reference will now be made to the drawings wherein like numerals refer to like parts throughout . fig1 illustrates a typical cooling fan assembly 100 used in one embodiment of the invention . in particular , the cooling fan assembly 100 includes a fan 101 that has a fan blade 102 that is mounted about a shaft 104 so as to be positioned within a housing 106 . the housing 106 is attached to a mount 108 that is mounted in the personal computer so that the fan blades 102 can cool an electrical component 110 . in the embodiment shown in fig1 the cooling fan assembly 100 is mounted on posts 112 that are attached to a printed circuit board 114 so that the cooling fan blades 102 blow cool air over an integrated circuit 110 that is mounted on the printed circuit board 114 . in this manner , the operation of the fan assembly 100 reduces the ambient temperature surrounding the integrated circuit 110 and thereby dissipates the heat that is produced by the integrated circuit 110 . it will be appreciated , however , that the fan 101 may be mounted in any of a number of well - known manners without departing from the spirit of the invention . for example , the fan may also be mounted in a sidewall of a chassis of the computer so as to allow the fan to draw cool air into the chassis from the outside environment or to expel hot air from the chassis to the cooler outside environment . a control circuit 120 ( fig2 ) is used to control the operation of the fan assembly 100 in a manner that will be described below . in one embodiment , the fan assembly 100 includes a dc fan 101 such as a model fba09a12m single speed fan manufactured by panasonic . it will be appreciated from the following discussion that the control circuit 120 of this embodiment can be used in conjunction with dc fan assemblies such as single speed fans that are mounted in any of a number of well - known manners within the chassis of a personal computer or any other electronic device requiring forced air cooling . referring now to fig2 the control circuit 120 is described in greater detail . specifically , the dc fan 101 is electrically connected between a positive voltage source 122 and a negative voltage source 124 . a zener fixed voltage divider 126 is connected in series between the dc fan 100 and the positive voltage source 122 . similarly , an electronic switch 128 is also connected in series between the dc fan 101 and the positive voltage source 122 so that the electronic switch 128 is connected in parallel with the zener fixed voltage divider 126 . the control circuit 120 operates as follows . the dc fan 101 receives the full differential between the positive voltage source 122 and the negative voltage source 124 when the electronic switch 128 is closed . alternatively , the dc fan 101 sees a smaller applied voltage when the electronic switch 128 is opened and the voltage is applied across the zener fixed voltage divider 126 . as is understood in the art , a zener fixed voltage divider 126 reduces the voltage that is applied across the dc fan by a known amount , v z . hence , by opening the electronic switch 128 , a reduced voltage can be applied across the dc fan 101 which thereby results in the fan 101 operating at a slower speed and producing less noise . advantageously , the voltage drop v z across the zener fixed voltage divider is generally independent of the current that is being drawn by the fan 101 . specifically , different fans from different manufacturers will draw different amounts of current . if a resistor network were used to produce the voltage drop for low speed operation of the fan , the resistor network would have to be tailored for each manufacturer &# 39 ; s fans . using a zener diode based voltage divider results in substantially the same voltage drop occurring regardless of the current that is being drawn by the fan 101 . fig3 is an electrical schematic which illustrates one embodiment of the fan control circuit 120 in greater detail . in particular , as shown in fig3 the electronic switch 128 in this embodiment is comprised of an open collector gate , p - channel mosfet . the mosfet switch 128 receives a speed control signal from a speed control circuit ( not shown ) through a gate 130 . the speed control signal can be a control signal that is provided by a processor of the personal computer . in that embodiment , the processor would typically receive a signal from a tachometer associated with the fan and use this signal to adjust the speed of the fan to optimize the fan performance . alternatively , one embodiment of a control system that can be used in conjunction with the fan speed control circuit 120 will be described in greater detail in reference to fig4 - 6 below . this embodiment of a control system senses the speed of one of a plurality of fans and then induces other fans within the system to operate at higher speeds upon sensing that a fan is no longer operating in a desired manner . hence , the speed control signal can be used to enable or disable the mosfet switch 128 . when the mosfet switch 128 is enabled , the 12 volts dc power from the power supply is applied across a biasing resistor 134 such as a 1 kohm resistor that is positioned in parallel with the fan 101 . when the mosfet switch 128 is enabled , the fan 101 effectively sees the entire 12 volts dc from the power supply across its input and output . this results in the fan 101 operating at a higher speed . hence , in this configuration , the fan 101 provides greater cooling capacity at a cost of increased power consumption by the fan assembly 101 , greater wear on the fan 101 and more noise . a noise filtering capacitor 136 may also be connected in parallel to the biasing resistor 134 and the positive and negative inputs of the fan assembly 100 in a well known manner . alternatively , the speed control signal can be used to open the mosfet switch 128 so that the fan assembly 100 only sees 8 . 7 volts dc which results in the fan 101 operating at a slower speed . as is understood in the art , once 3 . 3 volts are applied across the zener diode 126 , the zener diode experiences zener breakdown which effectively results in the zener diode operating as a low resistance conductor while reducing the voltage that is applied across the fan 101 to 8 . 7 volts . consequently , the control circuitry 120 is capable of operating a single speed dc fan 101 at a low speed by applying only 8 . 7 volts dc to the fan , thereby causing the fan 101 to spin at a slower speed and thereby prolonging the life of the fan and decreasing the noise produced by the fan . alternatively , the control circuit 120 can also be configured so as to apply 12 volts dc to thereby operate the fan 101 at a higher speed which will result in more cooling of the computer components . the circuit 120 of this embodiment also includes a biasing resistor 132 such as a 33 kohm resistor which is selected so that when the speed control input signal is to turn the gate 128 off , the gate is open and the resistor 132 between the gate of the mosfet 128 and the source of the mosfet 128 biases the mosfet in the off condition . however , when the speed control input is such as to bias the mosfet 128 to turn on , the gate pin of the mosfet 128 is pulled to ground giving the 12 volts across the gate - source junction and thereby turning the mosfet switch on and shorting across the zener diode 126 . it will be appreciated from the foregoing that this embodiment of the control circuit is capable of being used in conjunction with a dc fan so as to operate the dc fan 101 at a variety of speeds . the control circuit 120 can therefore be used to increase or decrease the speed of the fan 101 as required to achieve the desired cooling effect of the components while reducing the wear and tear on the fans and the noise produced by the fans . moreover , it will be appreciated that the use of the zener diode will allow the circuit 120 to be used with a plurality of different fans from different manufacturers while still providing the same voltage differential between low speed and high speed operation regardless of the current drawn by the fan . fig4 is a block diagram which illustrates another implementation of a control circuit 220 which is similar to the control circuit 120 described above in reference to fig2 and 3 . specifically , in fig4 the control circuit 220 is adapted to control the speed of a plurality of dc fans 202 which are substantially similar to the fan 101 described above . the control circuit 220 includes an electronic switch 228 similar to the electronic switch 128 that is described above . the electronic switch 228 is connected in series with the fans 202 which are connected in