Abstract:
Methods and circuits detect operational information about DC fans powered by pulse width modulation, such as detecting fan presence in a device and/or detecting rotational speed. Output pulses produced by the fan related to the rotational speed are utilized to produce temporary reductions of voltage at the input of a control circuit. The control circuit may count the temporary reductions per unit of time to detect the speed. A pull down resistor may be coupled to the input to pull the input to a continuously low voltage when the DC fan is not present to otherwise provide a pull up at the input, and the control circuit may detect a missing fan from the continuously low voltage. Additionally, or alternatively, a secondary voltage may be provided to the DC fan in addition to the pulse width modulation while a consistently high voltage is provided to the input of the control circuit. Accordingly, the DC fan continues to produce output pulses indicative of speed during the low state of the pulse width modulation, which enables the control circuit to continue to receive the temporary reductions at the input to determine the rotational speed with accuracy.

Description:
TECHNICAL FIELD 
     The present invention is related to direct current (“DC”) fans used in various devices. More particularly, the present invention is related to the detection of operational information of the DC fans such as the presence and/or rotational speed of the DC fan within a device. 
     BACKGROUND 
     Various devices utilize DC fans for purposes such as cooling components to prevent overheating. Computers are an example of a device that may utilize one or more DC fans to cool the components of the computer, such as a bank of hard drives. LCD projectors are another example, wherein the light bulb of the LCD must be cooled to prevent failure. The cooling from the DC fan is often critical to the continued operation of the device being cooled, so the DC fan must often be monitored for proper operation to prevent DC fan failure which would otherwise cause failure of the device itself. 
     To extend the life of the DC fan, a pulse width modulated voltage is typically applied rather than a continuous DC voltage. The pulse width modulated voltage provides a peak voltage for each pulse that is great enough to maintain an adequate rotational speed of the fan, but the pulses reduce the increased strain on the fan that otherwise results from operating with a constant voltage. A controller is often employed to monitor the rotational speed of the DC fan resulting from the pulse width modulation and adjust the duty cycle as necessary to ensure that the DC fan continues to adequately perform. 
     The controller includes an input that is pulled to a high voltage via a pull up resistor linked to a power supply voltage. In DC fans where the pull up resistor is internal to the fan, the pulse width modulated input to the DC fan is applied to the pull up resistor to pull up the voltage at the input of the controller. Where the pull up resistor is external to the DC fan, the pulse width modulated input to the DC fan or a separate voltage source may be applied to the pull up resistor to pull up the voltage at the input. The input is further connected to a transistor, typically internal to the fan, that is switched on and off by rotation of the fan, such as through the output pulse from a Hall effect sensor powered by the pulse width modulated input voltage to the fan. This on and off switching results in temporary reductions of the pulse width modulated voltage being received at the input and these temporary reductions are counted to detect the rotational speed of the fan. 
     The DC fans may be removable from the device. For example, a DC fan may fail and may need to be replaced or repaired. However, it is important to know when a DC fan is not present in a device so that harm due to overheating can be prevented. Where multiple DC fans are present, it is also important to know that one or more fans are not present since the remaining fans may not be able to provide sufficient cooling and because the duty cycle of the pulse width modulated voltage for the remaining fans will increase or even result in a constant voltage to allow the remaining fans to compensate for the missing fan(s). Thus, it is important that missing fans be replaced before a device is used or as soon as possible after the fan is removed. 
     Because for some DC fans the input to the control circuit is pulled up by the presence of the pull up resistor that is integral to the fan, when the fan is removed from the device then the input is no longer pulled up. This results in the input to the control circuit floating, or having no definite logical high or low value. This prevents the control circuit from detecting whether the fan is missing or present. Therefore, other cumbersome methods of detecting whether the fan is missing or not must be used, such as manual inspections of the devices prior to the devices being used. 
