In the late 1800s, Thomas Edison invented
the first commercially viable light bulb. Light bulbs like this work by passing large
amounts of current through a thin filament, which is basically a wire. The filament gets so hot that it starts glowing,
and emitting light. This process is very inefficient – less than
5% of the energy going into the bulb gets turned into light – the rest gets turned into
heat. A much more efficient source of light is a
light emitting diode, or LED. LEDs basically contain two specialized semiconductors
that are stuck together, and when you apply a large enough voltage across them, they emit
light from a process called electroluminescence. There is some heat produced, but overall the
process is a lot more efficient, and you can get a lot of light from a very small device.
On average, they last for over 10 years of
continuous usage, so you can see why they are popular. Nowadays we have LED flashlights, LED street
lamps, billboards and even LED light bulbs. But you don't have to be as big as Sony to
create LED circuits. In this video I'm going to show you the basics
of how to use an LED at home. First, get some LEDs. You can buy them anywhere that sells electronics,
but Amazon has them really cheap, and you can get dozens for just a few dollars. I'll put a link in the video description section.
I recommend getting some resistors to go along
with them, and I'll explain more about that in an upcoming video. So every LED has 3 important things you need
to know. The polarity, the forward voltage, and the
maximum current rating. Let's start with polarity, and this basically
means which way do you connect the LED in your circuit. All LEDs will have two leads, an "anode" and
a "cathode". The anode and cathode are sometimes abbreviated
A and C. The anode is the side that conventional current will flow into. In other words, you connect the positive side
of your power source to the anode. The cathode is where conventional current
will flow out from. So you will connect your negative side of
your power source to the cathode. For standard 5mm LEDs like this, there are
two easy ways to figure out the polarity.
The anode will have a longer lead, and the
cathode will have a shorter lead. Also if you look carefully, you'll see that
one side of the case has been filed down flat. The flat side is the cathode, and the round
side is the anode. So in this example I connected the positive
side of a power supply to the anode, and the negative side of the supply to the cathode.
The LED lights up as expected. If you get the polarity wrong your circuit,
don't worry, for low voltage projects the backwards LED blocks current from flowing,
and it just doesn't turn on. Ok, now let's talk about the forward voltage
of an LED. All LEDs need a certain voltage across them,
in the right direction, before any current can flow and they start emitting light. This particular white LED I'm using has a
forward voltage of 3 volts, so we need around 3 volts before it can do anything interesting. With the supply set to 0 volts the LED stays
off. With the supply set to 1.5 volts, it's still
not enough to turn the LED on. But as we get closer to 3 volts, the LED reaches
full brightness. Every LED will have a forward voltage that
is a little different, and here are some rough guidelines of what you can expect from different
LEDs. Once the LED is on, there will be a relatively
constant voltage drop across it. Next, let's talk about the maximum current
limit of LEDs.
In this example, I was using a special feature
of my power supply to limit the current going through the LED to a maximum of 30mA, which
is about as much as this LED can handle. But what happens if I take my foot off the
brakes? I'm going to set the power supply to 7.5 volts
and I'm going to remove all current limits. Let's see what happens! So if you significantly exceed the LED's forward
voltage rating, massive amounts of current will flow through the LED, and there's nothing
to stop it from blowing itself up. Obviously don't do this at home, it's actually
possible for LEDs to explode, sending tiny chunks of plastic and metal flying at your
face. You don't want to end up looking like this! Now you see why in my video on voltage a 9
volt battery was enough to instantaneously kill an LED – there needs to be something
to limit the amount of current to a safe value.
The current rating on every LED will be different. For example this high powered LED module can
easily handle 100 milliamps, but in general the standard 5 mm LEDs you are going to be
playing with at home are rated for 20 milliamps. So what do you do if you want to limit current,
and don't have a fancy adjustable power supply? You can use a thing called a resistor! And these literally cost a few pennies. I'll talk more about resistors and resistance
in my next video which will be linked here. In the meantime, leave a thumbs up, check
out the website, and follow me on Twitter and Facebook!.
