[asterisk-users] Quiet 24 port POE gig switch

[Steve Underwood]

Gordon Henderson wrote:
> On Mon, 2 Feb 2009, Steve Underwood wrote:
>
>   
>> Bernd Felsche wrote:
>>     
>>> Ian Cowley <ianc at moffat.co.uk> 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