SOLUTION: Keep fan running after shutdown

Yeah I saw this last week... pretty cool gadget... sorta like a turbo timer for a car... too bad pcmods almost always rips your off... don't think its worth $25 though. :Q

I didn't mean they rip you off by giving you crappy products... sorry... ... i meant their prices are a little too high... i was building a rheobus a looooong time ago and saw theirs... geez $45... so had to build my own and it cost me around $20... then i ordered a sheet of acrylic for $25... went to the hardware store the next day and saw the same thing for $7. My fault i guess for not looking around first... but that timer would look pretty trick though...

Here's one at a lower cost


Frankly though they are not needed. The cpu temp drops almost immediately after you shut down.

When the core isn't running, I am sure your average heatsink can draw away the heat from that small piece of silicon pretty easily.............. I wouldn't buy one.

Originally posted by: sumrtym
Without the fan running on your CPU, your system temps tend to shoot up after you shutdown, though by how much I couldn't say. That's only common sense....air blowing across hot heatsink, no air across the same hot heatsink.

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Hmmm....I don't think so. It's all about energy (heat) transfer. Heat from a CPU is the result of resistance to current flow when electricity flows through it. When you power down your system (key words POWER DOWN), there is no longer electricity flowing through the CPU, hence no new heat being generated. Since there is no further heat input when you shut down, he CPU can only get cooler, not hotter. Even without the fan running, the heatsink still acts like, well, a heatsink as it transfers heat from the CPU to the air (without a fan it relies on "free convection" rather than "forced convection"). While running, the CPU is always hotter than the heatsink otherwise heat would be transferred TO the CPU. After shutdown, the CPU will transfer heat (cool down) to the heatsink until they are roughly the same temperature (the CPU temp will be a tiny bit higher due to the thermal junction, but the difference is negligable in the grand scheme of things). Because of its low thermal mass relative to the heat sink, this will take a very short period of time. The heatsink and cpu will then remain at the same temperature as they cool down to room temperature. At no time does the CPU temp "shoot up" after shutdown - where does the energy to create the heat come from?

Umm, that's an interesting issue that I've never thought of. I think it's possible that CPU's temperature increases right after power-off. Heat generation and dissipation rates determine CPU's temperature. Right after power-off, energy generation stops but heat generation does not (though its "rate" decreases sharply), while the heat dissipation rate decreases because the fan stops. If the heat generated around power-off cannot go off as fast as before, CPU's temperature might increase. That's just my guessing, but I don't know if the temperature will actually 'shoot up'...

- mbuf

Fenris summed up what I was going to say. I don't see how your CPU would get hotter right after shutdown. No additional heat into the system.

Maybe I'm making an assumption here though. Is your CPU the hottest thing in your system? If the answer is no, maybe the heat that was being pushed out of the case with your case fans builds up a bit before adjusting to the lower heat dissipation of having no case fans on...

As soon as the system is shut down, heat generation stops. This means that the actual parts that generate heat (CPU die, etc) will start to cool.

When the fans stop, the parts that remove heat, e.g. heatsinks may well continue to heat up, heat from the CPU is transferred into the heatsink, and that heat is slow to be transferred to the air - but the CPU itself will not continue to heat up once the system is shut down.

however, when the case fans stop, heat may be trapped inside (and as heat is lost from the CPU etc.) the ambient air temperature may rise. This will cause cold parts of the system (e.g. floppy drive) to warm up after power is switched off - but these components will not reach the temperatures of the heat generating components.

I think the linked review at Holomaxx tells the best story. After shutting down they saw an immediate 6C drop in temperatures with or without this product. The first minute or two of running the fan drops it another 4C. There are actual numbers and decent graphs in that review. They did not see any sort of increase in temperature after shutdown.

The other review wasn't anywhere as good and was to sub-Powerhungry standards IMNSHO.

Originally posted by: mbuf
Heat generation and dissipation rates determine CPU's temperature. Right after power-off, energy generation stops but heat generation does not (though its "rate" decreases sharply), while the heat dissipation rate decreases because the fan stops. If the heat generated around power-off cannot go off as fast as before, CPU's temperature might increase. That's just my guessing, but I don't know if the temperature will actually 'shoot up'...

