[clug] ATX system fan connectors

Michael Cohen michael.cohen at netspeed.com.au
Sun Apr 8 05:00:31 GMT 2007

On Sat, Apr 07, 2007 at 08:24:52PM -0700, Michael Still wrote:
> So, what is "too hot" for a CPU core? Oh, and what cable do I need to

That depends of lots of factors. The rated temp range is a good indicator but I
have run cpus hotter than that before. The hotter the core is, the more mobile
impurities become and the shorter the time before failure. If you have a cpu
running at operating range it might last 20 years, if you exceed it it might
only last 2-3 years. Given the average cpu is not really expceted to operate
more than a couple of years its a moot point anyway (in 2 years time the cpu
will be worth less than $50). I once ran a box without any fans at all (i took
out the ps fan too). It was gastly to have a fully blown machine which makes as
much noise on as if it was off... That worked of for about 1-2 years (it was on
all the time at which point the ps died).

The operating range for the cpu depends on the technology used to make it - the
smaller the feature sizes the narrower the range amongs other factors. But it
really varies from cpu to cpu. A while ago i was running a laptop in a cupboard
with basically no ventilation. ACPI said the core temp was around 70 deg
sometime 75 deg. Turned out it was a pentium M chip which is rated to 100 deg
so it was absolutely no probelm at all. However a desktop chip might have
different rating as well.

Heat dissipation vs noise is always a trade off - with a mythtv its important
to be quiet but most cpus are over speced for playback (if you are not
transcoding on the frontends). In that case underclocking the cpu is an
attractive option. Clocking back by 10-20% might reduce power usage
significantly. This will also reduce the power drained by the mobo and the ram
really cooling down the box. In that case you can take the cpu fan out
altogether for a super quiet experience.

For example you can buy an 800mhz P4M laptop which draws 17 Watts at full speed
- thats less than a fluouro light globe - even if you put that under a doonah
it would still not overheat. (most laptops draw about 20-30watt while idle and
maybe 50watt while running hard). In fact, purely from the heat pov laptops are
best for mythtvs imho.

Most bioses will shut down the box if the cpu gets too hot so dont worry about
stressing it a bit - thats what its for... My current mythtv has a manual pot
for fan speed - when it gets too hot it sends me an email and I might remember
to turn the knob up a bit - otherwise its no big deal.

> connect from the system fan connector on the motherboard to the fan? Is
> there a limit to the number of fans I can hang off that connector?

Usually for a myth frontend you want as few fans as possible - you cenrtainly
dont want a jet engine running while you're trying to watch a quiet movie.
There is a maximum current the fan control can deliver if you put too many fans
on it you might blow that part of the mobo. Check your mobo specs as to what
total current they can deliver.


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