parallel . the electronic switch 228 is also connected in parallel to a zener fixed voltage divider 226 similar to the voltage divider 126 described above . the control circuit 220 also includes a microprocessor 230 which receives signals from tachometer outputs 240 that are attached to each of the fans 202 . in normal operation , the fans 202 see a lower dc voltage as a result of the zener voltage divider 226 being in series with the fans 202 . when the tachometer outputs 240 indicate that one of the fans 202 has slowed down , the microprocessor 230 is adapted to send a control signal along the switch control signal line 250 to the electronic switch 228 . the electronic switch 228 then shorts out a zener fixed voltage divider 226 . this results in the full voltage differential between a positive voltage source 222 and a negative voltage source 224 being applied across the inputs to the fans 202 in the manner that was described above . as will be described in greater detail below , the control system 200 is adapted to determine when one of a plurality of fans 202 is no longer operating such that the cooling system 200 is no longer providing adequate cooling to the computer . at which point , the microprocessor 230 provides a signal on the switch control signal line 250 to enable the electronic switch 228 . this results in the zener fixed voltage divider 226 being shorted out and a greater voltage being applied to the dc fans 202 . this results in the dc fans 202 that are continuing to operate to increase their speed of rotation , thereby increasing their cooling capacity . in one embodiment , the microprocessor 230 continues to monitor the tachometer output 240 of the plurality of fans 202 such that when the fan that is operating incorrectly comes back on line , the microprocessor monitor then sends a signal along the switch control signal line 250 to the electronic switch 228 which opens the electronic switch 228 . in this case , the voltage that is applied to the dc fan 202 is lowered as a result of these zener fixed voltage divider 226 being positioned in series with the fans . this will result in the fans 202 operating at a slower speed , thereby decreasing the wear and tear on the fans and the noise produced by the fans . hence , the control system 200 is capable of monitoring the speed of rotation of a number of fans when the fans were operating at a first speed . when the microprocessor 230 determines that one of the fans is no longer operating , the microprocessor can then increase the voltage to the remaining operating fans to thereby increase the speed of operation of the remaining fans 202 and to thereby increase the cooling capacity provided by these fans . consequently , the control system 200 is capable of correcting a situation where the failure of one or more of the fans would result in too little cooling capacity being provided by the remaining fans . fig5 is a block diagram which illustrates the operation and configuration of the control system 200 in greater detail . in particular , the tachometer outputs of each of the fans 202 are provided to a multiplexer 260 . in one embodiment , the fans 202 include intrinsic tachometers . in another embodiment , an external sensor is used with a simpler fan , in the manner described below in reference to fig5 to provide a tachometer output to the multiplexer 260 . the multiplexer 260 is under the control of the microprocessor 230 in the sense that the microprocessor 230 can , in a state 272 , induce the multiplexer 260 to sample each of the tachometer outputs 240 from the fans 202 . specifically , once the microprocessor 230 has induced the multiplexer 260 to sample a particular tachometer output 240 from a particular fan 202 , the tachometer output is provided to the microprocessor 230 such that the microprocessor 230 , in a state 274 , can sense the speed of the fan 202 . the speed of the fan 202 is directly proportional to the cooling capacity that is provided by the fan 202 . the microprocessor 230 can then determine , in decision state 276 , whether the speed of the fan 202 is above the desired limit . if the speed of the fan 202 is not above the desired limit , then the fan is not operating correctly and insufficient cooling capacity is being provided to the computer . consequently , the microprocessor 230 can then determine , in state 