     In addition to detecting the presence of a fan, it is important to detect the rotational speed of the fan so that its performance can be evaluated to determine whether maintenance or replacement is necessary. The speed of the fan is detected from the temporary reductions in the voltage pulled up at the input that are proportional to speed, but these temporary reductions can only be detected during the period of time that the pulse width modulated voltage is at the high state. This limited time of detection of temporary reductions is due to the rotation sensor integral to the DC fan requiring power from the power supply for the fan to accurately produce a pulse that results in the temporary reduction at the input to the control circuit, but no power is provided during the low state of the pulse width modulated voltage thereby limiting the time of accurate detection. For DC fans where the pull up is powered by the pulse width modulated voltage, then this limited time of accurate detection of temporary reductions is also due to no pull up voltage being present during the period when the pulse width modulated voltage is at the low state. 
     The output pulse train resulting in the temporary reductions that enables the control circuit to detect rotational speed is asynchronous relative to the pulse train of the pulse width modulated power supply. Due to this asynchronous relationship, erratic measurements result because during one high period of the pulse width modulation, there may be few if any output pulses while during another high period there may be several output pulses. These erratic measurements are not effective in monitoring the rotational speed of the fan since they arc inaccurate. Additionally, during the low periods of the pulse width modulation occurring during an output pulse, the rotational sensor may produce residual noise that appears as temporary reductions at the input to the control circuit since the sensor may be producing an output pulse which is briefly terminated by the low state of the pulse width modulation and which results in multiple temporary reductions rather than only one. This causes the input to the control circuit to detect a speed reading that is too high. 
     Thus, other methods of attempting to accurately detect fan speed are used. One technique is to apply a continuous voltage from the pulse width modulated power supply for a period of time when speed will be measured, so that the input is continuously pulled up, the rotation sensor is continuously powered, and the temporary reductions resulting from the output pulse may be measured consistently. However, such periodic application of continuous voltage from the pulse width modulated power source has significant drawbacks as well. This method is likely to be harmful to the fan, it results in audible noise that is distracting due to the variation in fan speed, and it also results in inaccuracy because the DC fan speed is higher and less constant for the measurement period than it is during the normal operating period. 
     SUMMARY 
     Embodiments of the present invention address these and other issues by providing methods and circuits that detect whether the fan is present through the input of the control circuit and that measure rotational speed at the input of the control circuit during both the high and low states of the pulse width modulated voltage. In certain embodiments, a pull down resistor may be provided at the input of the control circuit to provide a continuously low voltage indicative of a missing fan. In certain embodiments, a secondary voltage may be provided to the fan to power the rotation sensor, and pull up the input if applicable, during the low state of the pulse width modulated voltage to allow temporary reductions indicative of rotational speed to be detected at all times without altering the duty cycle of the pulse width modulated voltage. 
     One embodiment is a method of detecting operational information about a DC fan of a device where the DC fan is powered by pulse width modulation and produces an output pulse in proportion to rotational speed. The method involves providing a pulse width modulated voltage to the DC fan and providing a voltage to an input of a control circuit when the DC fan is present in the device. A temporary reduction in the voltage to the input of the control circuit is produced upon receiving the output pulse from the DC fan when present. The input of the control circuit is held at a low voltage when the DC fan is not present. It is detected through the control circuit that the DC fan is not present in the device when the input of the control circuit is continuously at the low voltage. The rotational speed of the DC fan when present in the device is detected through the control circuit from the number of temporary reductions in the voltage per unit of time at the input of the control circuit. 
     Another embodiment is a method of detecting operational information about a DC fan of a device where the DC fan is powered by pulse width modulation and produces an output pulse in proportion to rotational speed. The method involves providing a pulse width modulated voltage and a secondary voltage to the DC fan. A voltage is provided to the input of the control circuit. A temporary reduction in the voltage to the input of the control circuit is produced upon receiving the output pulse from the DC fan during each period of time that the pulse width modulated voltage is high and during each period of time that the pulse width modulated voltage is low. The rotational speed of the DC fan is detected through the control circuit from the number of temporary reductions per unit of time of the voltage at the input of the control circuit. 
     Another embodiment is a circuit for detecting operational information about a DC fan of a device where the DC fan is powered by pulse width modulation and produces an output pulse in proportion to rotational speed. The circuit includes a transistor operatively coupled to receive the output pulse and a controller having an input joined at a node with an electrode of the transistor. The controller is configured to detect the presence of the fan based on the input receiving a continuously low voltage or other voltage. A pull up resistor is electrically connected to the node and provides a voltage drop to the input upon the transistor conducting in response to the output pulse. A pulse width modulated power supply is electrically connected to the DC fan and a voltage source is electrically connected to the pull up resistor opposite the node. A pull down resistor is electrically connected between the node and ground and provides the continuously low voltage at the input upon the pull up resistor being disconnected from the node. 
