Power Supplies Demystified

Section 1: Why you should care about your power supply unit (PSU)

The power supply unit (PSU) is the engine on which your entire computer system is based. It is responsible for taking the high voltage mains power from your wall outlet and transforming it into the low voltage DC that your computer requires. Without electricity, your computer would be useless, but the importance of quality power supplies are often overlooked by the end user.

The reason for this is simple:

When a power supply is doing its job then the user should never have to know it is there.

Unlike RAM, storage space, CPU cycles, etc, a power supply should operate transparently from the time it is installed until the time the computer becomes obsolete. Unfortunately because of this transparent nature many users do not put much thought into their PSU, and thus problems ranging from intermittent errors to catastrophic cascading hardware failures resulting from inadequate or mediocre PSUs occur, and are often misdiagnosed.

Remember – if your PSU is causing your problems, then swapping out other components during troubleshooting will not only be a waste of time, but you may end up with an even larger stack of dead parts as they get damaged by the faulty power unit.

Section 2: Common Misunderstandings Regarding Computer Power Supplies

  1. Advertised wattage ratings are a good indicator of PSU capability and/or quality
  2. If a power supply is faulty or underpowered then the system will simply not power on and/or the problem will be obvious
  3. Power supplies with identical advertised specifications are equally good

Regarding misunderstanding #1:

In order to understand why wattage is a poor indicator of actual PSU capability one must first learn a very important electrical rule called “Joule’s Law”. Don’t worry, it is easy.

Power (watts) = Voltage * Amps

This means that wattage, which is the overall power, is directly proportional to voltage and current (amps).

Since there are many interdependent components inside of computers which rely on different voltages to run, and require different amounts of power, then the power supply must be able to not only convert power from the wall outlet but also split this power into different voltage ‘rails’ of different power capability.

Let us create a make-believe and simplistic (ideal) power supply. This supply has a 12V rail capable of 10A, a 5V rail capable of 5A, and a 3.3V rail capable of 1A. We will ignore negative rails for simplicity. This gives us a total power capability of:

P(watts) = (12V 10A) + (5V 5A) + (3.3V * 1A) P = 148.3watts

However, this does not mean that all 148.3W are available to all components. For instance, only 120W total is available on the 12V rail, and even if the other rails are barely utilized all of the 12V devices are limited to 120Watts total regardless of the total power capability of the unit.

Another key factor making wattage ratings misleading is that power output can be highly variable outside of ideal conditions. Due to poor component choices, poor quality, and poor design, many power supplies become increasingly unable to output their ‘rated’ power once temperatures and/or loads start to rise. Since increasing temperature results in lower ability to supply stable power to components, as well as increasing power requirements of the components themselves, it is easy to see how a ‘high wattage’ but poor quality PSU would result in instability and possibly hardware failures when a user attempts to run the supply at its rated power.

Regarding misunderstanding #2:

Power supply problems often lead to misdiagnosis during troubleshooting because they manifest themselves as errors in other components.

Regarding misunderstanding #3:

Build quality, internal component quality, and design are the most important aspects of a good power supply. It is much more important to get a reliable power supply than to get a bigger power supply.

Most power supply brands are simply repackaged units that have been sourced from a large manufacturer and cosmetically altered. Thus, a different model PSU from the same brand may be made by a completely different manufacturer and be of entirely different quality than others by the same brand. A google search for your make and model PSU and the letters ‘OEM’ will often reveal which manufacturer made the unit if it is a rebranded unit.

Section 3: I have a dodgy/suspicious PSU in my system. Should I replace it if everything is currently working fine?

That depends. Answer the following questions:

  • If the computer becomes unreliable will it have a significant negative impact on your life?
  • Is the hardware in the computer non-expendable?
  • Do you feel that a relatively small investment now is worth it to help ensure trouble free operation of your system on a hardware level?

If you answered YES to any of the questions above, you should seriously consider pro-actively replacing your power supply.

Section 4: What are good desktop PSU models?

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