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Switching Power Supplies A - Z, Second Edition
Switching Power Supplies A - Z, Second Edition, by Sanjaya Maniktala (Newnes), starting at $54.35
Home » Power
How Much Power Can a Generic 500 W Power Supply Really Deliver?
Author: Gabriel Torres
Type: Reviews Last Updated: March 15, 2008
Page: 1 of 6
Introduction

We decided to take a generic 500 W power supply and test it using the same methodology we use for “branded” units. The idea behind this review is to be as much educational as possible and answer a few very important questions: how much power a generic power supply can really deliver? What are the differences between a generic power supply and a “branded” unit? Is there any danger for my equipment to use a generic power supply? Why generic units cost so little? Read on and see our findings.

We call “generic” any cheap very low-end product that we can’t find out who the manufacturer is – normally because the manufacturer doesn’t want to be found! In the case of power supplies, generic units cost just a few dollars and usually come for free on low-end cases. No attention to the product finishing is given and they follow the traditional ATX layout, with an 80 mm fan on the rear and some venting holes on the front of the unit. They are smaller and far lighter than “branded” power supplies. If fact back in the days that good power supplies were hard to find one trick several technicians used to select a power supply among several was by selecting the heaviest one. They also have an AC outlet on the rear for you to connect your video monitor and this feature isn’t seen anymore on “branded” units.

Generic 500 W Power Supply
click to enlarge
Figure 1: Our generic 500 W power supply.

Generic 500 W Power Supply
click to enlarge
Figure 2: Our generic 500 W power supply.

Here we can see the first important difference between a good power supply and a generic one: cooling. Even when a good power supply uses just one 80 mm fan on the rear it has far more venting holes on the front (with several units completely replacing the front panel with a mesh), which improves airflow and prevents your computer from overheating. The power supply is a key element in the computer heat dissipation, as it is in charge of pulling hot air from inside the computer (through these holes) to the outside, through the power supply fan. In Figure 3 we illustrate this.

PC airflow and heat dissipation
click to enlarge
Figure 3: Airflow and heat dissipation on a typical PC.

Of course to cut costs generic power supplies don’t have PFC circuit (read more about PFC on our Power Supply Tutorial) and also we assume that they have a low efficiency, below 70%, but of course we will measure efficiency during our tests with this power supply. The higher the efficiency the better – an 80% efficiency means that 80% of the power pulled from the power grid will be converted in power on the power supply outputs and only 20% will be wasted. This translates into less consumption from the power grid (as less power needs to be pulled in order to generate the same amount of power on its outputs), meaning lower electricity bills.

Another main external difference between a good power supply and a generic one are the cables used. Generic power supplies use wires thinner than necessary, usually 20 AWG. The minimum required for today’s standards is 18 AWG. Also generic power supplies come with fewer cables than “branded” units. This generic power supply we bought, for example, had only two cables, one with two standard peripheral power connectors and another also with two standard peripheral power connectors plus one floppy disk drive power connector, plus the main motherboard cable and an ATX12V cable – no SATA power connectors (even though there are generic units with these connectors around) and no auxiliary power connector for the video card.

Power distribution is also a very important difference between generic units and “branded” ones. Generic units are usually based on the very first ATX specification, which was written at a time when computer power consumption was concentrated on the +5 V line. Nowadays power consumption is concentrated on the +12 V outputs, as the CPU (through ATX12V and EPS12V connectors) and video cards are connected to the +12 V output, not +5 V. So usually generic units have a higher current limit on the +5 V line while current good “branded” power supplies have a higher current limit on the +12 V outputs. There is also a major difference on how the +3.3 V output is obtained, but we will discuss this later in details.

But the most traditional “feature” of generic power supplies is that they can deliver far less power from what is printed on their label. Our power supply was labeled as a 500 W unit and one of the goals of this review is to check what the real wattage of this unit is.

There are several ways power supply manufacturers can use to label their power supplies:

  • Label the power supply with peak wattage, which can only be achieved during some seconds and, in some cases, in less than one second.
  • Measure the power supply maximum wattage with an unrealistic room temperature, normally 25° C (77° F), while the temperature inside the PC will always be higher than that – at least 35° C (95° F). Semiconductors have a physical effect calling de-rating where they lose their ability to deliver current (and thus power) with temperature. So a maximum power measured at a lower temperature may not be achieved when temperature is increased.
  • Simply lying, this is probably the case with generic units.

Now that you know what are the external differences between a generic power supply and a “branded” one, let’s see the differences inside.

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