this is how the total power usage is calculated.
## An A/C monitoring circuit
With the theoretical underpinnings of this project out of the way, we can begin looking at devices to measure the current and voltage of an A/C circuit. This project will use two kinds of sensors: current, and voltage. Current sensors will be installed in-line with every socket, while a single voltage sensor will be installed on each circuit. Given the formula above, we'll be able to do some quick math to get the wattage value from these numbers as well.