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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 back to Ask an Electrical Enginee. This is part three of the LEDs, and the final and maybe the best installment. We got so many questions about LEDs and we didn't want these videos to be so long that they cause people to die of old age before they finish them, so we split it up into three parts. So with that, we're going to jump right back into the questions about LEDs.

00:23 Why do LEDs need a resistor?

LEDs need a resistor because as you increase the voltage across them, they don't go, "oh, I have more voltage, everything's great...if I go higher voltage, everything's good". No, as it happens the voltage goes up a little bit linearly, but then the current starts to curve and go crazy. So if you can very finely control the voltage, then you don't need that because you can say, "okay, 1.7 volts, I'm good to go 1.8 volts, I'm good to go...and I just know I will never go over 1.8 volt because by the time I get up to 2 volts, I'm going to start causing damage because by then that current has gone a little crazy". But you stick a resistor in there, and you make it so that once you get over that forward voltage, you basically have a semi constant voltage drop over the LED and then you have the resistor itself and the resistor makes it so you have a linear relationship between your voltage and your current, so there is a protection device. It's there to make it so as your voltage increases past the forward voltage of the LED, the current is still reasonable and not spiking up like a madman. So that is why we use resistors with LEDs.

01:42 Can LEDs be wired in series?

Yes, they can be wired in series and that's a great thing to do because let's say you have a 9 volt battery, and you have your...I'm just gonna round this up to 2 because I'm terrible with math...your 2 volt LED, where you need 2 volts from this. Okay, so you have a 9 volt battery and two volts here. So what are you going to do with that rest of that 7 volts, that's where you need that resistor. But then you're wasting all of that power in the resistor that's trying to make it so the amount of current through this is okay, so since this is 2 volts, why don't I just put four of these in series and then it's 2 volts, 2 volts, 2 volts, 2 volts, which makes 8 volts. And then I have a 9 volt battery, I have 1 volt of overhead and then the other 8 volts will be dropped over these LEDs to make it so that I am having an LED that is basically producing four times the amount of light and I'm using the same amount of power. So LEDs in series – not only is it possible, it's recommended in a lot of situations. Again, going back to LEDs, the LED strips that you can buy for whatever purposes, that's usually what they're doing there, they are putting a bunch of LEDs in series to make it so that you have the equivalent forward voltage that you need for your system.

03:03 What's the difference between an OLED and LED TV?

So I don't actually understand OLED very well as a technology, like what drives it, but I do know what's going on with an OLED versus an LED TV. So OLED TV is basically just a huge screen of actual RGB LEDs. So when you have your millions of pixels, it's able to say "hey, I want just this pixel to be green on that RGB LED right there", so the OLED LED TV will make the pixel green and say, "huh, here it is, and it'll turn on as bright as it needs to and if it's not turned on, it will just turn itself off." And that is basically how an OLED TV works. It's literally just a bunch of LEDs that can output their own light.

An LED TV however, is not quite as cool. It has a screen over the front that has colors, and it controls those colors, and then behind it, it just has rows of LEDS that shine through and give it a nice uniform backlight. That is why OLEDs are so cool. With OLEDs, you can get a nice dark sky in The Dark Knight or when you're watching Interstellar and it's 90% darkness for some of those space scenes, with just a couple pin-points of light for the stars, the LEDs that need to be dark just turn off and so they are perfectly black. If you watch that same scene on an LED TV, all of the LEDs that are creating the backlight are always on. They have certain technologies for LED TVs where they'll turn off certain sections, but even then it's not as direct and when you need to produce a black output, what's happening is that screen in the front that's showing the colors is trying as hard as it can to block out that LED backlight but it's not perfect. You get some light that seeps through and that's why you don't get the same contrast ratios for LED TVs as you do with OLED TVs. Technically with OLED TVs you get infinite contrast ratios, the dark is completely black and then it gets as bright as needed and anything divided by zero is going to be infinite. Whereas LED TVs even with it being completely dark, it can't block out everything so some of that light seeps through and that's why the blacks will look a little bit white if you're good at noticing that sort of stuff.

05:17 Can LEDs overheat, explode, or catch fire?

Overheat, yes. Explode, yes. Catch fire...maybe? I've never heard of these catching fire but if it produces heat then technically it can catch fire, right? It's just a matter of scale. So LEDs can overheat and this is something you'll see at the junction where the two materials meet. Again, most of the electrons that are dropping over, they emit a photon but sometimes they create heat. And even though it's way better than incandescents, it's not ideal to have LEDs bulbs upside down....so if I took this and imagine this is an actual bulb, they want you to have the LED going up like this, because if you turn it upside down like that then the heat created at the junction and created in the electronics that are driving goes up and fries the electronics and that's actually a pretty common way for LEDs to burn out and that's one of the few things that an incandescent is better at, because the heat in an incandescent is in the bulb itself and the rest of it doesn't really matter – it's just conductors and there are no electronics that need to be worried about. But if you have an LED and you put it up like this, then the heat that's generated here and the heat in the electronics all kind of go up to the electronics and melt things and so that's where they can overheat.