280 , whether the fan speed is equal to zero based upon the fan speed that was detected in state 274 . in this embodiment , if the fan speed detected by the microprocessor 230 in state 274 is equal to zero , this means that the fan is being replaced . as will be described in greater detail below , once the fan is replaced , the voltage applied to the plurality of fans can be lowered to the lower voltage value . specifically , if the fan speed is equal to zero , then the microprocessor 230 sets , in state 282 , a bit entitled hot -- swap -- fan register equal to one . this indicates this fan is being replaced . subsequently , the microprocessor 230 then sets a bit in the fan -- fault register , in state 284 , corresponding to the fan 202 to one . setting the bit in the fan -- fault register to one results in a high output being provided , in state 286 , along the switch control signal line 250 ( fig4 ), thereby closing the switch 228 and shorting out the fixed zener voltage divider 226 causing the remaining fans 202 to operate at the higher speed . further , in one embodiment , the microprocessor 230 also sends a signal , in state 286 , to a non - volatile ram ( not shown ) recording that the fan 202 is no longer operating correctly . this allows a maintenance person to identify which fans are not operating correctly for subsequent replacement and / or repair of the incorrectly operating fans 202 . hence , the microprocessor 230 is capable of evaluating the speed of a particular fan 202 and determining whether it is underspeed . the microprocessor 230 also determines whether the speed is equal to zero or is simply less than the desired speed . the microprocessor 230 also induces a high output to be provided on the switch control signal line 250 to the electronic switch 228 upon sensing that any of the fans 202 is operating at a speed that is less than the desired limit . this results in a greater voltage being applied to the remaining fans 202 thereby increasing the speed of operation of these fans to offset the loss of cooling capacity due to the incorrectly operating fan . alternatively , if the microprocessor 230 determines , in decision state 276 , that the speed of the fan 202 is above the minimum limit , the processor 230 then determines , in decision state 290 , whether the hot -- swap -- fan register bit has been set for this particular fan 202 . if the hot -- swap -- fan bit has been set , that means that the fan was previously stopped and is now operating correctly which indicates the bad fan 202 has been replaced . this results in the processor 230 clearing , in state 292 , the bits and fan -- fault and hot -- swap -- fan registers corresponding to this particular fan 202 . the processor 230 then determines , in decision state 294 , whether the fan -- fault register is all clear and , if so , the processor 230 then sends a low signal on the switch control signal line 250 to the electronic switch 228 ( fig4 ) to open the electronic switch 228 . this results in the fans 202 seeing the lowered voltage as a result of the zener fixed voltage divider 226 being connected in series with the fans 202 . this results in all of the fans 202 operating at a lower speed thereby prolonging the life of the fan and also reducing the noise produced by of the operating fans . hence , the processor 230 sequentially evaluates the tachometer output lines of each of the fans 202 to determine the speed of the fan 202 . if any one of the fans is not operating at the correct speed , the processor 230 then sends a signal to the switch thereby disabling the zener voltage divider 226 and increasing the voltage to the remaining fans 202 to cause these fans 202 to speed up . the processor 230 continues to monitor the speed of operation of each of the fans 202 , including any fan that was previously sensed as not operating correctly . if the fans are operating correctly , then the processor 230 opens the switch 228 and positions the voltage divider 226 in series with the dc fans 202 to thereby reduce the voltage that is being applied to the dc fans . hence , the foregoing description has described several embodiments of a circuit for controlling the operation of fan which incorporates a zener diode divider that can be selectively switched into series with the power input of a fan to decrease the voltage that is