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Switching Power Supplies A - Z, Second Edition
Switching Power Supplies A - Z, Second Edition, by Sanjaya Maniktala (Newnes), starting at $55.25
Home » Power
Why 99% of Power Supply Reviews Are Wrong
Author: Gabriel Torres 391,735 views
Type: Articles Last Updated: April 19, 2010
Page: 3 of 5
Using an Oscilloscope

Another flaw from most power supply reviews posted around the web is not using an oscilloscope to evaluate the noise and ripple levels at the power supply outputs. Funny thing is, this equipment is not expensive (you can buy a Stingray DS1M12 PC-based oscilloscope for around USD 220). Therefore we have only two explanations for why websites do not evaluate power supplies using an oscilloscope: either they don’t have a clue on how to use one and are not willing to take the time to learn or are cheap and don’t want to spend a dime on their own business. Or both.

In theory the outputs of a power supply should present a perfect line when seen on an oscilloscope screen, but when you zoom in, you can see some “imperfections” on this line, in the order of some milivolts. ATX12V specification is very clear on the amount of noise and ripple a power supply may present: 120 mV for the +12 V and -12 V outputs and 50 mV for the +5 V and +3.3 V outputs (all values are peak-to-peak). So power supplies must not surpass these values and in fact we usually want to see outputs presenting half of these values or less to consider a power supply “excellent.”

In Figure 1, you can see a power supply with a low noise and ripple levels, while in Figure 2 you can see a flawed power supply, presenting very high levels of noise and ripple. Even a layman can see that the power supply from Figure 2 must do some harm to your computer. And it does. Units that present noise and ripple levels above specifications overload components from your computer (especially electrolytic capacitors from the motherboard and video cards) and may lead your PC to present random errors (random reset, crash, Blue Screen of Death, etc).

Power supply with low noise and ripple levels
click to enlarge
Figure 1: Power supply with low noise and ripple levels.

Power supply with high noise and ripple levels
click to enlarge
Figure 2: Power supply with high noise and ripple levels.

The problem, of course, is there are several websites recommending power supplies that have huge noise and ripple levels without knowing – because they simply don’t test this. Here is an excellent example of what we are talking about: Thermaltake TR2 750 W presents an outrageous noise/ripple level as you can see here, but got a “Top Rank Award” from this website. And like this website, there are several others like this around, recommending flawed products that can damage your computer.

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