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
mostly to embarrass me. I think that's what the point is.
Today's topic is LEDs and you're gonna learn
more about LEDs than you will from any other source.
GUARANTEED.
First question: 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?
A diode is something electricity goes one way and doesn't go another way.
That's really all you need to know you.
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 there you 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 just doesn't really work.
That's how a diode works on a more fundamental level.
In reality, all you need to know diodes electricity goes one way.
And the other way it's like, oh! can't quite make it!
Now, what does this have to do with light emitting diodes?
Only thing that it has to do with light emitting diodes is
that LEDs are diodes.
Like I said, two materials come together.
LEDs and how they work in particular is 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 as something 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 is to give us the photon.
So you want to drop more energy you get a bigger gap
and then you get something like an ultraviolet LED.
You want less energy where you like in the infrared or red
and you have a smaller step down and then as that electron goes
and it drops in the energy. That energy is released as LIGHT, instead of heat
which is what happens in a normal diode.
Next question: Why are LEDs better?
Wow!
Talk about wide scope in questions.
Why are LEDs better?
Big thing! They're very efficient.
Much better than incandescents.
Man, I could grab along this all day long!
and if you want, I think I've done another video where I did exactly that!
They're more efficient, they're tough, they can flicker on and off real good.
So they're good for communications.
They're good for dimming in a really power efficient way.
Did I mention they're tough?
I just love the fact that you can take them and throw them around.
Well I have an LED around here.. Nope that was an oversight.
They're cooler to the touch You can actually touch them
without burning yourself whereas incandescent,
I want you to see how long you can keep your finger on
even the lowest power incandescent bulb and 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 like 15+ minutes about it on some other video
So, knock yourself out.
When did LEDs become popular?
What? I, uh, okay
When did LEDs become popular?
They were invented like in the 1950s or something
but they were super expensive and not super bright
and then they figured out, oh hey we can make these a lot cheaper
and a lot brighter in like the 70s and that's when those red LEDs came out.
As I mentioned earlier, red LEDs there's smaller drop in energy
so those are 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
and so that's when they started getting into the mainstream of everything.
LEDs that you get all over the place that became popular when the blue LEDs
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 white.
Even a colorblind guy like me knows that, that's how you get white.
So the blue LEDs came out. Those were early 90s is when they figured that out
and they didn't become super mainstream and super popular until like 2000 on.
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 getting all over the place. They're getting so cheap.
They're getting so efficient that I mean it just it's getting ridiculous.
Next question: How much more efficient are LEDs than other options?
Let's compare them to obvious incandescent halogen.
Way more efficient.
Five, six times and that's at current levels.
It can get even more than I imagine in the future
and that's to the latest most efficient incandescent that they've been working on
trying to make those more efficient for a hundred years.
Compared to the original Thomas Edison one, I don't even know it'd be insane.
Compared to other things, fluorescent CFL stuff like that
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 and depends on the quality
of the electronics and your LEDs.
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 like
the gas discharge in the fluorescence.
They are way better in other ways so they're just as efficient as them
but better in every other aspect.
Moving on.
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're heating up an incandescent bulb, it's getting hot and getting cool
hot, cool, hot, cool, hot cool.
Every time you turn on and off the switch and that's one of the reasons
why you don't want to flick an incandescent light bulb.
And each time it does that, it's going for telephonic cracks
and once that tungsten filament cracks, well, it's completely useless.
A semiconductor is literally just two pieces of material sitting next to each other.
You're running some electronics through it.
There's no moving parts. Nothing's getting that hot in the junction itself.
Now the electronics to power it sometimes, those will get hot and those will break down.
And so, that's one really cool thing that you can tell what failed
the LED or the electronics.
Did it get dimmer and dimmer and dimmer and go out, then it's typically the LED.
Did it just go out or is it flickering stuff like that, that's actually
the electronics that are running the LED that went out.
So that's one good way to tell the difference.
LEDs are doing better than all those other ones simply because
they run coolers, they don't generate heat.
