I has someone ask me about AC power and how to get a constant DC supply out of it. I though I’d do a quick write up on how a bridge rectifier works and get a little bit more experience modeling circuits in the process.

For this example I’m showing the full wave rectifier. There’s also something called a half wave rectifier but the idea is essentially the same. The AC supply that comes out of your outlet in your house is 120V and 60Hz (in the US at least). Here’s a graph of what the voltage looks like that looks like over time. The time scale on this graph is 100ms or 0.1sec.

This graph shows the voltage varying from +120V to -120V in a sine wave just like it should be. The time between peaks on the graph is 16.66ms which will give you exactly 60 peaks per second (or 60Hz). In order to get a DC voltage from this we can use a bridge rectifier. This uses four diodes that are arranged in what’s called a diode bridge. One easy way to think of it is as a device that take the negateive portion of the sine wave and flips it over to be positive. Here’s a schematic of a full wave rectifier using a diode bridge connected to a 120V 60Hz source.

This is what the output voltage (Vout) will look like with this circuit.

Notice that the negative portion of the original sine wave is now flipped over. You can see this in the time between peaks (now 8.33ms). A good start but this would make a pretty poor power supply. The voltage now varies from nearly 0V to 120V. A good way to take care of this problem is to connect a capacitor across Vout and ground.

This gives the circuit a way to store all that voltage. Vout with this circuit looks like this

In this graph the blue and red waves show the two phases in the input voltage. And green is Vout. As each of the input waves increases you can see the Vout increasing and charging the capacitor. Then as the input voltage drops off the capacitor starts discharging until the next input wave recharges it again. This makes for a much smoother output voltage. A neat thing to see from this graph is that the capacitor is actually supplying the voltage a majority of the time. There’s a small ripple voltage of about 10V but that’s much better than a 120V ripple. This 10V ripple can be further filtered but I’ll that for another post.

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