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.

Thursday, May 7, 2009

Wherefore Art Thou, Camping?

Exactly how many doohickeys can you take with you camping and still be considered to be 'roughing it'? I know many people for who camping means 'in a camper', i.e. they need their flat panel TV and fridge full of beer or they're not going. I will sleep in a tent, so I consider myself somewhat superior to these people. But being an electrical engineer I just LOVE gadgets and I can barely stop myself from bringing them along. I know not to even THINK about bringing a laptop, but I'm still bringing our battery-powered fan. Of course, instead of batteries I'm using an emergency jumpstart car battery thingamajig with a cigarette lighter power adapter. If you want to power AC devices you'll need an inverter - just make it has sufficient maximum power (I've experienced the letdown of not having enough power firsthand when my friends and I figured out we couldn't run a smoke machine off of an inverter plugged into a car. Memories!) I haven't graduated to thermoelectric coolers yet but soon I will.

The question is how to you power all of your gadgets in the wild? A car battery is a good start but for a perfectionist like myself it just won't do. Car batteries are heavy. If you have to carry anything any distance you'll with you hadn't brought it. You can find better batteries than the lead-acid car batteries. Nickel-Cadmium, Lithium-Ion and Lithium Polymer are all better options (i.e. greater power density per kilogram). Technology is only one thing to look for when choosing a battery. There are four main things to look for in a battery: Voltage, Capacity, Current Capacity and Technology/Battery Type (Ni-Cad or Li-on or whatever).

Voltage is easy for the most part. Car batteries are 12V nominally and for the most part that is the magic voltage. Since your car runs on 12V everything that plugs into your car runs on 12V so there's lots of devices out there that will happily run on this voltage. Other popular voltages are 9V and 5V, but if you have a 12V source you're in luck because you can use dead-simple linear DC-DC conversion to get down to those voltages. Look up the LM7805, or in fact, everything in the LM78xx line (the last two digits are the nominal output voltage). They're really simple to use - for an electrical engineer anyhow. Fun fact: a battery is not a single unit - it's made up of cells. Depending on what the battery is made of, each cell has a different voltage. It's intrinsic - entirely dependent on the technology used in the battery. You get other voltages by stacking these cells up in series. Lead-acid is 1.5V per cell I believe, so a 12V battery has 8 cells in series. Another fun fact: it's not called a battery unless there is more than one cell used in it.

Capacity of the battery is also straightforward. It's measured in Amp-Hours. If my battery is rated at 7A-Hs then it can in theory deliver 1A for 7 hours before it's empty. Of course this is simplistic. Voltage degrades as the battery empties, so if you need 1A at 12V for seven hours you may not be in luck. After hour three the voltage may go down to 11.5V, then down to 11V next hour, etc. If you're using linear step-down conversion then you're out of luck for a couple of reasons:

  1. You need a certain minimum voltage difference between the input to the voltage converter. If your battery voltage goes too low then you won't meet this requirement and you won't get any power at all. This can be alleviated with a switched-mode voltage converter

  2. Assuming the power output is constant, lower volts means higher amps. As your voltage goes down your converter will draw more amps just to have the same amount of power and this will cause your battery to drain faster. Switched-mode voltage converters are not immune to this

The degradation in voltage is a function of the battery technology, so your mileage may vary.

Current capacity is distinct from the other capacity. Current capacity is the ability of the battery to source large amounts of current. Not all batteries are equal in this respect. As with any non-ideal voltage source a battery has a certain amount of internal resistance. This means as current flows out of the battery, power is dissipated in the internal resistance. This creates heat. Heat causes explosions, especially when it's applied to exotic chemicals. Remember when Dell's batteries were exploding? They got too hot. They got too hot because despite the fact that Dell spec'd the batteries with a peak current of say 5A for one minute, the manufacturer ignored that and went cheap on the batteries. This caused them to heat up when the laptop pulled the amount of power from them it thought safe. They got so hot that the chemicals got angry, and *POOF*! Exploding laptop. In general batteries are high-performers for current sourcing - that's the reason your emergency jumpstart kit works better than the alternator on your Yugo - current capacity.

