Full-wave rectifiers

I mentioned half-wave rectifiers in my last post as a pretty neat application of diodes. Unsurprisingly, it turns out we can do way better at converting alternating to direct current if we're willing to use some more diodes. Just like before, we want to input an alternating-current voltage source, which means the voltage difference between the two inputs oscillates over time. In the end, we would ideally like a completely constant voltage to be output. Well, check out this circuit:

Image adapted from electrician Joe Duncanson's highly informative blog.

(Notational note: The triangle-bar thing is the electrical symbol for a diode whose polarity will only allow current to flow in the direction the arrow points.)

As you can see, whether the top or the bottom input node is at a higher voltage, current can and will flow toward the upper output node. Then the current from the lower-potential bottom output node will flow through the diode diamond to the lower-potential of the two inputs. In this way, we ensure that the output voltage difference (voltage of the top output minus voltage of the bottom output) is always positive. More specifically, if the diodes are ideal, then the voltage we'd expect out of this circuit would be

Input and output voltages for an ideal full rectifier.

But as usual, the real world isn't quite so friendly. In reality, there's a bit of internal resistance in the diodes, so they don't start letting current through until there's enough voltage across them. As a result, the output voltage looks more like this:

Input and output voltages for a (slightly) more realistic full rectifier.

We're still not quite at the constant output voltage we set out to find, but we're a good deal closer than when we stuck with just one diode. Hopefully I can write a bit about capacitive filtering, which goes a long way towards smoothing this curve out, sometime in the near future.