Bill's Recycled, Fanless, Silent Woodbox Computer

Do-It-Yourself Systems
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THE POWER SUPPLY

The power supply (PSU) is a 140W micro-ATX design, small but adequate to drive the motherboard, a network card, the hard-disk and CD-ROM.

I couldn't find any figures for the efficiency of the PSU so, I measured the current draw on each of the supply rails (5V, 12V, 3.3V etc) with a DC clamp meter. Multiplying the current drawn by the voltage of each of the rails will give the power used by each rail. I estimated the power used at idle to be 28.5W. Measuring the mains input gave a power input of 46W, this equates to an efficiency of about 62%.

With the CPU running at 100% the whole machine is drawing about 60W which means I have about 22W (60W x 38%) of heat to dissipate in the PSU. I have not been able to find out which bits of the power supply dissipate the most heat but, clearly, the semiconductor parts are the most sensitive to temperature so, it makes sense to ensure that these part are adequately cooled.

It would be possible to remove the major semiconductors (i.e. switching transistor, diodes and regulators) from the PSU circuit board and mount and rewire them on a remote heat sink but, given the modest dissipation and small size of my PSU, I removed the fan and added heat shunts (made from short lengths of 4mm thick aluminium extrusion) between the semiconductor devices to the two heatsinks mounted over the fan orifice. I punched a few extra holes in the PSU case just to let the air flow through freely. (Editor's Note: Bill could have used use a pair of needle nose pliers to twist the bars in the grill to 90 degrees to greatly decrease the obstruction.)


Modified PSU cooled passively with external heatsinks.

The PSU is mounted at 90 degrees to normal to allow the heatsinks to sit vertically and to clear space above the CPU heatsink for a clean air flow. This has the slight disadvantage that the power connector now protrudes from the top! The PSU runs quite warm rising to about 48°C at full power.


Note the extra holes drilled in the casing of the power supply to aid cooling.

WARNING! Readers should be aware that a power supply can have high DC voltages in its capacitors even after power is turned off. Heatsinks can also be "live" in some power supplies. Exercise great caution if you choose to open up any PC power supply.



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