6.3. Why does my fan sometimes speed up and slow down? Should I replace it?
The fans have been a problem with the UNIX PC in a lot of
cases. What happens is the 12VDC fan is connected to a thermal switch
that will run the fan motor faster when the machine is hot, and then
slow it down when the machine cools. There have been occasions,
however, when a fan goes from fast to slow, and then slows all the way
to a dead stop. The fan tends to collect a lot of dust and dirt, and
sometimes when switching to the slow speed it goes so slow that it
stops and doesn't have enough "umph" to start up again.
Gil Kloepfer theorizes the reason for this happening is that
the 12VDC fan is constantly running with too little current due to a
resistor wired in series with the power supply when the thermal switch
is in its normally open position. When the fan is beginning to
malfunction, it will run subtly slower, but not enough to keep enough
air moving through the machine. The thermal switch kicks-up the
power, which causes the fan to move faster and cool the machine down
until the switch kicks-out again. This cycling effect keeps going
until the fan become excessively weak and clogged with dirt to where
it eventually stops working permanently. But he goes on to add that,
in any case, "I don't think it's just dirt."
The next stage is the machine overheats, the power supply cuts
out, and the machine powers down, then it powers up, and down, and up.
If you are lucky, you catch this before major damage occurs or you
take preventive measures beforehand. Most of the time the power
supply gets so hot, it warps the plastic case above it. This is a
sure sign that a UNIX PC has had a "meltdown" before.
What can you do? Well, you can replace the 12VDC fan with a
3" 120VAC fan. The 120VAC fans are more robust, they produce more air
current (~34CFM normally) and don't draw on the power supply any. One
drawback is that they are noisier. Radio Shack has an appropriate
120VAC fan for $16.95, cat. no. 273-242.
Some people can go years or forever without ever having a
problem, but it just takes that one day that you aren't around and the
machine goes into "meltdown" and then you'll be assured some damage
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