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Well, unless the laws of thermodynamics have changed radically since I got my engineering degree, what you're saying is contradictory. Heat generation and dissipation DO determine CPU temperature - it reaches a "steady state" where energy (lets only concern ourselves with heat here, not electricity) into the system (CPU and HSF) equals energy out. When energy in is greater than energy out, the temperature rises proportional to the energy difference and the thermal mass of the system. Vise-versa for energy out less than energy in. One thing to note - when electrical energy stops in the system, the production of heat stops. First law of thermodynamics - energy can neither be created or destroyed. It was being converted from electricity to heat. No electricity, no heat produced. The CPU will be hotter than the heatsink - the cpu converts the energy to heat and the heatsink transfers it to the air. When the computer turns off, the cpu is still the hottest component so it will transfer heat to the cooler heatsink until they are the same temperature (actually the cpu will be slightly hotter due to thermal resistance in the CPU/HSF interface) and will remain at the same temperature as they cool down to room temperature. The mass of the cpu (even if we include the heatspreader and/or ceramic/organic package) is still far less than the heatsink, especially if you have a big copper heatsink. The heatsink may slightly increase in temperature due to heat transfered from the cpu, but it won't go above the CPU temperature. Because of the loss of airflow due to the fan not being on, the RATE of heat transfer from the heatsink to the air will be less than before, but it will still greater than the zero heat generation of the CPU, therefore the system is no longer in steady state and is losing energy - cooling down. The air temperature may rise slightly in the case, but no more than a degree or two, even if there isn't good ventilation in the case. Free Convection (engineering term for "hot air rises") will move some air through a normal case, providing some cooling, along with normal radiative and conductive cooling. You've got a (maybe) 1oz processor and 30+ pounds of computer (and maybe a pound of air), it won't heat up a measurable amount from residual cpu heat.

If you want a more common analogy of what's happening, think of it this way. You have a cup of water in a pan on the stove. The stove is low enough that it's hot, but not boiling. The water will reach a steady state (lets ignore water loss due to evaporation for now) where the heat put in from the stove equals heat radiated out from the pan. Now, turn off the stove. Does the water suddenly boil? Not in my kitchen. Heat is going out of the system, not in. The stove and water is like the CPU. Now pour the cup of water in a gallon of room temperature water. It will warm it slightly, but nowhere near the original temperature of the water in the pan. This is like your HSF when everything is turned off. It will warm slightly from the CPU, but because it has so much more mass, the temperature increase won't be much and the overall temperature WILL be less than what the water in the pan was. Eventually it will go to room temperature.

Don't even get me started on the myth of "aluminum gets rid of heat better than copper"....

I'd like to trust the experiments at Holomaxx: the temperature goes down after power-off, they have numbers to back them up It's good to see them show some results.

One point I'm still not sure is, Will CPU still generate heat after power-off? Will the energy previously stored in the CPU's electronic components release (convert to) heat after power-off? If there is no such possibility, the temperature will absolutely drop. Otherwise, it'll depend on who transfers heat faster (heatsink or CPU's heat source).

- mbuf

Edit: it was too long, and typos

I have 2 temp sensors (hardcano II). One under the cpu placed in the center and the other between the fins of the heatsink. As soon as things turn off, the cpu temp starts to drop. Within seconds it's several c's lower. Within a minute or so the heatsink is warmer than the cpu. Clearly the heat is being transferred to the heatsink and the cpu is not getting any warmer. But never does the cpu temp ever rise after shutdown.


Those devices sound like a good idea, but I really don't believe they are necessary. But some still want something like that regardless.

Hhmmm... trying to recall the proper terms.... The thermal mass of the cpu is extremely small, although the heat output is large. That's why fan cooled heatsinks are required in the first place. The thermal mass of heatsinks is large, as is their ability to dissipate heat into the air. When the processor shuts off, that's instantaneous, but the fan will continue to spin for a little while. So the actual temp of the cpu will drop instantaneously when shut off.

It's an interesting gizmo, but I suspect it would do a better job if applied to a case fan. And I'm not sure that job is even desirable. Under extreme conditions, thermal shock can be a problem, over stressing the physical properties of electronic components- they'll crack..... slow heating/cooling cycles help maintain physical integrity.....

I'm glad I can reflect on this 23 years later.

Back when I was a teenager building my PC in a ThermalTake Mozart TX case, I was excited to use a rheobus and a Shutdown Power Protector. I thought it was genius to keep the fans running after shutdown, believing it would protect my overclocked system from overheating.

However, reading about thermodynamics now, I realize that setup was counterproductive.

But, what lead me here is that I'm currently building a fully custom PC case built to be cooled by an Air Conditionning Unit, which would be pushing cold air through a duct into one of the side panels, which would then be exhausted from the top panel.

The case would be divided into two compartments to guide the airflow through the GPU fans, and through the CPU AIO's radiator.

I've seen many videos of Youtubers trying this for fun, but I'm wondering if anyone has ever done this as a daily.

I'd also like to have the AC unit be managed just like any PWM fan by the motherboard Smart fan curves.

I have been trying this on my current PC case, an old HAF500, which has a partial mesh side panel and is quite appropriate for testing purposes.

I've been able to reach single digit temperatures in mid summer, but I was playing with luck, a wine hygrometer, and full negative airflow (all fans switched to exhaust only) to prevent humidity.

I'm wondering if the Power Shutdown Protector would be appropriate for safe shutdowns in this specific setup, either for the exhaust fan, both exhaust and intake, or only on the CPU AIO pump and fans, to prevent any humidity from building up inside the case after shutdowns.

Cheers.