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 Post subject: Powersupply fundamentals and broken math ?Posted: Sat Jan 12, 2008 5:54 am

Joined: Sat Oct 11, 2003 6:01 am
Posts: 2
http://www.silentpcreview.com/article28-page2.html

Are the efficiency tables totally wrong or is my math broken.
THE EFFECTS OF EFFICIENCY table shows some weird figures

So if we have a efficiency of 80% and draw of 400 the actual loss in power is 400*0.2 = 80W or 400* 1.2 - 400 ..

in that table the figure is 100W

So how am I calculating wrong?

For example, a power supply that requires 100W AC input to produce 70W DC output has an efficiency of 70%. In this example, 30W is lost as heat within the PSU.

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 Post subject: Posted: Sat Jan 12, 2008 8:17 am

Joined: Thu Feb 10, 2005 11:20 am
Posts: 2198
Location: TN, USA
The correct math is (DC output / power from wall) = % efficiency but the table doesn't tell us power from the wall.

To find power from the wall we get (DC output / % efficiency) = Power from wall.

To find waste heat we have (Power from wall - DC output) = watts wasted.

Now if we use 100 W DC output / 80% efficiency it equals 125W. 125W - 100W = 25 watts wasted not the 20W you might have though of. The key is that you have to divide by efficiency in this case not multiply.

So the table in the article is most likely correct. We at least know that the 25W number in the intersection of 100W and 80% is correct.

Code:
THE EFFECTS OF EFFICIENCY
Efficiency        Power Lost as Heat
@100W output @200W output @400W output
85%          18W          35W          70W
80%          25W          50W         100W
75%          33W          67W         133W
70%          43W          86W         174W
65%          54W         108W         216W

If we use 400 W DC output / 65% efficiency it equals 615.38461538461538461538461538462W.

615W - 400W = 215W watts wasted, the table rounded up to 216W but maybe he used data for efficiency with more decimal places or maybe he just prefers to round up.

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