     Another embodiment is a circuit for detecting operational information about a DC fan of a device where the DC fan is powered by pulse width modulation and produces an output pulse in proportion to rotational speed. The circuit includes a transistor operatively coupled to receive the output pulse and a controller having an input joined at a node with an electrode of the transistor. The controller is configured to detect the speed of the fan based on the input receiving voltage having temporary reductions corresponding to the output pulse during periods when the pulse width modulated voltage is high and low. A pull up resistor is electrically connected to the node and provides a voltage drop to the input to provide the temporary reduction upon the transistor conducting in response to the output pulse. A pulse width modulated power supply is electrically connected to the DC fan and a voltage source is electrically connected to the pull up resistor opposite the node. A secondary voltage supply is also electrically coupled to the DC fan and provides a secondary voltage that is less than the pulse width modulated voltage when high. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a DC fan with an internal pull up resistor and a circuit coupled to the fan for detecting fan presence and rotational speed. 
     FIG. 2 shows a DC fan with an external pull up resistor and a circuit coupled to the fan for more accurately detecting rotational speed without requiring an adjustment to the duty cycle. 
     FIG. 3 shows a DC fan with an internal pull up resistor and a circuit coupled to the fan for detecting fan presence and more accurately detecting the rotational speed without requiring an adjustment to the duty cycle. 
     FIG. 4 is a graph of the signal received at the input of a controller according to the circuitry of FIG. 1 to detect fan presence and rotational speed. 
     FIG. 5 is a graph of the signal received at the input of a controller according to the circuitry of FIG. 2 to more accurately detect rotational speed. 
     FIG. 6 is a graph of the signal received at the input of a controller according to the circuitry of FIG. 3 to detect fan presence and more accurately detect rotational speed. 
     FIG. 7 is a graph of the signal received at the input of a controller according to prior art circuitry or the circuitry of FIG. 4 where residual noise results in inaccurate rotational speed detection. 
     FIG. 8 is a graph of the signal received at the input of a controller according to the circuitry of FIG.  5 . 
    
    
     DETAILED DESCRIPTION 
     Various embodiments of the present invention described herein provide circuitry for detecting the presence of a DC fan where the DC fan utilizes an internal pull-up resistor that is removed from the circuit upon the DC fan being removed from the device. Additionally, various embodiments of the present invention described herein provide circuitry for more accurately detecting the rotational speed of the DC fan, including DC fans with an internal pull up resistor and DC fans with external pull up resistors. 
     FIG. 1 shows a DC fan  102  that includes a propeller  104  driven by a DC motor coil  106 . A sensor  108  at the coil, such as a Hall effect sensor, receives power from the power supply powering the coil  106  and produces an output pulse each time the sensor crosses paths with a magnetic pole of the DC fan (not shown). This output pulse is therefore proportional to the rotational speed of the fan  102 , and the speed can be determined by knowing the number of magnetic poles, and therefore the number of output pulses that occur for each revolution of the fan  102 . 
     In this embodiment, to provide reliable speed measurement, the sensor  108  sends the output pulse to the base of a bi-polar transistor  110  to momentarily switch the transistor  110  on. The transistor  110  is in an open collector configuration, as the collector has a high resistance load from the input  128  of a microcontroller  120 . The input  128  is pulled up to a voltage by a pull up resistor  112  internal to the fan  102 . The pull up resistor  112  may receive the same input voltage as the coil  106  in some embodiments or may be decreased through a voltage divider (not shown) to reduce the voltage at the pull up. Accordingly, the node interconnecting the pull up resistor, collector of the transistor  110 , and input  128  of the controller  120  is pulled up to the voltage being input to the fan  102 , or a reduced voltage where a voltage divider is present, when the transistor  110  is off. Upon the transistor  110  being switched on momentarily by the output pulse biasing the transistor  110  toward saturation, current is drawn from the input power source of the fan  102  through the pull up resistor  112  and transistor  110  to cause the collector voltage, which is the voltage at the input  128  of the controller  120 , to drop to a typically logical low value near zero. 