provided to the fan . this allows the fan to operate at a lower speed thereby prolonging the longevity of the fan and also decreasing the noise produced by the fan . in another embodiment , a cooling system is provided which incorporates the zener diode control circuit and a plurality of fans . the cooling system may also include a microprocessor that senses the speed of operation of the plurality of fans and , upon sensing that one or more fans are no longer operating at a desired speed , the microprocessor closes a switch to short out the zener bridge divider so that the remaining fans receive a higher voltage to offset the loss of cooling capacity resulting from the loss of the fan . hence , although the foregoing description of the invention has shown , described , and pointed out fundamental novel features of the invention , it will be understood that various omissions , substitutions , and changes in the form of the detail of the apparatus and method as illustrated as well as the uses thereof , may be made by those skilled in the art without departing from the spirit of the invention . consequently , the scope of the invention should not be limited to the foregoing discussion , but should be defined by the appended claims . appendix a______________________________________incorporation by reference of commonly owned applicationsthe following patent applications , commonly owned and filedoctober 1 , 1997 , are hereby incorporated herein in their entiretyby reference thereto : application attorney dockettitle no . no . ______________________________________ &# 34 ; system architecture for remote 08 / 942 , 160 mnframe . 002a1access and control of environ - mental management &# 34 ;&# 34 ; method of remote access and 08 / 942 , 215 mnframe . 002a2control of environmental manage - ment &# 34 ;&# 34 ; system for independent powering 08 / 942 , 410 mnframe . 002a3of diagnostic processes on acomputer system &# 34 ;&# 34 ; method of lndependent powering 08 / 942 , 320 mnframe . 002a4of diagnostic processes on acomputer system &# 34 ;&# 34 ; diagnostic and managing distrib - 08 / 942 , 402 mnframe . 005a1uted processor system &# 34 ;&# 34 ; method for managing a distrib - 08 / 942 , 448 mnframe . 005a2uted processor system &# 34 ;&# 34 ; system for mapping environ - 08 / 942 , 222 mnframe . 005a3mental resources to memory forprogram access &# 34 ;&# 34 ; method for mapping environ - 08 / 942 , 214 mnframe . 005a4mental resources to memory forprogram access &# 34 ;&# 34 ; hot add of devices software 08 / 942 , 309 mnframe . 006a1architecture &# 34 ;&# 34 ; method for the hot add of 08 / 942 , 306 mnframe . 006a2devices &# 34 ;&# 34 ; hot swap of devices software 08 / 942 , 311 mnframe . 006a3architecture &# 34 ;&# 34 ; method for the hot swap of 08 / 942 , 457 mnframe . 006a4devices &# 34 ;&# 34 ; method for the hot add of a net - 08 / 943 , 072 mnframe . 006a5work adapter on a system in - cluding a dynamically loadedadapter driver &# 34 ;&# 34 ; method for the hot add of a 08 / 942 , 069 mnframe . 006a6mass storage adapter on a systemincluding a statically loadedadapter driver &# 34 ;&# 34 ; method for the hot add of a net - 08 / 942 , 465 mnframe . 006a7work adapter on a system in - cluding a statically loadedadapter driver &# 34 ;&# 34 ; method for the hot add of a 08 / 962 , 963 mnframe . 006a8mass storage adapter on a systemincluding a dynamically loadedadapter driver &# 34 ;&# 34 ; method for the hot swap of a 08 / 943 , 078 mnframe . 006a9network adapter on a system in - cluding a dynamically loadedadapter driver &# 34 ;&# 34 ; method for the hot swap of a 08 / 942 , 336 mnframe . 006a10mass storage adapter on a systemincluding a statically loadedadapter driver &# 34 ;&# 34 ; method for the hot swap of a 08 / 942 , 459 mnframe . 006a11network adapter on a system in - cluding a statically loadedadapter driver &# 34 ;&# 34 ; method for the hot swap of a 08 / 942 , 458 mnframe . 006a12mass storage adapter on a systemincluding a dynamically loadedadapter driver &# 34 ;&# 34 ; method of performing an exten - 08 / 942 , 463 mnframe . 008asive diagnostic test in conjunc - tion with a bios test routine &# 34 ;&# 34 ; apparatus for performing an 08 / 942 , 163 mnframe . 009aextensive diagnostic test inconjunction with a bios testroutine &# 34 ;&# 34 ; configuration management 08 / 941 , 268 mnframe . 