Wednesday, May 13, 2009

Power Power Everywhere...

But specifically from the sun. We will never run out of power (never being equivalent to several billion years) because of the sun. Consider that most of our power comes from consuming things. Basically we burn things - we turn complex hydrogen-carbon chains into simpler ones and live off of the resulting energy output. Coal, wood, sugar - it's all the same. Combustion. The sun is better. The sun is powered by gravity itself. By the very shape of the universe! There's a lot more energy out there than sitting in every oil field, forest or coal mine. But how do we access it? The simplest method is by laying in the sun and getting warm. As much as I like a tan, I LOVE electrical power! So I use solar panels.

Let's talk about them for a second. Solar panels are a lot like batteries in some ways. You start out with solar cells which are maybe an inch square of real estate. They're made of silicon in an extra-special arrangement which causes them to produce voltage and current when light strikes them. But how much of each? There's essentially two main parameters that matter. The first is open-circuit voltage, the second is short circuit current. They're fairly self-explanatory: if you shine sufficient light on the cell and don't put an electrical load on it then you'll measure the open-circuit voltage across its output; if you shine a light on it and put a short circuit on the output then you'll measure the short-circuit current through the output. In between those points is not a straight line, but more of a graph with a knee. Take a look at the graph I show below and you'll see what I mean.

You'll notice that the cells are almost constant voltage sources except when you attempt to draw currents close to the short-circuit current. You can either use a simplistic model of a constant voltage source or you can use a more complicated model that accounts for the actual behavior more:

If my calculations are correct, Il is the short-circuit current and the open-circuit voltage is equivalent to Il*RSh. Props to Wikipedia for having such awesome images that I can steal for free. I like free.

So that's your single solar cell. They're a lot like the cells of a battery as I mentioned last time: to get a useful voltage out of them you have to put a lot in series. The difference between these and batteries is that you also typically have to put several sets of cells in parallel to get a decent current. I have an 18V solar panel made out of perhaps 32 solar cells - 8 in series and four sets of these in parallel. Its open-circuit voltage is 18V and short-circuit current is 300mA. This actually makes it rather useful for charging batteries!

When charging batteries you need two things at the same time: voltage and current. Well lucky for us voltage and current together are POWER! And we can get power out of our solar panel! Success! Just connect wires to things and make it go! Well, not that easy. You need specific voltages and specific currents to charge our batteries. I have some Ni-Cad batteries that need to be charged at a maximum rate of (off the top of my head - don't hold me to this) their total capacity divided by 10. So I have 5 batteries that have a total capacity of 6A-Hours, so the max rate I charge them at is 6/10 = .6 Amps. And for charging these batteries current is the most important part: as you push more current into them their voltage goes up and you stop putting current into them when they reach about 13.8V. So you need your voltage to be at least above whatever the battery voltage is at the moment. Just figure you'll need at least 13.8V to get this to work.

So we have a specific power IN from the solar panel and we need different power OUT. My first guess is to use an LM317 in constant current mode. However this has problems. The main one is that as you try to draw constant current out of the solar panel its voltage will drop. And the LM317 is a linear device. For linear devices the rule of thumb is that the current into the device is the same as the current out, and the voltage out is less than the voltage in. Thus, you will need to make sure that your solar cell voltage stays above the battery voltage. Good luck, because even if you do make sure of that you will be dissipating the 'extra' voltage from the panel IN the LM317 as heat - thus losing it. Solar panels aren't amazing power sources, so I'd rather not waste any of the power that it does generate.

You can also do power transformation with a switched-mode device. The rule of thumb for switched-mode devices is 'power-in equals power-out'. The only loss is due to efficiency. Another great thing about switched-mode devices is that they can produce nearly any output voltage off of its input voltage. So you can always keep your output voltage above the battery voltage even if the solar panel voltage goes below it. The trick is that switched-mode power ICs are usually set up as constant voltage supplies. They employ a feedback voltage to set the output to the right voltage. They're a little control system! I love control systems! SOOOOO CUTE! It's contained in a single IC! Adorable!

Anyway, I know about control systems and I reckon that I can modify the feedback signal so that it's based off of the current going to the batteries instead of the voltage at the output. I would current sense resistor to monitor that current and then use a differential amplifier to amplify and scale the voltage from the current sense resistor. This would be the new feedback signal to the switched-mode power supply. Thus it would regulate its voltage to make sure that the proper current goes into the batteries.

My fingers are yet again getting tired so I'll mention that this is just the battery charging circuit. You still need an output stage connected to the batteries to power things off of the batteries. Also, there's an improvement on the simple battery charging circuit I described here. It's called a Maximum Power Point Tracker that gets even more power out of the panel than before.

1 comment:

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