Diodes

I got the chance to play with some diodes recently, and I came away with the fact that diodes are really cool. Abstractly, a diode is simply an electric device (not electronic, I believe, because it doesn't involve transistors or logic gates or any fun stuff like that) which prefers for electric current to pass through it in just one direction. In theory, if you apply a forward bias to an ideal diode, that is, you apply a voltage to it such that electric current "wants" to flow in the preferred direction, then the diode should behave just like a wire. But if you apply a reverse bias, trying to force electric current through the other way, then the diode opposes you with (theoretically) infinite resistance. Naturally, nothing behaves quite that nicely, and in real life, diodes can be broken down if you apply too strong a reverse bias. (The exception is Zener diodes, which are designed to be reverse biased, and they have the cool property that when you run current through them in reverse bias, the voltage across the diode itself is constant - weird, right?) The idea remains, though, that you can use a diode as a sort of one-way gate in electric circuits, and this turns out to be a very handy property.

For example, one application of diodes is in rectifier circuits, which convert alternating current (AC) into direct current (DC). In other words, they can be used to allow current to flow in only one direction in a circuit. The simplest type is a half-wave rectifier, which consists of one boring diode connecting your input voltage (AC) to your output voltage (DC). In the ideal case, this results in an output voltage identical to the input, except never negative, like this:

Input and output voltages for an ideal half-wave rectifier.

But, as usual, I don't have access to the magical ideal physics stockroom, so I have to make do with real-life, physical diodes. And those come with a little bit of internal resistance. That means that it takes a non-zero positive voltage across them to actually get current to flow, and when current does flow, some of the voltage is dissipated into heat inside the diode, leaving only a fraction of it to contribute to the voltage across the diode. So in reality, the output voltage from a half-wave rectifier looks more like this:

Input and output voltages for a real, physical half-wave rectifier.

There's a lot more you can do with diodes, but even this simple example is pretty neat. Someday, I might write up full-wave rectifiers and capacitive filter circuits. Those are pretty cool, too. Conclusion: physics and electronics are awesome.