As an electrical device, the computer needs power in order for its components to operate properly. The device responsible for supplying power to the computer is the power supply. In a short way, we could say that the main function of the power supply is to convert alternating voltage (a.k.a. AC) which is supplied by the electrical power system into continuous voltage (a.k.a. DC). In other words, the power supply converts the conventional 110V or 220V alternating voltage into continuous voltage used by the PC electronic components, which are: +3,3V, +5V, +12V, -5V and -12V. The power supply is also present in the PC cooling process, facilitating the airflow inside the case.

There are two basic power supply designs: linear and switching.

Linear power supplies work by getting the 127 V or 220 V from the power grid and lowering it to a lower value (e.g. 12 V) using a transformer. This lower voltage is still AC. Then rectification is done by a set of diodes, transforming this AC voltage into pulsating voltage. The next step is filtering, which is done by an electrolytic capacitor, transforming this pulsating voltage into almost DC. The DC obtained after the capacitor oscillates a little bit (this oscillation is called ripple), so a voltage regulating stage is necessary, done by a zener diode or by a voltage regulator integrated circuit. After this stage the output is true DC voltage.

Although linear power supplies work very well for several low-power applications – cordless phones and video games consoles are two applications that come in mind –, when high power is needed, linear power supplies can be literally very big for the task.

The size of the transformer and the capacitance (and thus the size) of the electrolytic capacitor are inversely proportional to the frequency of the input AC voltage: the lower the AC voltage frequency, the bigger the size of those components and vice-versa. Since linear power supplies still use the 60 Hz (or 50 Hz, depending on the country) frequency from the power grid – which is a very low frequency –, the transformer and the capacitor are very big.

Building a linear power supply for the PC would be insane, since it would be very big and very heavy. The solution was to use the high-frequency switching approach.

On high-frequency switching power supplies, the input voltage has its frequency increased before going into the transformer (10-20 KHz are typical values). With input voltage frequency increased, the transformer and the electrolytic capacitor can be very small. This is the kind of power supply used on the PC and several other electronic equipments, like VCRs. Keep in mind that “switching” is a short for “high-frequency switching”, having nothing to do whether the power supply has an on/off switch or not…

If you want to learn more about the PC power supply internals, please read our Anatomy of Switching Power Supplies tutorial.

The power supply is probably the most neglected component on PC. Many times, when buying a computer, we just take on account the processor clock, the motherboard model, the quantity of installed memory, the hard disk storage capacity, and we forget about the power supply, which, in fact, is the one who supplies the "fuel" for the PC parts to operate properly. A power supply of good quality and with enough capacity can increase the durability of your equipment. Just to have an idea, a high-quality power supply will cost less than 5% of the PC total amount. On the other hand, a low-quality power supply can cause several intermittent problems, which are mostly difficult to be solved. A defective or bad-intentioned power supply can lock the PC, can result in hard disk bad blocks, can result in the infamous "blue screen of death" errors and random resets and freezings, added to many others problems.

On this tutorial we will talk about the aspects which have to be taken into account when buying a power supply. You will notice that not always it is ideal to buy a power suplly with “more watts”.