010amethod for hot adding and hotreplacing devices &# 34 ;&# 34 ; configuration management 08 / 942 , 408 mnframe . 011asystem for hot adding and hotreplacing devices &# 34 ;&# 34 ; apparatus for interfacing buses &# 34 ; 08 / 942 , 382 mnframe . 012a &# 34 ; method for interfacing buses &# 34 ; 08 / 942 , 413 mnframe . 013a &# 34 ; computer fan speed control 08 / 942 , 447 mnframe . 016adevice &# 34 ;&# 34 ; system for powering up and 08 / 943 , 076 mnframe . 018apowering down a server &# 34 ;&# 34 ; method of powering up and 08 / 943 , 077 mnframe . 019apowering down a server &# 34 ;&# 34 ; system for resetting a server &# 34 ; 08 / 942 , 333 mnframe . 020a &# 34 ; method of resetting a server &# 34 ; 08 / 942 , 405 mnframe . 021a &# 34 ; system for displaying flight 08 / 942 , 070 mnframe . 022arecorder &# 34 ;&# 34 ; method of displaying flight 08 / 942 , 068 mnframe . 023arecorder &# 34 ;&# 34 ; synchronous communication 08 / 943 , 355 mnframe . 024ainterface &# 34 ;&# 34 ; synchronous communication 08 / 942 , 004 mnframe . 025aemulation &# 34 ;&# 34 ; software system facilitating the 08 / 942 , 317 mnframe . 026areplacement or insertion of de - vices in a computer system &# 34 ;&# 34 ; method for facilitating the re - 08 / 942 , 316 mnframe . 027aplacement or insertion of devicesin a computer system &# 34 ;&# 34 ; system management graphical 08 / 943 , 357 mnframe . 028auser interface &# 34 ;&# 34 ; display of system information &# 34 ; 08 / 942 , 195 mnframe . 029a &# 34 ; data management system sup - 08 / 942 , 129 mnframe . 030aporting hot plug operations on acomputer &# 34 ;&# 34 ; data management method sup - 08 / 942 , 124 mnframe . 031aporting hot plug operations on acomputer &# 34 ;&# 34 ; alert configurator and manager &# 34 ; 08 / 942 , 005 mnframe . 032a &# 34 ; managing computer system 08 / 943 , 356 mnframe . 033aalerts &# 34 ;&# 34 ; computer fan speed control 08 / 940 , 301 mnframe . 034asystem &# 34 ;&# 34 ; computer fan speed control 08 / 941 , 267 mnframe . 035asystem method &# 34 ;&# 34 ; black box recorder for informa - 08 / 942 , 381 mnframe . 036ation system events &# 34 ;&# 34 ; method of recording information 08 / 942 , 164 mnframe . 037asystem events &# 34 ;&# 34 ; method for automatically report - 08 / 942 , 168 mnframe . 040aing a system failure in a server &# 34 ;&# 34 ; system for automatically report - 08 / 942 , 384 mnframe . 041aing a system failure in a server &# 34 ;&# 34 ; expansion of pci bus loading 08 / 942 , 404 mnframe . 042acapacity &# 34 ;&# 34 ; method for expanding pci bus 08 / 942 , 223 mnframe . 043aloading capacity &# 34 ;&# 34 ; system for displaying system 08 / 942 , 347 mnframe . 044astatus &# 34 ;&# 34 ; method of displaying system 08 / 942 , 071 mnframe . 045astatus &# 34 ;&# 34 ; fault tolerant computer system &# 34 ; 08 / 942 , 194 mnframe . 046a &# 34 ; method for hot swapping of net - 08 / 943 , 044 mnframe . 047awork components &# 34 ;&# 34 ; a method for communicating a 08 / 942 , 221 mnframe . 048asoftware generated pulse wave - form between two servers in anetwork &# 34 ;&# 34 ; a system for communicating a 08 / 942 , 409 mnframe . 049asoftware generated pulse wave - form between two servers in anetwork &# 34 ;&# 34 ; method for clustering software 08 / 942 , 318 mnframe . 050aapplications &# 34 ;&# 34 ; system for clustering software 08 / 942 , 411 mnframe . 051aapplications &# 34 ;&# 34 ; method for automatically config - 08 / 942 , 319 mnframe . 052auring a server after hot add of adevice &# 34 ;&# 34 ; system for automatically config - 08 / 942 , 331 mnframe . 053auring a server after hot add of adevice &# 34 ;&# 34 ; method of automatically config - 08 / 942 , 412 mnframe . 054auring and formatting a computersystem and installing software &# 34 ;&# 34 ; system for automatically config - 08 / 941 , 955 mnframe . 055auring and formatting a computersystem and installing software &# 34 ;&# 34 ; determining slot numbers in a 08 / 942 , 462 mnframe . 056acomputer &# 34 ;&# 34 ; system for detecting errors in a 08 / 942 , 169 mnframe . 058anetwork &# 34 ;&# 34 ; method of detecting errors in a 08 / 940 , 302 mnframe . 059anetwork &# 34 ;&# 34 ; system for detecting network 08 / 942 , 407 mnframe . 060aerrors &# 34 ;&# 34 ; method of detecting network 08 / 942 , 573 mnframe . 061aerrors &# 34 ; ______________________________________