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Inside a Power Supply

written by: Daniel Barros•edited by: Lamar Stonecypher•updated: 5/30/2009

Our coverage of Power Supply Units takes you now beyond just which one to buy - into the darkness and to the inner-workings of the PSU. Curious? There's much more information inside.

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    A Tale of Two Currents

    The computer power supply is perhaps one of the least understood and most mystifying elements to the traditional PC builder. What’s inside the box? How does it supply power to all those components? Let’s take a look at how exactly a PC power supply works by going inside.

    The Power Supply Unit (often termed just Power Supply or PSU) of a computer is the source of all its power. If we think of the CPU as analogous to the human brain, the PSU is comparable to the human heart. Rather than pumping blood however, this supply pumps electrical current.

    A long time ago, not in a galaxy far, far away, but rather here in the United States, there were two geniuses vying for the public’s imagination when it came to electricity.

    In one corner, we have the father of modern electricity and the lightbulb (along with many other inventions), Thomas Edison. Edison, aside from being an electrical whiz with his creations at Menlo Park, was also a huge proponent of DC or Direct Current power. In a DC system, low voltages are used and the waveforms of the electrical current are all in the same direction. Without getting overly technical, this one-direction current flow is what’s responsible for the power inside the computer you’re reading this on. Without DC current, low-voltage electronics wouldn’t have ever made it into homes today.

    In the other corner, we have the eccentric (and often misunderstood) genius, Nikola Tesla. Tesla is often portrayed in movies and books as being a sort of mad genius, when really; he was more of a genius with some nervous tics and paranoia. His idea for electricity revolved around Alternating Current, or AC power. In an AC system, electricity can travel enormous distances without losing voltage – this was the necessary breakthrough for the modern electrical grid.

    Why am I telling you this tale of AC vs DC? Your home today gets AC power piped in through the electrical outlets on your walls. However, your PC can only use DC power, so you need to change AC to DC, which involves taking the 120V (or more if you’re in Europe) outlet’s voltage and changing it to much smaller voltages for inside the PC.

    Inside your power supplier are mainly two types of technology, the first and most prominent being an army of rectifiers. These rectifiers are what convert AC to DC power by canceling out the negative portion of the waveform in the AC current. Along with a rectifier, there are also transformers inside that are capable of varying the voltage for multiple uses.

    Apart from the electrical current switches and generators, there is also a considerable amount of connectors running internally through the power supply. The PSU not only changes the voltage of the incoming AC current while converting it to DC, but then it also splits this current amongst several different outgoing sources. This is so that you can have power to both your SATA HDD and your IDE CD Drive without having to have two separate power supplies.

    One final point to consider is what the power supply uses as its “Power Rating”. Rather than giving you how much electrical current and voltage it can use at a given time, it’s much more convenient for the manufacturer to write the number in Watts, or the amount of work that the PSU is capable of doing. This makes for a problematic situation when buying a PSU because you need to be aware that the manufacturer will lie in order to get you to buy their PSU. They’ll advertise a PEAK rating of 600 Watts only to have you discover that the true rating is somewhere in the 300 Watt area. How can this be? The manufacturer has a choice of PEAK or CONTINUOUS wattage, and continuous is the only one that will give you a real-world idea of what the PSU is actually capable of.

    Before we part ways, let me just give you a small disclaimer. While I’ve told you what’s inside a PSU, unless you’re an electrician (or know an electrician), don’t go opening up the PSU to play around with its innards or try and fix it when it breaks. Sensitive electrical equipment has a big chance of breaking things apart from the equipment itself.