As you probably guessed LEDs are probably my favorite passive component when it comes to electronics. Normally, when you order them you get nice bags of red LEDs and blue LEDs and green LEDs, you know exactly what you have.
Then halfway through the semester, or every day in my life, you end up with a bag that is just a big question mark of LEDs that you didn't feel like putting back in the right bag. That's going to be a good launch place for us to talk about them.
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The first thing I want to mention is that even though these are all five millimeter LEDs you will see that some come with very different lead lengths so the top is the same, it's five millimeters in diameter or called a T1 and three-quarters package, but they're different heights so look out for that if you're using them for something particular. This is a three millimeter or T1 package; you can see the difference there. So if you're ordering a bunch of colors and you accidentally end up with these instead of these it makes a big difference.
These are all considered indication LEDs rather than lighting LEDs so they usually have a forward current of about 20 milliamps for all the different colors. If you're going to actually do a lighting application you would want probably surface mount lighting specific LEDs, those take more like a third of an amp or an amp and output a lot of light. These are just nice for making your circuits look pretty, for checking the status with certain pins, etc.
So what I want to show you here is that they are constant current devices not constant voltage devices. With resistors you put a voltage across them you get a certain current out. These work a little bit differently, if this is current and this is voltage theoretically as you go higher in voltage (0.1V, 0.5V) the current isn't actually increasing until you get to the threshold and then shoots straight up. It's actually a little bit more of a slope, you'll start to see some light at lower voltages, but the idea is that once it hits that voltage it will just accept more current rather than drop a voltage.
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So I have the power supply here set up to give me 5 volts which, if you're not limiting the current, will blow up any one of these. Most of them have a forward voltage closer to 2V, 3V, or 3.5V depending on the color. Every LED with two leads like this has a longer lead and a shorter lead. The longer lead is called the anode and is the positive lead, shorter lead is the cathode and the negative lead. If I limit the current to 20 milliamps which, I've done here, I can leave it at 5 volts and you'll see that when I turn that on, the top is always the brightest part of the LED; don't look directly into that. You'll see that even though we've given it 5 volts, it's only drawing 2.8 volts because I'm only letting it draw 20 milliamps. If I do go to current and start increasing that a little bit, you see that voltage goes up because the current is going up. Yet there is a maximum limit, the voltage is not going up nearly as quickly as the current is going out. That was about 3.2 volts at that current 50 milliamps.
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This one, a different color, very different forward voltage. You can see how it's actually kind of annoying to swap out colors mid-design because you have to go through and find different current-limiting resistors in order to not hurt any of these LEDs. I have a cheater way; this is a 3 volt battery, it's just a simple little 2032 3 volt battery. Because this is such a small battery, it's not going to be able to give enough current to actually hurt any of these LEDs. You just slide one on and you can see the color really quickly. Even though this is a red LED, it has a forward voltage closer to 2 but it's not going to hurt itself. Even these that have a higher voltage they'll still draw enough voltage and current to show you the color. To avoid all that stress, if you're going to use a white LED like this in all of your designs, you can color it with a sharpie to cut other colors. It's not going to be perfect, but if you feel like coloring rather than doing math you can still end up with a red LED. If you're building your electronics kit that's why I recommend have a bunch of these five or three millimeter white leds on hand because then you know the forward voltage, you know that they want about 20 milliamps, you can test them all with the 3 volt battery, and you can color them as desired.