[MLB-WIRELESS] Access Point in a box

[Donovan Baarda]

On Fri, Nov 29, 2002 at 01:54:31PM +1100, lkhoo at csc.com.au wrote:
> Michael said:
> 
> >Hi there,
> >I am toying with the idea of moving my newly  acquired DWL900AP+ into a
> nice fibreglass electrical enclosure and putting it up  my TV antenna mast.
> My main concern is heat.? I can imagine that the  inside of the box >can
> get quite hot even on a slightly sunny day.? How do  you all find this?
> (those that have this type of arrangement). The last thing I  want is my AP
> dying from heat exhaustion!!
> >i had two ideas to combat the problem,?first  is to pump cool air from
> inside the house, up?20mm flexible conduit,  circulate it in the box then
> out through a vent in the bottom.
> >Second, and much more technical, is to use a  Peltier semiconductor heat
> pump to basically turn the box into a mini  fridge.? This would require a
> separate power cable as they draw more  current than could be >supplied via
> PoE.
> >Any thoughts??
> >Michael.
> >VK5ZEA.
> 
> How about some insulation inside the box? Possibly use two boxes with a
> "sandwitch" of insulation inbetween the two boxes(packingfoam/peanuts, roof
> ceiling batt?).

The problem with insulation is it...insulates. This means it prevents heat
getting _out_, as well as in.

The AP will produce heat. If the box is perfectly insulated (not
possible...), then the heat can't get out, and the temperature will rise
until the AP melts itself enough to stop drawing current.

Two things you need to do; prevent heat getting in, and let heat get out.

To prevent heat getting in; 

1) try to place the box in the shade. 

2) Use a silver metalic box instead of a black one, or paint it silver or
white.

To get heat out; 

1) ventilate it so the hot air can get out... requiring that you let cool
air in to displace the hot air. This risks getting rain etc in the box.

2) conduct the heat out by using a metalic box and put heatsinks on it,
making sure the heatsinks are in the shade. Black heatsinks radiate heat a
bit better, but can act like a solar-array in the sun, so silver heatsinks
are probably better.

3) pump the heat out, using some sort of heat transfer thingy. This can be
as complex as something like the liquid cooling setups people are starting
to use on PC's, or a peltier. Note that peltier's are very neat in that they
are extremely compact and easy to use. 

You had concerns about the current consumption of a peltier; a peltier will
"pump" as much heat as the current you give it allows. This means it will
just pump less heat if you don't give it enough current. I doubt you need to
freeze the AP, so a under-driven peltier could be more than enough.

Peltiers are very thin. The tricky part is making sure the cold side is
sucking heat from your AP, and the hot side is radiating heat out. You can
do this by mounting the peltier on a metal box, mounting a heatsink on the
peltier, then insulating the rest of the box (you don't want the peltier
just cooling the air outside the box). If you are worried that air inside
the box is not transfering enough heat to the peltier through the box, you
can mount another heatsink inside the box to "collect' the heat inside.

I suspect a silver metal box with big silver heatsink(s) might be enough. If
not, you can add an underdriven peltier between the box and heatsink, and
insulate the rest of the box.

Ventilation is scary because it lets the water in, but it is amazing how
effective ventilation can be at cooling. 

-- 
----------------------------------------------------------------------
ABO: finger abo at minkirri.apana.org.au for more info, including pgp key
----------------------------------------------------------------------

To unsubscribe: send mail to majordomo at wireless.org.au
with "unsubscribe melbwireless" in the body of the message