Can the 3b1 power supply handle 2 fans

Floyd Davidson floyd at
Fri Mar 22 16:05:43 AEST 1991

In article <6744 at bone13.UUCP> murphyn at motcid.UUCP (Neal P. Murphy) writes:
>renkel at motcid.UUCP (Will Renkel) writes:
>>the 7300 had the same powere supply setup and 2 fans.  In fact, dont know why
>>the second fan was rfemoved from the 3b1.
>The second fan was removed because it didn't add to the cooling. In fact, it
>would appear that the machine runs cooler with just the one fan pulling air across
>the power supply.
>The 3B1 has a power supply that yields an extra 20-50W of power - needed for the
>bigger disk drives, I imagine.

The second fan, if it blows outward, causes a vacuum in the center
area of the motherboard.

The way around that is to put the fan in the way fans should be,
blowing inward so the air can be filtered.  Unfortunately the
fan by the power supply will spread the heat all around the
machine if it is reversed to blow inward.

Replacing the fans with AC units takes the load off the DC
supply, which is nice, but they are always at full speed and
thus make more noise.  An AC fan will be more reliable.  If the
machine starts up cool enough that the DC fan is on low speed,
and the fan is old and the bearings worn, the fan may not start.
The fan itself then heats up and becomes impossible to start,
even when it gets full voltage for high speed (which it will as
soon as the power supply overheats).  The power supply overheats
and shuts down, and recycles itself.  Sometimes the case above
the power supply melts...  Some times the power supply melts...

What I've had for three years is two AC fans, with the second
one blowing inward and with a filter on it.  I put the second
fan in before the DC fan went bad, which it eventually did.
No problem...  It was then replaced with an AC fan.  My 3b1
sounds like a 747 but I don't worry about a meltdown either.

Floyd L. Davidson  |  floyd at   |  Alascom, Inc. pays me
Salcha, AK 99714   |    Univ. of Alaska      |  but not for opinions.

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