Your LED Questions Answered! (Part 1)

Published

Ask an Electrical Engineer is a series where frequently asked questions from non-engineers will be answered by an engineer. For this video, our main topic is all about LEDs (Light Emitting Diodes).

Transcript:

Hey guys, welcome to Ask an Electrical Engineer, the series where all the non-engineers like to come up with a series of questions, make sure that I have no idea what they are, and then put all this engineering stuff around me to mostly to embarrass me...I think that's what the point is. So today's topic is LEDs and you're going to learn more about LEDs than you will from any other source, guaranteed [not guaranteed].

00:24 How do LEDs (light emitting diodes) work ... and what's a diode?

That's two questions, guys, two. First of all, what's a diode? So a diode is something where electricity goes one way and doesn't go the other way. That's really all you need to know. You don't even need to know that for understanding how an LED works. If you want to get a little bit more complicated, it's because you get these two materials together and the way the energy of electrons works, you put a voltage one way and they're like "wheeee" and they go where they're supposed to, but you put the voltage the other way and it's kind of like making it so they're trying to climb up a hill and it just doesn't really work. So that's how a diode works on a more fundamental level. But in reality, all you need to know about diodes is that electricity goes one way, and the other way, it's like, "ugh, I can't quite make it". Now what does this have to do with light emitting diodes? Only thing that it has to do with lighting emitting diodes is that LEDs are diodes.

So the way LEDs work is that they get that material so that when an electron is going from one to the other, it drops in just the right amount of energy that it emits a photon. So basically, you can kind of think of it like it's stepping off a curb, and as it jumps off a curb, it loses a certain amount of energy and we can control how much of a drop it is, and therefore how much energy it requires until it gives us the photon. So if you want to drop more energy, you get a bigger gap and then you get something like an ultraviolet LED. If you want less energy where you're in the infrared or red spectrum, you have a smaller step down and then as that electron drops in energy, that energy is released as light instead of heat, which is what happens in a normal diode.

02:05 Why are LEDs better?

Wow. Okay, talk about a wide scope of questions. First big thing, they're very efficient, much better than incandescents. I could ramble about this all day long. And if you want, I think I've done another video where I did exactly that. LEDs are more efficient, they're tough, they can flicker on and off really well, so they're good for communications. They're good for dimming in a really power efficient way. Did I mentioned they're tough? I just love the fact that you can take them and throw them around. They're cool to the touch – you can actually touch them without burning yourself whereas with incandescents...I want you to see how long you can keep your finger on even the lowest power incandescent bulb...tell me how that feels. You can get all sorts of cool colors. And I mean, they're just cool! If you want a better response, I got another 15+ minute video about it, so knock yourself out!

02:58 When did LEDs become popular?

They were invented in the 1950s or something. But they were super expensive and not super bright. Then they discovered, "oh hey, we can make these a lot cheaper and a lot brighter" around the 1970s (when the red LEDs came out). And as I mentioned earlier, with red LEDs, there's a smaller drop in energy – so those were the first ones that came out and they were super popular. Instead of being like $20-30 bucks, they dropped down to a couple cents per LED. That's when LEDs started getting into the mainstream of everything. Now, the color LEDs that you get all over the place became popular when the blue LEDs is came out. So as the energy level stepped up, you got red LEDs, you got green LEDs, and then finally with blue LEDs, you can mix RGB to get any color including white. Even a colorblind guy like me knows that that's how you get white. So the blue LEDs came out in the early 1990s and they didn't become super mainstream and super popular until around 2000 onward. And I mean, they're just getting more and more popular. So when did LEDs become more popular? Maybe the right answer to that is, now. They're just appearing all over the place. They're getting so cheap. They're getting so efficient that I mean, it's beginning to get ridiculous.

04:14 How much more efficient are LEDs than other options?

Let's compare them to the obvious: incandescents and halogens...way more efficient! Five, six times and that's at current levels. It will be even greater than that in the future I imagine. And that's comparing LEDs to the latest, most efficient incandescent that they've been working on trying to make more efficient for a 100+ years. So compared to the original Thomas Edison bulb, I don't even know...it'd be insane. Compared to other things like fluorescent and CFL, it's actually about the same, maybe a little bit more efficient. Sodium, mercury, mercury vapor gas discharge style, again, about the same it really depends on your application, depends on the quality of electronics in your LED. The cool thing about LEDs is not only are they more efficient than those other options, but the ones where they're kind of in line with the efficiency of the gas discharge and the fluorescents. So they're just as efficient as these other types and way better in every other aspect.

05:12 Why do LEDs last so long?

That's a good question. I really like that question. In our experience, we find that things that move, don't last as long as things that don't move. When you flip the switch of an incandescent bulb, it's getting hot, getting cool, hot, cool, hot, cool and each time it does that it's going and going until it cracks and once that tungsten filament cracks, it's completely useless...it' one of the reasons why you don't want to quickly flip the switch to an incandescent light bulb. Now a semi conductor, it's literally just two pieces of material sitting next to each other and you're running some electrons through it, there are no moving parts, nothing is getting that hot in the junction itself. The electronics that power it will get hot, and those will break down. It's a cool thing that you can tell what fails – the actual LED or the electronics. If it gets dimmer and dimmer and dimmer and goes out, then it's typically the LED. If it just goes out or is flickering, that's actually a problem with the electronics driving the LED.

