Second fan, power supply questions

[Howard E. Motteler]

Concerning adding fans in a 3b1:

Fan power is not straight off the 12v supply.  I have heard that it is
dropped through some thermal sensing circuitry.  Or it could just be a
resistor.  (I have the hardware manuals, but they don't include any
info on the power supply.)  With the stock fan, I measured (as I
recall) about 9-10 volts with an open (cool) box.

Due to the (sensor or resistor) drop, if you add more load, the
voltage to each fan will drop slighly.

The 67 meg miniscribe draws 2 amps at 12 volts (quite a bit...), and I
didn't want to add too much extra load.

What I did was to buy a PAIR of the radio shack 3" DC fans.  These
draw a bit less current, about 120 (each) milliamps as opposed to
about 200 for the stock fan.  I also put a ten ohm half watt resistor
in series with each, so both fans are running very quietly on about
8.5 volts.  Total air through the machine is about the same or
slightly increased, and noise is slightly less.

If you need more cooling you could probably leave out the extra
resistors, and one new fan in parallel with the stock fan should also
be ok, especially if the current power supply is the same as that
with the old 2 fan verions.  Extra cooling will allow a supply to put
out more current, up to a point.

As someone else mentioned, if you want really moby cooling, you could
replace the DC fan with a 3" AC fan, or even 2 AC fans (available from
radio shack or any electronics supply house).  This would make more
noise, but takes some load off the supply while providing more
cooling.

						Howard Motteler