All electronic circuits require some source of power in order to operate. For a digital watch or cell phone, that power comes from a battery. But for devices that connect to a wall outlet, some means of converting the 115 volts down to a level that the device can actually use must be provided. A power supply is the obvious solution, and it does a variety of other things in the process.

    Though a transformer can adjust the voltage, most circuits will operate on DC only. So another conversion must also be performed. This is where the diode comes in, though in most power supplies at least two diodes are used. If only one diode is placed into an AC circuit, then only half of the available electricity will be used. Remember, half the time the electricity is going in one direction, and the other half it is reversed. So if a single diode is used the current passes only when the AC happens to be going in the favored direction, wasting the other half of the current flow.

    There are many ways to construct a power supply, some using one diode, some using two, and others using four or more. I don't see a lot of value in belaboring all the possibilities, and I hope you'll understand if we move along to other, more important aspects of power supplies. For example, regulation and filtering.

Once an AC voltage has been converted to its DC counterpart, the job is still not quite complete. Unless additional steps are taken, the voltage will still fluctuate. even though the direction is no longer changing. The original AC voltage is constantly varying in level gradually reaching a maximum, then progressing towards zero, and again approaching a maximum in the other direction. Therefore, even though the diode can block the voltage when it goes the wrong way, the level is still changing.

Earlier I mentioned that besides its frequency capabilities a capacitor can also be used to store electricity, much like a rechargeable battery. It is precisely this property that will help us out here. How much current can be stored, and for how long, depends on the size of the capacitor. But once an adequate value has been chosen, this capacitor will continue to supply voltage when the incoming level drops below its maximum. A power supply that uses capacitors this way is said to be filtered, though the purity of the DC that results depends on several factors.

    The most important consideration is how much current is being drawn by the powered device. That is, if the capacitor needs to provide only a small amount of current, then it won't be depleted very much between cycles. Similarly, when a higher frequency of AC input voltage is used, the capacitor won't have to provide the current for as long. But no matter how large the capacitor is, some amount of fluctuation will still occur. For circuits such as a PC that require a very stable voltage, the only real solution is to employ regulation.

    A voltage regulator is a fairly simple circuit, though again, I won’t go into great detail since it's just the principles that matter here. However, the general idea of a regulated supply is that it's capable of putting out slightly more power than is actually required, and uses some type of sensing circuit to constantly monitor its own output. When the powered circuit demands more juice, the sensor adjusts a built-in electronic valve to turn it up. Likewise, if the AC input varies -- for example your air conditioner just kicked in -- that can be compensated for as well.

    So far we've assumed that a power transformer will be used to drop the original 115 volts down to something more manageable for the rest of the supply. But there's a problem with most transformers: they're big and heavy. Worse, the lower the frequency to be transformed, the more iron that is needed for the transformer to work efficiently. This is where switching supplies come in, and at last we're getting to something digital!

    Since a computer's size and weight are sometimes as important as its speed, savvy designers will do everything possible to keep those to a minimum. Further, since electronic components are less expensive than copper and iron, a power supply that is twice as complex may still be cheaper to build if it can use a smaller transformer. With that in mind, let's look at how a switching power supply operates.

    Most switching supplies begin with an extremely crude normal power supply. No regulation, minimal filtering, not even a transformer -- it plugs right into the wall! Then, a circuit called an oscillator is connected to this "cheap" supply to create a new AC voltage, though at a much higher frequency. (The oscillators you may have encountered probably had a big dial, and produced a sweeping audio tone. The same idea applies here too, although the frequency is not adjustable.)

    In this case the oscillator is used to rapidly switch the crude supply on and off, thereby simulating an AC voltage. But the whole point is that since the AC is artificially created, we can make it any frequency we'd like. And if we choose a high enough frequency, only a very small amount of iron is needed in the transformer that is ultimately required.

Another important reason many PC designers favor a switching power supply is because it also provides an improved means of regulation. As we saw earlier, all power supplies have the capability to put out slightly more power than is actually needed, with the slack being discarded by the circuits that sense the output voltage. However, an unfortunate side effect of throwing away the extra current is that it is always dissipated as heat, which also wastes power. But a switching supply can do this more intelligently, by varying the duty cycle of the AC voltage it creates.

    Since creating AC from DC in a switching supply is simply a matter of turning the original power rapidly on and off, the relative times may be easily varied. That is, if the output voltage is sensed as being too high, then the "off" time can be made slightly longer. And when it falls too low, the "on" time is adjusted instead. The duty cycle of a power supply is always expressed as a percentage, with 50% meaning on for exactly half the time.