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Power Consumption

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raretech   12-11-2003 at 08:47:55 AM

Plase forgive the ignorance of this question. Up till recently, I had thought that power supplies "burned" current "as needed", meaning, if you have a 400 watt power supply, and the components it's powering only require 300, that's how much energy you'll use. Recently, someone told me that wasn't the case, claiming the rating was a like a light bulb, meaning if it's rated for 400 watts, that's how much power it burns all the time. That pretty much goes against what I thought I understood about electronics, but this guy seems like he would know better than I. Anyways, is this the case, that a 400 watt power supply always burns 400 watts? thanks!

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Crashman   12-12-2003 at 03:38:40 AM
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You're both wrong. But at least you're closer! Power supplies have an efficiency, usually at around 70% load it peaks. Loads below that are a little less efficient, and load above it are a lot less efficient.

Your 300W load will consume around 400W from the power grid using a 350W power supply, around 405 watts using a 400W power supply, around 410 watts using a 450W power supply, etc. These are very rough numbers since each power supply has different efficiency.

So you're closet, the power supply will always draw power proportionally to how much is output. The efficiency does change depending on load, but the load continues to be the major determining factor. That's because these power supplies use TRANSFORMERS, which only consume x amount of power to put out y amount of power.

So you're closer to correct. Your friend probably learned his ideas from another field, such as audio equipement, where a Class A amp varies input with output, but a Class B amp is "full volume" all the time (only varying the input signal using a pre-amp).

Your friend is 90% wrong, and you're 90% right, congrats.

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raretech   12-12-2003 at 07:35:16 AM
- 0 +

"You're both wrong."

Excellent, that will make it easier to tell him...

Thanks for taking the time to write that. Very clear explanation, makes perfect sense. I did think there was a little overhead involved, but I didn't think it'd be quite that much.

"Your friend is 90% wrong, and you're 90% right, congrats."

I'll keep that part to myself. :smile: I just wanted an answer, it was one of those stupid things that kept buggin me... Thanks again...

<i>8088 4.77 mhz
640k RAM
CGA, 12" monitor
20mb mfm harddrive
PC Speaker, DOS 2.0</i>

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upec   12-13-2003 at 01:54:09 PM
- 0 +

Actually most PSU runs most efficient when they are at 25-75% of peak load since they are not design to be running at peak load constantly.

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Crashman   12-13-2003 at 07:32:42 PM
- 0 +

You might be right. I was thinking of the exact load that came out with the exact highest efficiency for a certain power supply I saw tested, and trying to make a generalization based on it.

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