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.
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