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Oct 25, 2006 by Bill Todd (btodd at cis dot co dot uk)
An English electronic engineer who likes making things, Bill Todd made many modifications, and created a new wood case to take this modest old Pentium III system far along the road to silent nirvana. His ingenious journey involved recycling and creating of all kinds of parts including old electronics heatsinks, home-made damping gel packs, scraps of plywood, and even wheels from an old scooter. Two years after he first built this passively cooled system, it's still working silently away, even after a leak in a gel pack next to the hard drive.
- Mike Chin, Editor
The noise that most PCs make is very annoying and I have often wondered what it would take to make a truly quiet PC. When I found an old 1Ghz Pentium III motherboard (MSI 6315), a chassis, a spare HDD and an old micro ATX power supply, I thought I would try to make a truly silent PC.
The end result of my work on this P3 system.
It began life more like....
Given the relatively low power dissipation of the system, I decided to try and cool everything passively without any fans. This entailed a lot of customization and optimizing parts for convection cooling.
COOLING THE PROCESSOR
Normal Pentium CPU heat sinks tend to have closely spaced fins, which require a fan to force the air to move and remove the heat. The heatsinks used in the electronics industry, however, are usually rated for convection cooling (i.e. with the fins vertical in free air). This rating describes how hot the heatsink will get according to how much heat is dissipated by it, so the unit of thermal resistance is given in degrees per watt (°C/W). To calculate how big the heatsink needs to be, I would need to know the amount of power the 1GHz Pentium 3 uses (it's safe to assume that all the electrical power the CPU uses will be dissipated as heat) and the safe maximum temperature for the CPU. Checking the Intel website, it gave figures of 29.9W and a temperature of 69°C. If I assume an ambient temperature (inside the case) of 30°C, then I can calculate I need less than...
...which is the thermal resistance between the CPU and the air.
This has to include (in my case) the thermal resistance of the processor to heat-shunt junction, heat-shunt to heatsink junction and the conductivity of the copper heat-shunt itself. The thermal resistance of the junctions is almost impossible to calculate, since it depends on the area of contact and thickness and conductivity of the jointing compound, but if care is taken to mate or flatten the contacting surfaces it is not too difficult to reduce them to less than 0.1°C/W.
Because the size of heatsink would be much larger than the socket, I decided to use a 40mm diameter copper block to act both as a spacer, lifting the heatsink clear of the motherboard components, and as a heat-shunt to move the heat from the CPU chip into the heatsink. The thermal resistance of the block is calculated from the formula T ÷ (l × A) where T = thickness, A= area and l = the thermal resistance of copper (385 W/m K). Plugging these numbers in, we see that the copper block barely adds 0.005°C/W to the thermal resistance, so I will need a heatsink better than 1.3°C/W.
I asked few friends to look in their junk pile and before too long Bill Hall gave me a large 120x120x60 mm black anodized heatsink with a rating of about 1°C/W. (Cheers Bill)
Oversized open-fin anodized heatsink on the CPU.
The copper block was lapped flat on a surface plate with fine grade abrasive paper then polished to a mirror finish on one side. The other side of the block was lapped to the heatsink with fine grinding paste and then polish to mate the surfaces exactly. I then mounted the block to the CPU socket using a stainless steel spring like a normal Pentium heatsink and fan. The aluminium heatsink was then mounted with a couple of M5 screws, to the chassis in such a way as to just rest on the copper block without unduly stressing the CPU. The screws go between the chassis motherboard tray and the part of the large heatsink that overhangs beyond the top edge of the motherboard. A small bracket on the bottom of the heatsink keeps it in place when the PC is moved.
To work well, passive or fanless cooling needs a clean airflow path, clear of obstructions. To this end I cut two large square holes in the bottom of the chassis, and matching holes in the outer wooden case, then cleared room above the CPU, by moving the CD-ROM drive as far forward (out of the case) as possible and, to clear space behind it, mounted the power supply vertically. The result is a CPU temperature that remains below 60°C (35 above ambient) even after a prolonged thrashing.
View from above CPU heatsink area; it's open for convection.
View of heatsink from the underside of the case: Open again, for convection intake.
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