Technology is a good one. I'm not into chemicals and ions and things but it's somewhat exciting to see the effect that all of that sciencey stuff has on actual performance and use. As I said before, technology affects all sorts of things on a battery: nominal battery voltage, voltage degradation over time, capacity to weight ratio, discharge capability, charging method, discharge behavior, etc. In general your newer battery technology has better capacity to weight ratio, less voltage degradation, higher capacity and greater current capacity. What it doesn't have going for it is simplicity and reliability. Simplicity in that they're more difficult to charge. Lead-acid is dead simple: apply 13.8V to a 12V battery and let it go for a while. It will charge completely. Lead-acid is surprisingly hard to damage as long as you don't exceed its recommended voltage. Newer technology is harder - it's rather finicky about how much current you push into it and what voltage and you must stop trying to charge it when it's done or else you'll break it. As for reliability consider that you can discharge a lead-acid battery almost down to 0V and it will still live - just charge it up again (deep cycle maritime batteries are the best for this). Most other battery types can't handle this. If you let them get too low then they're dead.

My fingers are tired so I'll leave until a different time the discussion of the solar-powered power unit I'm making. For my camping gadgets of course!

Monday, May 4, 2009

Firmware in all things

So today I'm home from work for a bit to wait for the repairman (or repairwoman) to come and take a look at my dishwasher. It has lights flashing on the front and won't do anything no matter how many buttons I press. You may be wondering why I don't just fix it myself. Despite all of the experience I have with fixing household appliances (none) and my familiarity with similar devices (huh?) I've decided that it's just not worth my time to try to fix the thing myself (although having to stay home from work to wait for the repairman somewhat negates that position). I tried some things. The manual says that it may be a bad heating element and to check the wiring. I checked the wiring (well, looked at it anyhow) and nothing. Some advice online said to try pressing a sequence of buttons in rapid succession - nothing. I gave up after that.

Why? Because it's obvious this isn't a hardware problem. And if it's not a hardware problem it's a just plain HARD problem. Mechanical systems are easy - they either work or they don't. No ill-defined states, no invalid inputs, no built-in tests. If something can't happen it physically can't happen. If one gear is moving then the gear in direct contact with it also has to be moving. If something sounds wrong it's probably directly linked to the problem - just follow the connections.

But somewhere in this dishwasher is firmware - code. Code breaks all of the rules. Impossible things happen in code all of the time. Jump to the wrong memory address? You could end up executing impossible code. 'This shouldn't happen! I never called this function! It's impossible!' Mess up with pointers or indices? You could start reading impossible values. 'Umm, an unsigned 8-bit integer CAN'T be 1024....' Use a case fall-through instead of explicit checks? 'That's IMPOSSIBLE! That value isn't handled! Why isn't it going to default...'

Code is arbitrary. I have a flashing light. That's not a symptom of the problem - it's an indication. I need a manual to tell me what it means, and if that doesn't suffice I need the REAL manual - the one they only give to the service technicians (well, sell anyhow). It means whatever they tell me it means. A mechanical device is not arbitrary. If my engine is overheating it doesn't do something illogical like lock the drive shaft automatically - it just starts heating up. All according to rules Mr. Newton figured out (differential equations baby!). You can track it and explain it with rules you find in your high school physics book. The code only follows the rules that Mr. Programmer set forth and he's not bound to adhere to any standards and even if he was he wouldn't tell you. Furthermore, the state of a machine is obvious and can be ascertained by observation (perhaps complicated observations, but still, observations). Code need not give any indication of state and typically doesn't, or it's not very useful (green light - good, red light - bad!)

And if you aren't rigorous in your code you can have undocumented behavior. It's entirely possible for you not to be able to reach one of your states or not be able to leave it if you make simple mistakes. And if your testing doesn't catch it then you'll have to reboot your dishwasher or some other such silliness.

Computers are very powerful, but simple. They execute instructions at memory addresses. Everything else is defined by the designer. If he or she is incomplete in the definition of the system then irrational behavior follows. Mechanical systems are bound by the laws of physics: they cannot perform physically impossible functions, they must move smoothly between states that are defined by their physical characteristics alone, and failures are characterized by smooth transition to a new state(one which is obviously broken). Software doesn't follow these rules and that makes it much more difficult to troubleshoot.

PS: It was bugs Living on the electronics. Dirtying them up. I'm not a dirty person I swear. It wasn't a vague error code or random impossible to get to state, it was just flat out broke. At least even computers have flat-out broke modes...