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 still 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 less than most of the other sources.
That's why they last so much longer.
Are LEDs AC or DC?
Excellent question!
Part of the confusion there is because you take those light bulbs
that look like the old-school incandescent and you put them into an AC socket
and they produce light. So you're like sweet they're AC.
Nope.
Again, those bulbs on the bottom they have electronics
that's converting that AC to DC.
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 in the 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 get light
get light, get light.
But even then, you notice how that's not you get good light.
You get brighter, dimmer, brighter, dimmer.
Of course, that's cycling at 60 or 120 Hertz a second
if you take that bottom portion and flip it over to the top.
So it's happening too fast for us to really see.
But that's not the most efficient way in terms of actually driving it.
But at the same time it's super cheap, it'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 that makes it so it runs on the AC.
Next question: What our LEDs made from?
Okay, that is wide.
They're made of different materials.
Going back to that thing of how far you want the electron to drop
for the amount of energy that it drops you need different materials.
Gallium arsenide pops into my mind.
I think that's red.. no, is that blue..
Dude, I don't memorize this stuff.
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.
The way that you change that is by doping them and throwing some impurities in there
that sometimes make them a little bit more conductive or a little bit less conductive.
And then, 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.
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
releases that energy as a photon.
Moving on.
Are LEDs polarized?
Polarized...
Yes.
I'm just thinking the North Pole, you know.
Polarized doesn't matter which way that you put it in, definitely it matters a lot.
Thank you Taylor.
LEDs you can actually see that there is one leg that is slightly longer
Oh, this is kind of weird. Maybe this one's been trimmed a bit.
If you look inside, which you probably can't see, you notice that on 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, the cathode I believe.
And then this small thingy is the anode.
And if you hook this up backwards, it's just not going to do anything.
If you hook it up backwards and put a huge amount of voltage over
you get to blow it up and it's kind of fun because it'll overheat right there
and then possibly pop but that's actually surprisingly hard to do
that only seems to happen when you don't want it to.
But yes LEDs are polarized because they are diode.
It very much matters which side is positive and which side is negative
because it will not work if you put it in backwards.
Moving on.
Will 24V LEDs work on 12V?
No.
LEDs they don't conduct at all until you reach their forward voltage.
Until you hit a certain point that you have enough energy to come over
overcome the barrier between the two portions, the two materials
that are touching each other.
So, if you have a 24V LED, it's designed to wait until it has 24V
before it crosses that barrier and goes up.
Mostly when you see these 24V vs 12V LEDs, they're talking about the strings of LEDs
and usually what they do is they divide those strings up.
So, you have like, "I'm gonna have 6 LEDs be in series at a time and then
6 LEDs times 1.7 is about 10V or something like that.
And then if I say, "hey actually I want to make this a 24V string"
then they do 12 LEDs in series.
The overall voltage drop across all 12 of those, again will equal around 20ish volts.
If you take a 12V LED string and put 24V across it, it will work.
And you might not even destroy it. You will, in the long term.
But if you put 12V across a 24V rated LED string, it's just not gonna do anything.
You're not gonna harm it.
Nothing's gonna happen.
But if you flip it and have 24V over 12V, you're at best going to shorten
the length of time that string is going to work.
At worst, it's gonna have some fireworks
or just fires.
Moving on.
How do LEDs change color?
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 produced that color.
That's all they can do.
Wait. You say, "Josh that doesn't make any sense".
I've seen RGB LEDs that change their color.
Ha! They don't.
In an RGB LED, they actually have 3 LEDs in there acting at the same time.
You have it red, green and blue.
And as you control them and make one brighter and one not so bright,
dimmer, that's the word I was going for
and change that ratio between them, they create 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.
Once we get past that, the way you do change LED colors
is by making your red or your green or your blue brighter or dimmer
to get the necessary color that you want.
The actual LEDs themselves do not ever change color.
Though they do get a little bit warmer sometimes depending on how much electricity
you go oh man don't make me back up on this.
They do not change color. Nope I'm sticking with it.