(This document is part of the PC-Clone Unix Hardware Buyer's Guide. The Guide is maintained by Eric S. Raymond ; please email comments and corrections to him.)

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Power Supplies and Fans

Power supplies can matter but quality is cheap; give preference to those with a Underwriter's Laboratories rating. There's some controversy over optimum wattage level; on the one hand, you want enough wattage for expansion. On the other, big supplies are noisier, and if you draw too little current for the rating the delivered voltage can become unstable. And the expected wattage load from peripherals is dropping steadily. The big old 300-watt supplies that were designed for running several full-height 5.25" floppies and hard disks are overkill in these days of portable-ready lightweight 3.5" drives. 200 watts is good enough these days, and the new breed of compact 200W supplies is quieter to boot.

About that annoying fan noise, ask if the power-supply fan on a target system has a variable speed motor with thermostatic control --- this will cut down on noise tremendously. If not: I have seen a rave about, but haven't used, a thermostatic fan controller called "The Silencer". This tiny device mounts inside your power supply and connects to the fan's power leads. It automatically varies the fan motor speed to hold a 79 to 82F temperature. The basic model is: $49.95 + $4.95 (S&H). For details, write:

    Quiet Technology Inc.
    500 Executive Center - Suite 3C
    P.O. Box 18216
    West Palm Beach, FL 33416
    407-683-6200
    1-800-SILENCE

Warning: installing this may void your warranty!

Also, be aware that a thermostatic sensor basically measures the temperature at the sensor (typically within the power supply box) and makes sure there is enough airflow to keep the power supply from overheating. However, the sensor does not know a thing about the temperature in certain hot spots likely to develop in a PC case (CPU, between SIMMs, between drives mounted in vertically adjacent bays).

This can be a problem, because in garden variety tower cases there often isn't enough airflow to cool all components effectively even if a single fan is going at full speed. This is especially true if your computer has lots of add-on cards or hard disks (not much airflow between cards or between drives). Note that the fan in the power supply was basically designed to cool the power supply, not the components in the case. Not providing additional fans is a sign of cheapness. On tower PCs with "expensive" engineering (e.g. HP Vectra, Compaq) one will find one to two extra fans besides the one in the power supply.

So the bottom line is, use thermostatic controls if you can to cut noise. But if you want high reliability, use two or more fans. Modern designs will often have a small auxilliary fan mounted right over the chip! This is a good feature to ask about in advance.

The noise produced by a fan is not just a function of the speed with which it turns. It also depends on the nature of the airflow produced by the fan blades and the bearings of the rotor. If the blades causes lots of turbulent airflow, the fan produces lots of noise. One brand of fans that, I'm told, has a reputation for being much more silent than others even if going at full throttle is the German manufacturer Pabst. Their fans are offered in US computer magazines.

Eric S. Raymond