The goal: Understand how much power a solar panel actually makes.
The most straightforward way to measure DC power is to give your source a resistive load and measure voltage across it. Power = Voltage^2 / Resistance.
Challenges we face: The problem with solar panels is that they can't be approximated as constant-voltage or constant-current. So just one resistor isn't enough. Further, I don't want to have to run power or networking outside. So whatever device does this measuring it needs to itself run off of the solar power and either save that information or convey it wirelessly.
Solution: First, let's use an 'Arduino Yun'. This is an arduino printed on the same PC board as a linux machine with the serial connection and software already made between them. So the linux machine can do the networking side of it while the arduino can do the more typical microcontroller functions; turning the resistors on and off and measuring their voltage.
For power, we can use voltage regulators 7812 and 7805 to bring the 38V of the solar panel down to 12V where a battery will store it and then down to 5V where the arduino will consume it. With a large battery, it should have no problem lasting the night even in the low-light winter.
We also want monitoring, manual control, and safeguards.
Monitoring: For monitoring, I'll put an LED on the power coming in from the solar panel, and then one on each of the resistor networks to see if they get pulled low.
Manual control: For manual control, let's add three light switches (because they're cheap) to turn on overall power from the solar panel, power from the battery to the microcontroller, and then turning on or off the monitoring LEDs. Having LED control lets me turn them off when they'd otherwise be sapping power. Having two levels of power control let me boot the microcontroller only if I want to debug, or boot it first to get around uncertainty in startup, or not boot if the battery is dead, etc.
Safeguards: The resistors could take as much as 260W at peak but I don't want to buy and don't need a 300W resistor because I'll only be pulsing that power on for a moment to take a reading. Then it will have plenty of time to cool down. Unless something breaks... if power does stay on while the resistor is turned on, it could heat up, burn, and short out other parts. So I'll put in fuses in a few key parts to make sure if it does go melty it doesn't stay melty. I could also have a secondary system montor heat, but the complexity of the machine increases dramatically if I do that.
Results
It had two problems: First, it doesn't have enough grainularity of load. At full sunlight with 2ohms it did 16V and about 124W. But then at 8ohms it did 28V and about 100W. It should have been doing at least 200W and perhaps it would have been had it been at the ideal voltage of 24V. Perhaps solar cells have a much tighter wattage curve than I knew.
Second, it melted itself. Something bad happened and the 2ohm resistor must have stayed on. It was on until it melted the acrylic holding it in place, charred part of it's outer surface and eventually internally ruptured it's connection thus ending the short.
So the data was not good enough and the device was damaged. Need a new plan. :)
