Hey guys, welcome back to "Ask an Electrical Engineer" - this is part three
of LEDs and is 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
caused people to die of old age before they finished them, so we split it up
into three parts. So, with that, we're gonna jump right back into the questions
about LEDs. All right, moving on. Why do LEDs need a resistor? Okay, cool.
LEDs need a resistor because, as you increase the voltage across them, they
don't go, "Oh, I have more voltage everything's great. Now everything is
happy. If I go to a higher voltage, everything's good." No, it goes where the
voltage goes up a little bit linearly but then the current starts to curve and
go crazy crazy 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'll never go over 1.8 volts,
because by the time I get up to 2 volts I'm gonna start causing damage, because
by then that current has gone like this and has gone kind of crazy. But you stick
a resistor in there and you make it so once you get over that forward voltage
then you have basically 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 it's there as 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 have resistors with
LEDs. So, moving on. Can LEDs be wired in series? Yes. Just kidding, yes, it 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 one point... I'm just going to round this up
to two because I'm terrible with math, your two volt LED, where you need two
volts to run this. Okay, so you have a 9-volt battery and two volts here so
what are you gonna do with the rest of that seven volts? That's where you need
that resistor but then you're wasting all of that power in that 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 that makes 8 volts, and then I have a 9 volt battery,
I have 1 volts of like 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
producing basically four times the amount of light and 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 the
strips of light, 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. Moving on. 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 and so when you have your
millions of pixels up there and it says, "Hey, I want this pixel to be green." And
that RGB LED right there, the OLED, will turn on the green pixel 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 turn itself off. And that is an OLED TV. It's literally just a bunch of LEDs
putting out light. An LED TV, however, is not quite as cool. An LED TV, 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 to give it a nice uniform backlight. And
that is why OLEDs are so cool is because OLEDs, you want a dark sky and dark
night, you're watching Interstellar and it's 90% darkness for some of those,
with just a couple of pinpoints of light, those LEDs, those OLEDs just turn off and
so they are black black black. Now, you watch that same scene on an LED TV and
all of the LEDs that are creating the backlight are still on. They're all on
and they've done some things where they'll turn off certain sections
but even then it's not as direct. And when you have black, what's happening is
that screen in the front that's showing the colors, it's 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. Because technically OLED TVs, you get infinite
contrast ratio. The dark is completely black, just absolutely dark, and then it
gets bright. And anything divided by zero is going to be infinite.
Whereas LED, even with it being completely dark, it can't block off
everything, so some of that light seeps through. And that's why the blacks will
look a little bit white - if you're super super noticing that sort of stuff. Next
question! Can LEDs overheat, explode, or catch fire? Overheat? Yes. Explode? Yes. Or
catch fire? Maybe. I've never heard of anything 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 still see. The LED, the
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 incandescent, they don't like you to take
LED, so like 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 it in the bulb, it 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. Because the heat in an incandescent is
in the bulb itself and the rest of it doesn't really matter. It's just, you know,
conductors and there's no electronics that need to be worried about. So, if you
have an LED and you put it up like this, then the heat that's generated here and
the heat and the electronics, all kind of go up to the electronics and melt things.
And so that's where you can overheat. Now, the explode...
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, I couldn't even destroy an LED
on purpose. And then it turned back on. Yeah, I don't know what in the world is
going on with these LEDs. They're just here to mock me. There's nothing in the
world that'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 or 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. Next question. Can LEDs power solar
panels? Do you know how solar panels work? Do you understand that LEDs use power
from the solar panels? They don't power the solar panels. Solar panels generate
electricity. LEDs use electricity. Oh, man, I really did think that that was just a
stupid question. So, with that in mind, can using an LED, if I shine an LED on a
solar panel, which is probably how that should have been phrased (I'm not
accepting any of the blame on that misunderstanding) if you shine an LED on
a solar panel it will produce electricity. Now. It depends on the LED,
how efficient you are and the solar panel. Solar panels are designed to be
most effective with the sunlight they are expected to receive. And you can
even... I've read about solar panels where they say, "Hey, if you're in cloudy areas
and the clouds are absorbing certain amounts of that light, certain
spectrums of the light, then you want to use these solar panels because they're
designed for the light that's going through." That's not visible, that's gonna 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 with that, your LED,
you could have an LED like on this one, it's only emitting red so it's not
gonna 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 power on the solar panel.
Again, poorly worded question, not my fault at all.
Moving on. How are LEDs protected in an electrical circuit? So the most basic one
is just throwing in a resistor. You just have that resistor and it makes it so
any spike in voltage or any even just slight increase in voltage does not fry
your LED. And then a lot of the stuff is the same things that you're going to be
protecting everything else in your circuit. I mean you've got your TVS's,
MOV's, fuses, breakers, you have your capacitors, that are gonna be absorbing
any stray current and trying to suck up any voltage spikes, stuff like that. So,
really other than making sure that you don't overvoltage 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. All right,
moving on!
How do LEDs dim? Oh, coo. So the two ways is 1) 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, it
makes it how bright it is. So, roughly, if it's on half the time, off half the time
it's going to look about half as bright. And that's pretty cool because it's
pretty easy. Now, the drawbacks are 1) 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. So 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 ten minutes later you can have an LED
doing whatever you want in terms of brightness, flickering on and off. It does
have that complication it also, depending on how big the LED is, you could actually
start to create some electromagnetic interference because you're pushing the
current on and off so much and you're getting that oscillation that is
actually putting out small, basically radio waves, and that can disrupt other
electronics and you're gonna start to have some conformance issues. And.. it
just gets ugly so something like this, that's not a big deal and probably the
way you want to do it. But once you're getting into stadium lighting or
something crazy where you're getting amps of current and you don't want to
have, you know, ten amps of current flicking on and off a hundred times a
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 that IV curve, your voltage versus current curve, then you just
change the voltage and measure that 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. Next
question! What are you hoping LEDs can do in five years? I'm not very smart, nor
am I... incredibly smart...
I'm just proving my point now... Creative! Creative, that's the word I'm looking for
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 then that's fantastic. And the
thing is, the market just finds places for them as the LEDs become
better. People will have brilliant ideas "Oh 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" I mean
that's exactly how it's been up to this point but 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 give out so fast and I'm
really excited to see where they go because LEDs are awesome and they're
only gonna get better with time. Next question. What are the coolest uses
you've seen for LEDs? Hmm... I think the coolest thing I've seen and this is kind
of ridiculous cuz I just made fun of them, is the fact that my kids have some
shoes that my wife got for, I think, it's five bucks. 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 in a disco mode. My four-year-old loves to go and do
the disco party mode and it's flashing all these different crazy things and
honestly one of the dumbest things ever. And what is so cool about that
is the first red LEDs, they came out, they're like 200 bucks a pop and now
they're so cheap that we just stick them into our kids shoes and my wife can get
them on clearance for a couple of bucks and they're 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, 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... they're everywhere! They can do so many different
things and they're so cheap and so ubiquitous, they're just cool. So
that's what I think is cool. Yep. All right guys, that's all the questions
we got! If you like this video, leave a like, if you have any questions, leave it
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