Archived from groups: alt.comp.hardware.pc-homebuilt (
More info?)
Peter I. Hansen wrote:
> Hello
>
> Does a PC PSU require a fan to be connected?
It can be run safely for only a few minutes without the fan.
> If so, would it be ok to shortcircuit the connector? The reason
> is that I'm installing a thermostate which handles the fan speed
> and thus it's not connected to the PSU anymore.
Don't short the connector or you'll overload the +12V output (if the
fan runs at full speed all the time) or possibly destroy the transistor
in any fan speed control circuit (if the fan speed varies with
temperature). A thermostat is meant to be wired in series with the
device it controls, meaning you need to cut the fan's red wire in the
middle and connect its freed ends to the thermostat's terminals. The
thermostat must be mounted securely and away from the high voltage
section of the power supply to prevent any possibility of electric
shock. That means do NOT mount it on the large heatsink next to the
big cylindrical high voltage capacitors because that heatsink is
insulated from high voltage by only 0.2mm of material, or it may even
be connected directly to 160 volts. However the other large heatsink,
close to all the wires that exit the power supply, is a safe mounting
point, and if you use it the thermostat should be rated to close (not
open, as most do) at about 75-80 degrees Celcius. If you instead mount
the thermostat on the power supply case, use a much lower temperature
rating, like 35-45C.
The problem with controlling the fan with a mechanical thermostat is
that the fan will alternate between complete silence and maximum
loudness, so it may be better to use a thermistor rated for 30-50 ohms
at 25 Celcius.
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