As I am an electronics technician with a local (Western Canada) electronics manufacturer, I am quite familiar with the use of thermistors.
<BR>The computer PSU design calls for several DC outputs, I.E. 5VDC @ 15-amperes, 12VDC @ 5-amperes, etc. To acheive these requirements
<BR>and remain physically small, the manufacturers use switching designs. Basically, the 120VAC power supply is filtered for internal/external
<BR>powerline noise, rectified into DC, and filtered with DC capacitors. This raw DC voltage is the DC supply for the main switching transformer and the
<BR>switcher control circuitry. The switcher frequency varies, but will be around 40 kHz (40,000 Hertz).
<BR>The output windings are generally fed into air-core wound coils. These coils are pulsed by the control circuitry and thereby "store" the energy.
<BR>Thus, these coils will get hot with the loading of the MoBo, CPU, HDD, RAM, etc. The thermistors add a level of safety protection for the PSU, in that if
<BR>the coils get too hot, the thermistor resitance will change, the cooling fans will run faster, thereby cooling off the coils and the rest of the PSU.
<BR>Adding an variable potentiometer to manually adjust the voltage to the fan is an interesting idea, but has one drawback: the PSU now has no automatic
<BR>protection against generated heat within the PSU design. If the PSU fails, the designers hope the switcher will fail in the OFF state (not always). However, the
<BR>PSU could always fail in the ON state, provide high voltages and wreck havoc on the MoBo, CPU and the rest of your computer. Most manufacturers
<BR>(if they are smart) have proection on the DC powerlines to guard against high voltages. These devices are called zener diodes and/or transorbs. Both provide
<BR>protection against over-voltages. At best, these devices should be in the PSU design and at least, be on the MoBo.
<BR>In my 30-years of experiences with power supplies, both linear and switching, I would recommend against fooling with the thermistors. The
<BR>thermistor is a protection device, like a fuse. You wouldn't bypass a fuse, would you?