Gordon Henderson wrote: > On Mon, 2 Feb 2009, Steve Underwood wrote: > > >> Bernd Felsche wrote: >> >>> Ian Cowley <[email protected]> wrote: >>> >>> >>> >>>> Beware PoE switches that can't handle Class 3 (15W) on all ports. >>>> Most have fans because 24 (or 48) x 15W is hot! >>>> >>>> >>> That's the power supplied .. which'd be at the far end of the wire. >>> >>> The efficiency of the PSU plays a big part in the heat dissipation. >>> The push to compact dimensions doesn't help ... a 400W or >>> thereabouts PSU with 24 independent outputs in 1U height? I suppose >>> if the switch were quite deep it could be workable and quiet. >>> >>> The problem isn't simply of being "fanless". But being quiet. >>> Preferably below 32 dBA at 1 metres for most offices. >>> >>> You can do that by using fans other than the tiny, whiney, 40mm fans >>> that vibrate at 6000 to 18,000 Hz. A couple of 80 or 120 mm muffin >>> fans at the back or front, pushing air in (hence the deep >>> dimensions), but the top and bottom would need recesses to allow >>> sufficient airflow when the positions above and below are filled. >>> >>> >> So, size does matter after all. :-) >> >> 24 x 15W => 360W. Its not that big a supply really, and spread across a >> 1U case its not that dense a supply. A 360W desktop PC supply can be >> pretty quiet, so its sad none of the 1U chassis supplies are. Probably >> if they used a large impeller fan they could get the noise down. I guess >> they assume these things will be in cupboards or data centres where >> nobody cares. This is a poor assumption. >> > > I think you might be missing what Bernd Felsche wrote - 24 * 15W is indeed > 360W, but the power supply will not be dissipating that - the phones at > the far-end will. A modern switched mode PSU ought to be more than 90% > efficient, so that means the PSU should only be dissipating 30 watts or > so. Easy enough to keep cool with little or no fans. Same for those PC > PSUs - the PSUs themselves really shouldn't be dissipating that much power > (as heat). I suspect some early PSU makers just put fans in "because". > I think you definitely are missing what I wrote. I said its a 360W power supply, which it is. Its dissipation should be comparable with a 360W PC supply, though the per port power control will add a bit to the total dissipation.
Very few supplies are >90% efficient, and they only hit their peak efficiency at some magic load settings. An efficiency of 70-80% is far more likely. Try looking at some specs for PC supplies. > Early ethernet switches did get hot - because of all the switching going > on in their chips, so it wouldn't surprise me if most of the heat coming > out of them was actuall the Ethernet part of it - esepcially at Gb levels > than the power convertors... > Most current Gig-E chips get hot when plugged into a Gig-E device, but run very cool when plugged into a 100M device. A lot Gig-E stuff actually overheats badly in such circumstances. If the sales of Gig-E switches rises, a lot of people are going to find their motherboards roasting. :-) Steve _______________________________________________ -- Bandwidth and Colocation Provided by http://www.api-digital.com -- asterisk-users mailing list To UNSUBSCRIBE or update options visit: http://lists.digium.com/mailman/listinfo/asterisk-users