     This momentary drop to nearly zero is seen as a temporary reduction in the input voltage at input  128  by the microcontroller  120 . This temporary reduction is counted at the microcontroller  120  for a particular unit of time to provide an indication of the rotational speed of the DC fan  102 . However, because a pulse width modulated voltage is provided to the fan  102  as the input voltage, the input  128  is pulled up to the pulse width modulation voltage which is asynchronous relative to the output pulses. For example, the sensor  108  may be passing magnetic poles that would otherwise generate multiple output pulses during the low state of the pulse width modulated supply. This results in the input  128  not receiving the proper number of temporary reductions since the input  128  receives the one reduction, which is the low state of the pulse width modulation and which is likely to be different than the number of output pulses that should be counted during that low state period. This inaccuracy is illustrated in FIG. 4, which is discussed in more detail below. 
     The pulse width modulated power supply includes a field effect transistor  114  (“FET”) having a source connected to a voltage source  116  with a voltage (e.g. 12V for a 12V fan) great enough to cause rotation of the fan  102 . A bias resistor  118  is connected between the source and the gate of the FET  114 , and the gate and resistor  118  are further connected to a collector of a bi-polar transistor  122 . The microcontroller  126  has a pulse width modulated output  126  that feeds the base of the transistor  122  to turn it off and on according to the desired pulse width modulation scheme. When the transistor  122  is turned on, current is drawn through the resistor  118  to create a voltage difference between the source and the gate of the FET  114  which results in the FET  114  turning on to provide the pulse width modulation. This pulse width modulated voltage provided to the fan  102  has a peak voltage and current supplied by the source  116  rather than from the output  126 , which is typically unable to provide the necessary power to the fan  102 . 
     Although this circuit of FIG. 1 may not accurately detect the rotational speed of the fan  102  without altering the duty cycle or providing some other firmware based solution, this circuit of FIG. 1 does provide an accurate indication of whether the fan  102  is present or not within a device. As discussed, the pull up resistor  112  is internal to the fan  102 , so when the fan  102  is removed then the pull up resistor  112  is also removed. This results in the voltage at the input  128  being pulled to a logical low value by a pull down resistor  124 . This pull down resistor  124  is connected between the input  128  and a logical low voltage, such as system ground. Thus, when the fan  102  is removed, rather than floating, the input  128  is pulled down to a continuously low value such that the microcontroller  120  detects the continuously low voltage indicating that the fan  102  is not present. This pull down resistor  124  may be orders of magnitude greater in resistance than the pull up resistor  112 , such as two orders greater so that when the fan  102  is present the pull down resistor  124  draws relatively little additional current from the voltage source  116 . 
     FIG. 4 illustrates the signal received at input  128 . The input  128  is pulled up to the pulse width modulation (“PWM”) level. As shown, six separate pulses  402 ,  404 ,  406 ,  408 ,  410 , and  411  of the pulse width modulation have been provided while the fan  102  is present. Because the voltage at the input  128  is fluctuating from below V Lo  (logical low; e.g. 0.8V) to above V Hi  (logical high; e.g. 2.5V), the microcontroller  120  detects that the fan  102  is present. However, after the sixth pulse  411 , the fan  102  is removed such that the pull down resistor  124  pulls the input  128  to a continuously low value, below V Lo . Upon the microcontroller  120  detecting that the voltage has not returned to above V Hi  after a predefined period, such as after two periods  418  and  420  of the PWM input, then the microcontroller  120  detects that the fan  102  is not present, and may provide some indication such as a blinking LED or other signal. 
     FIG. 2 illustrates a circuit that more accurately detects the rotational speed of a fan  202 . The fan  202  includes a propeller  204  driven by a coil  206  having a rotational sensor  208  that produces output pulses in proportion to rotational speed. A transistor  210  is switched on by the output pulse. In this embodiment, the fan  202  has an external pull up resistor  212 . This pull up resistor  212  is connected to a secondary voltage output from a secondary voltage source  214 . As another alternative, the pull up resistor  212  may be connected to another voltage source (not shown) besides the secondary voltage source  214 . 
