[asterisk-users] Quiet 24 port POE gig switch

[Gordon Henderson]

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".

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...

Gordon