On Mon, 2007-10-01 at 11:11 -0400, ed@xxxxxxxxxx wrote: > On Mon, 1 Oct 2007, Karl Larsen wrote: > > > > So if you buy a 450 Watt power supply to run a server that does nothing > > most of the time, the power supply will not draw 450 watts. It will draw > > perhaps 100 Watts. This is good. The power supply is lightly loaded and will > > last much longer than if it is drawing 400 watts. > > > > So do not worry about what the power supply is rated at. It will adjust to > > a smaller load. > > > > Everything you said was true, and it still does not help to have a lightly > loaded 450 watt power supply drawing 100 Watts... If your target is to > draw 30 watts from the wall. In fact you validated my point that matching > the power supply is better then over buying excess capacity. > > ed > > p.s. You're not the only one here with an EE degree. > These supplies are switching supplies, not linear. You should check some of the draws, I think you will be surprised about how little some of them draw (admittedly there are various design strengths out there.) In general, a linear supply is about 20-35% efficient, meaning that a 450 watt supply would draw about 90 watts all the time. But these new switchers are about double that or more, meaning that the supply will draw somewhere between 20 and 50 watts. Moreover that draw will basically be independent of the size of the ultimate supply, simply existing to control the switcher itself, and not the drive current. Technology exists in some forms of "buck control" that could boost the on line efficiency further reducing the drain. Power stuff is evolving even faster than much of the other technology, but is less glamorous, so doesn't get as much of the press. Regards, Les H
