Hi, I'm Steve Jones and I'm going to explain how you might wire an LED or light emitting diode into a circuit. LED stands for light emitting diode. Now a diode, if I spell it correctly, diode, a diode is a device which allows electric current to pass through it only in one direction so it has a certain amount of safety in it that if you connect it the wrong way round it actually isn't going to do very much, but the one problem about an LED, a light emitting diode is that it is a current driven device. There are different ratings, but usually an LED will take something like thirty mA, a very small current, a very very small current indeed. So, this very small current traveling through this light emitting diode causes it to give out light and of course that light can be a different colors, the common ones, red, blue green. If you want a yellow, then you mix a red and a green. You actually put two diodes together and it'll look yellow, but actually it's two diodes, a red and a green. But the most important thing is to regulate the current flowing through here, and to regulate that current, through the light emitting diode, you need to make sure that the voltage between the ends of it, the voltage, by Ohm's law is I times R. You've got to put in a resister which will limit the current, because the light emitting diode has a very small resistance. And therefore this resistor is very important. If the current you want is thirty mA, thirty mA, that's thirty over one thousand amps, multiplied by the resistance. Let's say I'm using as I might use a twelve volt supply, then that is like a car battery, twelve is equal to thirty... by rearranging the equation now bring this to the other side, I get twelve times one thousand over thirty and, well if we cancel it down, it's one thousand two hundred over thirty... over three which is... gives us a resistance of four hundred ohms. And that will be a typical value for the resistor in a circuit where you're operating with an LED. This resistor is always incorporated into the LED or should be. If you buy an LED array, you'll often find that the resistor is already built in. If you want to use several of these, you tend to buy an integrated circuit in which there may be seven or eight resistors, all the same value, it makes is much simpler to connect these in. If you want more light, you have more LEDs and if I connect another one by the side, here's my second light emitting diode, I put it into my circuit in the same way and I still need here four hundred ohms. This one of course is four hundred ohms as we've already calculated. So each LED needs a four hundred ohm resistor. Or, if the current capacity is quite high, it is actually quite reasonable to connect the four hundred ohm resistor to a series of LEDs, each of which is placed up to the twelve volt line in this way. So you've got a series of separate LEDs all connected through one high powered resistor, that is a resistor which can take a high current. Either of these will work, the resistor here is what is controlling the current so when I'm connecting in my light emitting diodes, I must have a resistor for each light emitting diode to limit the current to thirty mA. And that is how I wire up my LED in an electrical circuit.
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