PSU Theory and Types of Power Supplies

Power Supply Theory

As it is, nothing good will happen if you stick an LED into an AC outlet (And I beg you not to try). The LED will likely be gone in a matter of milliseconds and you will have some crispy fried fingers. And they won't smell or taste like chicken.

So if we can't just stick a circuit board into an outlet and expect everything to be intact afterwards, what use is it?

Well, first of all there is a very good reason we use AC in our outlets and not DC. With AC there is a limit to how much current a rectifier can draw. If it were DC, a short in the wiring would cause worse results, faster.

At any rate, our first order of business for getting usable power is isolating the dangerous mains voltage.

Mains Isolation

We don't make circuits to operate directly off of the mains. If we do, it's likely that we'll get some more crispy fried fingers.

So we use a transformer. This also has the advantage that we can adjust the transformer to not give us more power than we need. For example, all I want is a 12V adapter so that I can run my shaver at home. Of course, the shaver should come with a wall adapter. But let's pretend that it doesn't.

You can imagine that for a 12V hand-held shaver, 120V might be a bit more than enough. It might be good for some 4th of July sparkler, but let's face it. You want to shave your face, not burn it off. So in order to step down the voltage so that we don't fry ourselves, we can use a 10:1 transformer.

In case you didn't know, 10:1 is the primary to secondary winding ratio. This means that if 120V is applied to the primary, the voltage coming from the secondary will be 12V.

We still don't have quite what we want, however. The output of that transformer is AC, not DC.

So to progress further in attaining that ever elusive usable power, we need to rectify our AC.

Rectifiers

Rectifiers are used to convert isolated AC into a DC output.

[Diagram of a rectifier]

To keep diode switching noise low it is recommended to bypass the AC end of the rectifier with a 10nF or 100nF cap (with proper voltage ratings), which will absorb the transients.

[simulation plot of output of rectifier (loaded)]

Filtering

Okay, the rectifier is a good step. However, the output is rather irregular. It keeps dipping when he input goes low. What we need here is a filter, and to be more specific, a reservoir cap.

A reservoir cap is a very large capacitor that helps a great deal to smoothen the output of the rectifier. The capacitor stores charge when the output of the rectifier is at normal voltages, but when the voltage drops it will release its charge and fill in the gaps.

[diagram and simulation of rectifier with reservoir cap (loaded)]

So now it's not too bad, perhaps enough for a shaver. But lets say we want to stretch it further and see just how smooth we can get it.

We need a voltage regulator. (note: a suitable power supply can be maid with just a rectifier and plenty of filter caps and calculation and it is done regularly on the forums. However, money can be saved with a more in-depth approach.)

Voltage Regulation

A voltage regulator takes an unsteady voltage and regulates it to a steady one.

There are three main types of regulation:

Pass Element Regulation (Or linear regulation)
Shunt Regulation
Switching-Mode Regulation