Last updated 06 March 2008
1) Cooling the processor (08/05/2004)
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 heat-sinks used in the electronics industry are usually rated for convection cooling (i.e. with the fins vertical in free air). This rating describes how hot the heat-sink 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 heat-sink 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
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 heat-
(69°C - 30°C) / 29.9 = 1.3°C/W
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 heat-sink 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 of the size of heat-sink, I have used a 40mm diameter copper block which acts both as a spacer, lifting the heat-sink clear of the motherboard components, and as a heat-shunt to move the heat from the CPU chip into the heat-sink. The thermal resistance of the block is calculated from the formula T/ lA 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 heat-sink of about 1.1°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 anodised with a rating of about 1°C/W. (Cheers Bill)
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 heat-sink 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 heat-sink and fan. The aluminium heat-sink 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. A small bracket on the bottom of the heat-sink keeps it in place when the PC is moved.
To work well, passive or fan-less cooling needs a clean airflow, clear of obstructions . To this end I have 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.
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 heat-
To work well, passive or fan-
