 Hi, I'm John Foote. I'm a recovering scientist. Next slide. Okay, I'm going to talk about driving high power LEDs. Now most of you have probably played around with the little LEDs, right? Anybody done that? Like down the hardware hacking section? Okay, well I'm going to talk about these big hockey LEDs. Next slide. Okay, so if you don't do the little LEDs and you hook it up to a voltage source, like a battery or something, you know you've got to put a resistor in there. Anybody tried it without a resistor? Yeah, what happens? It blows the hell up. It's great fun. Try it. Okay, so next slide please. Okay, what do you do with these big hockey resistors? Well, you can put a big ass resistor in there, but that may not be the best way to do things as the next slide shows. Because this sucker will get hot, right? If you look at the numbers, remember power equals current times voltage. We got a lot of current. We got a lot of voltage drop across this. This sucker is going to get hot, and even if it's only dissipating 5 watts, think about the size of that resistor. Think about 100 watt light bulb. How hot does that get? How much smaller is that resistor than 100 watt light bulb? So it's going to get smoking hot, and furthermore, you're wasting power. It's inelegant. Next slide please. Okay, so this is how you don't do it, okay? People think, well, I can make a nice little adjustable voltage supply, and I'll just dial in the voltage just perfectly so we'll get the right current through the LED. No, next slide please. The reason why is because the current relationship for a diode is exponential. If you diddle around with this little voltage, you might get huge current swings, and that might be too much current to blow up the LED. Next slide please. Okay, so diodes want current, and next slide I'll show you a couple ways to get them. Okay, you can do a linear current source using a couple of these different things. These work great, however, these are basically smart resistors. They also get hot. If you can use a voltage regulator, put a big honking heat sink, it'll work great, but you might, once again, we're wasting power. Okay, so how do we do this right? Next slide please. We'll show you. Okay, this is a SEPIC buck converter that stands for Single Ended Primary Inducted Converter. There's the inductor, right? It's that curly thing that you might have slept through in physics class. Okay, this is a resistor. Now, this is a tiny little resistor. It's less than an ohm, so it's not taking any power. Here's a switch that's not taking any power. So any power that comes from the voltage source down the ground is hopefully going to go into that diode. And this kind of looks a little gnarly, but let me show you how it works. It's actually pretty easy on the next slide. Okay, so there's a switch here, and that switch can either be closed or open. When it's closed, the current comes from the power source down through the inductor, through the LED, and into ground. Okay, and when it's open, this diode kind of recirculates it. Now, inductors are kind of like flywheels for current. They like to keep a constant current going. They store energy in the magnetic field. Next slide please. Okay, so this is how the thing works. There's a smart little chip that knows when to close and open the switch. This chip is looking at the current through the diode, and you can measure that from the voltage across the sense resistor. If that voltage is too small, that means the current's too small, so he closes that switch, charges up the inductor, gets more current flowing, and when that gets too much current, he opens the switch, it recycles, and the current still goes through the LED. Next slide please. Okay, so this is an actual circuit that I built. This is that smart chip. This looks kind of gnarly, but now you know how it works, right? This is just a switch. This is a sense resistor, and it's kind of monitoring the sense resistor, and it's either turning on or off that switch depending on feedback, and you can turn the whole thing on and off if you want to dim your LED using pulse width modulation. Next slide, and this is the money slide. There's no way you can possibly read that, but this is the... Oh, I'm sorry, sorry. Okay, this is the URL. This is a whole bunch of... everybody wants to make these chips because everybody wants to replace the light bulb with these more efficient LED things, so there's a whole bunch of chips you can buy that basically do that little transistor man thing, and that is pretty much the end of my presentation. That's me. Thank you very much. This is going to stay in here for the rest of the lightning talks.