What makes passive CPU heatsinks...passive?

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Lakitu

Limp Gawd
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Jun 8, 2004
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Just as the topic says, I've wondered why there are passive heatsinks, like why do they not need a fan, but other heatsinks do? Couldn't like any heatsink work as a passive one?

Summary: What's the difference between a regular and passive heatsink?
 
Usually passive heatsinks have a much greater surface area than heatsinks with fans. With more surface area, the heat can dissipate more easily.
 
Passive heatsinks are usually for quiet systems. They don't need a fan because theres enough heat dispersion already.
 
So..they are mainly for processors that produce so little heat that the airflow in the case can get rid of the heat without any active cooling mounted on the heatsink?
 
make sure you don't overclock with passive cooling though... well i've heard some passive cooelrs can handle it. i don't know.. i haven't been in the heatsink loop for about a year and a half...
 
Take a thermodynamics class and pay attention during the convection heat transfer part

or google it..it can be interesting
 
When Talking About Cpu, Chipset And Vga Cooling,passive Means No Fan On The Heatsink As Apposed To Active Cooling Which Has A Fan On The Heatsink.
 
Lakitu said:
So..they are mainly for processors that produce so little heat that the airflow in the case can get rid of the heat without any active cooling mounted on the heatsink?
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both my vid card and north bridge arnt known for being cool
RancidWAnnaRIot said:
make sure you don't overclock with passive cooling though... well i've heard some passive cooelrs can handle it. i don't know.. i haven't been in the heatsink loop for about a year and a half...
Click to expand...

passive north bridge and video card here... passive means you MUST have good case cooling... cant have any hot zones or your gonna fry stuff(bacon kind of fry)
passive doesnt mean hot.. my graphics card peaks at 60C... at 500 mhtz core... maximum voltage bios allows... thats a lot of heat and it still runs cooler than a 5900 does stock
north bridge is covered by cpu cooling(literaly...covered :p )
and yet my pc is nearly silent... if i shove my finger in the psu fan i can hear my seagate barricudas... (some people say they make no noise at all.. thats wrong :p )

the problem is finding a passive cpu heat sink... i dont know.. definetly not passively cooling prescotts... or A64 FX's
 
Lakitu said:
Just as the topic says, I've wondered why there are passive heatsinks, like why do they not need a fan, but other heatsinks do? Couldn't like any heatsink work as a passive one?

Summary: What's the difference between a regular and passive heatsink?
Click to expand...


Passive = it needs no fan to cool....
Passive also = A Fried Pentium Prescott processor :eek:
 
Vapor1000 said:
Passive = it needs no fan to cool....
Passive also = A Fried Pentium Prescott processor :eek:
Click to expand...
there serving those now? mmmm deep fried prescott
 
I know that passive has no fans and that it needs good cooling and whatever else you guys have said, I mean what is special about passive ones that a non-passive one can't do? Couldn't I use just any heatsink without a fan, instead of buying a special passive one?
 
You can use any heatsink as a passive heatsink by removing or disabling the fan, yes.

However try it and look at your temps. They won't be pretty. Its all about that heatsinks specific characteristics. All heatsinks however, whether meant to be actively cooled or not, are in the same field - none can 'do anything special,' unless you count heatpipes. If you put a quarter on your CPU die it will be a passive heatsink. If you put a fan on it too it'd be an active heatsink. However it wouldn't work well - why not? Because it has little mass and little surface area. But lets say you could theoretically push 150,000 CFM of room temperature air across the quarter. Maybe it would work now. Thats what I mean by compromise.

If you want passive cooling you are minimizing the air flow factor. You may be comfortable walking outside in 25 deg F weather with just jeans, boots and a hooded sweatshirt. However if there is considerable wind outside as well you will get cold fast. The temperature will feel a lot colder, but could be the same.

Thus a heatsink meant for passive cooling will have to maximize factors such as mass and surface area. If I could attach my CPU to a block of solid copper the size of my car weighing in the thousands of pounds, then I bet my temps would be lower than they ever are even if the air is mostly stagnant.

Main point: There is nothing 'special' about a passive heatsink. If you took an active heatsink meant for a Prescott and put it on a Pentium Mobile Dothan chip without the fan, it would work great as a passive cooler. There is nothing special about a heatsink that makes it work passively. Its all about your specific requirements.
 
Passive heatsinks do differ from normal heatsinks: the fin layout. Usually, passive heatsinks have a thick base and have thick fins that are spaced far apart. This allows maximum effeciency in transfering heat to a large amount of air without the heat being as dense. If there are too many fins in a small area, then it realistically wouldnt allow for any kind of natural convection.

If you look on the back of an amp for a subwoofer or something, you will notice it is usually a big chunk of extruded aluminum with big fins spaced far away. This works great passively.
 
As for CPUs that make lots of heat. Xeons, Prescotts, etc. can all, and are passively cooled in servers. 1U rackmount(1.75" high) cases use passive cooling, along with blower fans in the case.
 
They arent passive if they have blowers in the case, they are cooled indirectly from forced airflow. (the blower may be in the front of the case, but it is eventually channeled throught the heatsink.)

Look on the inside of a monitor or big screen TV to see real passive heatsinks.
 
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