As far as exploding..I have had an LED explode on me I tried to get another led to explode on me and it failed, which was kind of embarrassing actually that I couldn't actually destroy an LED on purpose...and then it turned back on I don't know what in the world is going on with these LEDs, they're must just be here to mock me.

There's nothing in the world it's completely safe. I mean, I can be sitting right here at this desk and get hit by a meteorite. There's nothing I can do and I kind of feel like it's the same thing with these bulbs. Yes, there is the chance that something could explode and catch fire. But it's pretty darn unlikely and I have a feeling that there's going to be a significant amount of human intervention involved and not very wise human intervention at that.

07:22 Can LEDs power solar panels?

If you shine and LED on a solar panel, it will produce some electricity. Now, it depends on the LED, how efficient you are and the solar panels. Solar panels are designed to be most effective with the sunlight that they are expected to receive. And I've read about solar panels where they say, "hey, if you're in cloudy areas and the clouds are absorbing certain amounts of certain spectrums of light, then you want to use these solar panels because they're designed for the light that's going to actually make it through the clouds". Whereas if you're living somewhere in California, or Arizona, where it's just bright, sunny skies all the time, then you want the direct sunlight that you're going to be getting. So if you have an LED like this one, it's only emitting red, so it's not going to be super efficient because your solar panel is expecting a whole spectrum of light. And this is giving out a very, very narrow band of light. But you make it bright enough and as long as a solar panel is at all tweaked to accept it, then you should be fine. And it should create some power on the solar panel.

09:18 How can LEDs be protected in a circuit?

So the most basic way is just throwing in a resistor, which makes it so any spike in voltage or even a slight increase in voltage does not fry your LED. The other means of protection are the things you're doing to protect everything else in your circuit. I mean, you got your TVS's, MOVs, fuses, breakers, you have your capacitors that are going to be absorbing any stray current and trying to suck up any voltage spikes and stuff like that. So really, other than making sure that you don't over-voltage your LED and get it into that crazy current spike area of the current voltage curve, then you just protect them like you'd protect any other electronic component.

10:13 How can LEDs be dimmed?

So there are a couple of ways to dim an LED. For the first way, you can turn the LED on full brightness and then totally off really fast. And the way that works is our persistence of vision makes it so even if it's flickering on and off, it looks like it's on. But depending on how much it's on versus how much it's off, changes the appearance of how bright it is. So roughly, if it's on half the time, off half the time it's going to look about half is bright. And that's pretty cool, because it's pretty easy. Now the drawbacks are, you need something that's flickering on and off like that, so you need some sort of driver and if you want any flexibility, it has to be something somewhat intelligent. That's why Arduinos are so popular, because they're really, really easy to do this with. I mean, you can pick up an Arduino and 10 minutes later, you can have an LED doing whatever you want in terms of brightness and flickering on and off.

It also depends on how big the LED is. You can actually start to create some electromagnetic interference, because you're pushing the current on and off so much, and you're getting oscillation that is actually putting out small, basically radio waves, and that can disrupt other electronics and you're going to start to have some conformance issues and so it just gets ugly. So for something simple, that's not a big deal, and probably the way you want to do it, but once you're getting into something like stadium lighting or something crazy, where you're getting amps of current and you don't want to have, you know, 10 amps of current flicking on and off 100 times per second – that's when you get into the second way of dimming an LED and that's simply by controlling that voltage directly. So if you know what you're doing with the I-V curve (voltage versus current curve) then you just change the voltage and measure the current and make sure, "okay, I'm creeping up the voltage, okay I hit my current" and it's good to go and that way you're getting a smooth persistent output and and you're not getting those huge current spikes and radio frequency emissions and stuff like that.

12:22 What are you hoping that LEDs can do in 5 years?

I mean, if LEDs could just keep on doing what they're doing, but even better and last even longer and be even more efficient, that would be fantastic. And the thing is, the market just finds places for them. As LEDs become better, people will have brilliant ideas and think, "oh, wait a second, now that it's this cheap, I can do that, and now that it's this efficient, I can do that." And that's exactly how it's been up to this point. But that's that's really it. I don't have anything in particular that I am hoping that LEDs can do in five years. I'm just hoping to see them get even better even more robust. I'm hoping that we'll figure out the electronics so that they don't die so fast. I'm really excited to see where they go because LEDs are awesome, and they're only going to get better with time.

13:30 What are the coolest uses you've seen for LEDs?

I think the coolest thing I've seen, and this is kind of ridiculous because I just made fun of them, is the fact that my kids have some shoes that my wife got for five bucks that have LEDs all around the bottom. And they come with a little remote and you can make them be red or green or you can make them do a disco thing. My four-year-old loves to go and do the disco party move and they start flashing and doing all these different crazy things. And honestly it's one of the dumbest things ever. And what is so is so cool about it is, the first red LEDs when they came out were around 200 bucks a pop. And now they're so cheap that we just put them into our kids shoes, and my wife can get them on clearance for a couple of bucks and my kids are literally walking on them. Whereas you go back 50 years, and not only were those colors not in existence, but those shoes would have been thousands, or tens of thousands of dollars, even if they were just red LEDs. And so I think that's one of the coolest things is that they're just everywhere. They can do so many different thing and they're so cheap and so ubiquitous. So that's that's what I think is cool.