...
I did not read your whole response before replying the first time. I've got
a variety of 60 and 120 mm case fans sitting in a drawer, and I've probably
got an old AC-DC transformer somewhere also. I was thinking I could do it
with stuff I've already got. If I can't find an old transformer, a new
transformer would still be cheaper than that 3" 120V fan.
We can't see the cabinet so can only assume your holes and
fan position are complimentary.
If the gigabit switch is all that is in this cabinet, and it
isn't some old monster sized thing that uses a lot of power,
a reasonably small fan at low RPM should be sufficient, no
need to create more noise, dust buildup and fan wear going
overboard (unless you realistically plan on adding more
significant heat generating gear to this cabinet.
Tiny fans, and thinner ones, tend to be balanced worse and
may have poor bearings (relatively speaking). An 80mm x
25mm might be ideal, ran at lowest RPM that will allow the
fan to spin up reliably, unless the ambient environment
outside the cabinet is also hot, then a bit higher airflow
might be useful.
You could use the Radio Shack AC output wart and add your
own bridge rectifier and capacitor to make it DC output for
the fan, but why? Just get an AC-DC wart instead if you
didn't have one suitable.
Keep in mind that a "12V, 500mA", AC, or AC-DC wart that is
unregulated as many are, will float above the labeled
voltage with no load on it... by roughly
(voltage * 1.41) - 1.3V = unloaded voltage
The 1.3V takes into account the voltage drop from a couple
of diodes forming the bridge rectifier if you were
wondering. Point is, if you chose an AC wart then rectified
and smoothed it with diodes and capacitors, or bough a wart
with these parts already inside (thus the AC-DC type), since
the fan is using less than 500mA, the resultant voltage the
fan would see will still be floating higher than 12V.
Do you see what I'm getting at? A 12V 500mA DC output wart
would supply, perhaps closer to 14V to a typical low RPM
fan. You could use a method of fan speed controller, to
throttle it back down like one of the more elaborate PWM
controllers (but you won't need constant readjustment most
likely and it is a large increase in cost for the project)
or more simple method like an inline series resistor or IC
regulator chip like an LM7809 (for 9V, so the lower voltage
results in lower speed operation).
IMO, the better alternative is to just pick a lower voltage
AC-DC wart, and ideally one with a fair amount of current
margin so it's not running hot long term. For example with
the typically spec'd 12V, 0.15A computer case fans you might
seek a 5V, 200mA or higher current wart. The fan might
consume some small amount less than 100mA, might still run
a little faster/louder than required for the job.
If you want to use only parts you already have (since that
Radio Shack wart is expensive for what it is, as are all
Radio Shack warts, you can buy a basic wart online and have
it cheaper even with shipping charges added on)
For example,
http://www.bgmicro.com/index.asp?PageAction=VIEWPROD&ProdID=11223
though being 7.5V, the fan might spin faster than needed.
The following is quite overkill but still a great value and
might be most versatile,
http://www.goldmine-elec-products.com/prodinfo.asp?number=G8606
but really you probably having something suitable, a wart
from an old cordless phone or some other consumer
electronics device. If the voltage rating is much higher
than 8-9V (keep current rating at 200mA or more) then you
probably want a controller added, I would use a series
resistor in the 1-240 Ohm, 2W range (depending on what wart
voltage @ fan load was), making assumptions that your fans
are typical brushless DC as found in a computer case.
Most AC fans are pretty high speed, I'd think them the worst
alternative considering that, the price, and having to run
the AC into the cabinet which is a slight addt'l risk and
the cord should be routed with safety in mind. IMO, avoid
running AC into a cabinet unless you are going to do it to
electricial code, for reasons other than powering the fan
(to have a multioutlet strip inside for powering the other
gear using AC power).
