We’re gonna go over really quick on

how to calculate the resistor that you need to put in series with an LED

so you don’t blow it up. The reason we need this is because once

you pass the forward voltage of an LED, a slight increase in the voltage makes

a huge increase in the current and we don’t want to blow things up. So we put a resistor in series with it

that keeps it under control and it makes it have a little bit more

of a linear relationship between the voltage and the current though

it’s not exact, it’s better. So I have six steps for choosing

the resistor but two of them are sanity checks and

the other four are really simple. So definitely don’t stress about it. The first thing you need is to find out

the forward voltage. That’s either from the datasheet,

a generic source like our color and forward voltage table on our

website or from simple experimentation. I bought this from SparkFun and I don’t

know if they still do this because I did it a couple of years ago,

but they have some of the basic data you need in the package itself. It’s pretty cool. So I can see straight from here that the

voltage drop of this LED is 2.2V. That makes it easier than going and

looking in the datasheet. But if you don’t know, it’s worth it. So that is the first thing: find out what the

forward voltage drop of your LED is. Now, the second thing is to find out

what the max current is. They’re should also be from the datasheet

and you can almost always go past the max current and still have it work

but you’ll be shortening its life. Looking on here, this says I max (current max) is 20

milliamps which is very typical for this size of LED. I’m actually not sure if I’ve ever seen one

that’s different than 20 milliamps on these. If you get a bigger LED or a smaller LED,

you want to check that datasheet to make sure that you don’t blow this. The third thing is actually a

continuation of step two and that is we know that this can have 20 milliamps

before it starts to damage it. But how much do I actually want?

Do I need it to be that bright? Can I use less current? Step three is find out how much

current you actually want and that could be something where you

just have to do a little bit of trial and error and see how much current is needed

to give it the brightness that you want and that’s all you have to do. Once you know what the forward voltage

drop is and you know how bright you want it to be, how much current

you need to have go through it making sure that you don’t go over

the maximum current, you do step four which is figuring out what the

resistance will require to hit that current. You can use Ohm’s Law and just math it

which is voltage source minus the forward voltage at the desired current and then

voltage over current. In that case, we have the voltage source,

we’re just going to assume 5V because that’s very common and

just keeps the math simple. The voltage drop on this is 2.2V so,

5V minus 2.2V gives you 2.8V and then you just divide that by the 20 milliamps. So we actually have an app or something

on our site where you can first see this all laid out visually, gives you the

equation and you can type it in. So voltage source is 5V,

the voltage drop of the LED is 2.2V and then the current of the LED

is going to be 20 milliamps. Doing that and that way I don’t have

to do the math in my head, gives us a hundred and forty ohms. So that is basically how you figure

that out and that’s step four. The next two steps are just sanity checks. And number five is making sure the

resistor that you physically, actually using has a high enough

power rating. I’m gonna use a quarter watt resistor

with this and looking at my calculations here, it’s showing that I’m only going to be

consuming about 44 milliwatts. I can handle 250 milliwatts, this says,

I’m only going to be consuming 44 milliwatts.

I’m good to go. And then number six, the other sanity check

is just making sure that everything makes sense. So, if you have a 12V battery source and a 3V LED,

why not put three or four of them in series three or four LEDs in series instead

of all of them in parallel with huge power wasting resistors

for the rest of the voltage. If your max current is 1 amp but

you’re only driving it to 1 milliamp, why don’t you just use a

smaller cheaper LED. If this is just a project you’re doing

for the fun of it, then efficiency in saving a couple pennies

isn’t such a big deal. Just go and have some fun. But if you are planning on something

that’s a little bit more logical a little bit more sensical,

take that time to think How much power am i burning here? Is this oversized? Is this undersized?

And then go from there. So with that, I actually want

to do this really quick. We calculated a hundred and forty ohms. But in reality, unless you’re doing high

precision resistors, they’re at a different rate. So you don’t get 100, 102, 104, 105,

you get the 100, 142, 150, 100 like the more random numbers. I’ll probably only be able to find a 150 ohm

resistor to go with this but that’s okay. If you can’t get the exact resistance

if you’re already pushing the limit you want to go up a little bit in resistance

so 150 ohms will be perfect for this. I’m just gonna grab my power source

here and turn it on. So it’s gonna be 5V because

that’s fairly common with USB. I’m gonna hook this up to the anode.

Spread that out. And then I’m going to hook up to the

resistor and hook up to the cathode and we’re gonna

see what happens. We’re getting a bright shiny

green LED and not only that but just as you can see here I’ve got 5V

and we’re at 19 milliamp which is about exactly what we expected with

a 150 ohm resistor. We’re gonna be burning much less

than this quarter watt resistor can handle. And I think we’ll be good to go.

So that is what happens in action. All those steps really quick: First, find the forward voltage. Second, find the max current. Third, figure out what current you want as long as

it doesn’t go over that max current. Fourth, use Ohm’s Law to find out

what resistance you need. Fifth, make sure that your resistor is rated

for the power dissipation that you’re actually going to be

putting it through. Sixth and final is just make sure that

your whole circuit makes sense. That you’re not doing something

crazy and inefficient or really expensive that just

doesn’t need to happen. And that’s it! It’s really straightforward. Don’t even need all those steps. If you just get rid of those

sanity checks, you’ll be fine. If you liked this video, give us a like.

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