So that's one thing that LEDs are doing better than all those other alternatives – they simply run cooler. If they were perfectly efficient, nothing would be hot at all, it would all be straight photons coming out. But in reality, it's not perfect, even though they're much much better. So sometimes when those electrons drop, instead of emitting a photon, they'll generate heat, but they do it much, much less than most of the other sources. And so that's why they last so much longer.

06:47 Are LEDs AC or DC?

Excellent question. Part of the confusion here is because you take those light bulbs that look like the old school incandescents and you put them into an AC socket, and they produce light. So you're like, "Oh, sweet, they're, AC". Nope, again, in the bottom of those LED bulbs, there are electronics that are converting AC to DC. Now in reality, they can run on AC. But again, electricity can only go one way in a diode not both ways. And with AC, it goes one way then another way, one way then another way. And so if you don't have something in there, and you just take an LED and hook it straight up to an AC signal, it'll give you light, and then it won't give you light (while negative), give you light and then it won't give you light. So you can easily put a bridge rectifier in there to make it so you continually get light. But even then you notice that it's not good light. You get brighter, dimmer, brighter dimmer – of course that's cycling at 60 or 120 hertz a second, so it's happening too fast for us to really see. It's not the most efficient way in terms of actually driving the LED, but at the same time, it's super cheap. And sometimes that's better than getting some sort of rectifier in there that's taking this signal and making it a beautiful crisp DC signal. So LEDs are DC. But they can run on AC just not as well. And they can hook straight into your AC because almost every led out there has the electronics to make it work with AC.

08:20 What are LEDs made from?

Okay, that is wide. They're made of different materials. And I again, going back to that thing of how far you want the electron to drop for the amount of energy that it dropped. You need different materials. Now gallium arsenide pops into my mind, I think that produces red or blue. Dude, I don't I don't memorize this stuff, but it's a variety of semiconductor materials. And maybe that's what I should focus on. Semiconductor materials are materials that sometimes conduct like a conductor and sometimes they don't conduct, like an insulator. And the way that you change that is by doping them and throwing some impurities in there that will make them a little bit more conductive or a little less conductive. As you change the voltage across them, you'll make them go from a state of not wanting to conduct to a state of wanting to conduct. Now, the way you get really cool things out of semiconductors is you take those semiconductors of different types, and you put them together. And the way they act at that junction really changes them. You can get all sorts of cool stuff besides LEDs by that junction. So semiconductors are two materials, you put them together and then they behave in such a way that when the electron goes across, it drops in energy and releases that energy as a photon.

09:36 Are LEDs polarized?

Definitely, and it matters a lot. With LEDs you can actually see that there is one leg that is slightly longer and then if you look inside, which you probably can't see, you notice that on the one side, it's this tiny little stick coming up and then there's this other side that has this big cup. And that big cup is the negative side of the cathode, I believe, and then this small thing is the anode. And if you hook this up backwards, it's just not going to do anything. Now if you hook it up backwards and put a huge amount of voltage over it, you get to blow it up and it's kind of fun because it overheats right there and then goes "pop". But that's actually surprisingly hard to do. It only seems to happen when you don't want it to but yes, LEDs are polarized because they are diodes so it very much matters which side is positive and which side is negative because it will not work if you put it in backwards.

10:41 Will 24v LEDs work on 12v?

No, LEDs don't conduct at all until you reach their forward voltage – until you hit a certain point that you have enough energy to overcome the barrier between the two portions (the two materials that are touching each other). So if you have a 24 volt LED, it's designed to wait until it has 24 volts before it crosses that barrier and goes up. Now mostly when you see these 24 versus 12 volt LEDs, you're talking about the strings of LEDs and usually what they do is they divide those strings up. So it's like, "Hey, I'm going to have six LEDs be in series at a time and then six LEDs times 1.7, is about 10 volts or something like that". And then if you say, "hey, actually, I want to make this a 24 volt string", then you do 12 LEDs in series and the overall voltage drop across all 12 of those will equal around 28 volts. So if you take a 12 volt LED string and put 24 volts across it, it will work and you might not even destroy it...you will in the long term, but if you put 12 volts across the 24 volt rated LED string, it's just not going to do anything. You're not going to harm it. Nothing's gonna happen. But if you flip it and have 24 volts over 12 volts, at best it's going to shorten the length of time that that string is going to work and at worst you're going to have some fireworks or just fires.

12:14 How do LEDs change color?

That's a myth, they do not change color! LEDs are actually completely incapable of changing color. Their color is defined by the materials that are involved. Once those LEDs are manufactured, they produce that color. That's all they can do. "Wait", you say? That doesn't make any sense. I've seen RGB LEDs that change their color. Ha, they don't! So in an 'RGB LED", they actually have three LEDs in there acting at the same time – you have red, green, and blue. And as you control them and make one brighter and one not so bright (dimmer) it changes the ratio and it creates different colors. But in reality, all you're doing is adding or subtracting those colors to get exactly what you want. There isn't an actual change of the color of the LED. So once we get past that, the way you do change led colors is by making your red or green or your blue brighter or dimmer to get the necessary color that you want. The actual LEDs themselves do not ever change color. They do get a little bit warmer sometimes depending on how much electricity goes...oh man, don't make me back up on this. No, they do not change color. Nope. I'm sticking with it.

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