     The secondary voltage source  214  also provides a voltage through the diode  216  to the input to the fan  202  in parallel with the voltage from the pulse width modulation supply. The secondary voltage source  214  provides an output voltage (e.g. 5V for a 12V fan) that is sufficiently small so that it does not provide sufficient voltage to significantly impact rotation of the fan  202  such that the duty cycle approach to increasing motor life is not undermined. The output voltage of the secondary source  214  is sufficiently great to power the sensor  208  so that output pulses may continue to be generated even while the pulse width modulated voltage is at a low state. Where the secondary voltage source  214  is being used to provide the pull up voltage for the input  230 , then the secondary voltage is great enough to be recognized as a logical high by the microcontroller  224 . If another voltage source is providing the pull up voltage for input  230 , then this other voltage must also be great enough to be recognized as a logical high by the microcontroller  224 . This allows the microcontroller  224  to detect temporary reductions, such as going from logical high to logical low, at the input  230  so that the rotational speed can be detected from counting the number of temporary reductions per unit of time. 
     To provide the pulse width modulation to the fan  202 , a voltage source  220  with a voltage capable of rotating the fan  202  is provided to a FET  218 , and a bias resistor  222  between the source and gate of the FET  218  is included in conjunction with the bi-polar transistor  226  so that when the transistor  226  is on, the FET  218  is turned on to provide a pulse of voltage to the fan  202 . The transistor  226  is switched on by a pulse width modulated output  228  of the microcontroller  224 . The high state of the pulse width modulation is greater than that of the secondary voltage source  214 , but the secondary voltage source  214  is electrically isolated from the high state of the pulse width modulation voltage by the diode  216 . 
     In addition to providing the secondary voltage to the rotation sensor so that pulses can be produced during the low states to prevent temporary reductions from being missed where the signal to the input has a higher frequency than the pulse width modulation frequency, the secondary voltage further prevents the residual noise pulses that create false output pulses where the pulse width modulation has a higher frequency than the frequency of accurate temporary reductions at the controller input. This situation is shown in FIG. 7, the pulse width modulation may have a frequency greater than the accurate number of temporary reductions that should be occurring for certain situations. The accurate points and duration for temporary reductions are indicated with a tick mark and bracket. If the pulse width modulation, shown in dashed lines as pulses  502 ,  504 , and  506 , also goes low once or more during each bracketed period, then the sensor briefly stops producing an output pulse for the low period of the PWM which results in the signal at the input briefly returning to high, as can be seen between temporary reductions  508  and  510 . This brief return to high at the controller input is followed by another temporary reduction  510  resulting from the PWM going back high which causes the sensor to again produce an output pulse. The two output pulses produced by the sensor resulting in a first temporary reduction  508  and second temporary reduction  510  correspond to only a single magnetic pole (as indicated by a single tick mark) such that there is an extra output pulse which leads to an extra temporary reduction  510  at the controller input. This extra temporary reduction  510  results in the inaccurate speed reading which is too high relative to the actual speed of the fan. 
     As shown in FIG. 8, since the rotational sensor remains powered during the low states of the pulse width modulation between the pulses  502 ′,  504 ′, and  506 ′ shown in dashed lines, the temporary reduction  508 ′ at the controller input remains low during the entire bracketed period corresponding to one magnetic pole. Accordingly, the controller counts only a single temporary reduction  508 ′ for this one magnetic pole which results in an accurate detection of rotational speed. 
     FIG. 5 illustrates the signal at the input  230  which allows the microcontroller  224  to accurately detect the rotational speed. The pulse width modulation pulses  402 ′,  404 ′,  406 ′, and  408 ′ match those of FIG.  4 . FIG. 4 will be discussed in relation to FIG. 5 to further illustrate the inaccuracies of the input signal of FIG. 4 for the circuit of FIG.  1  and the lack of inaccuracies of the input signal of FIG. 5 for the circuit of FIG.  2 . 
     As shown in FIG. 4, temporary reductions  410  are created in the input signal during the high state of the pulse width modulation. The ticks along the time axis correspond to the output pulses that are being created during the high state and that would be created during the low state except for the lack of power being provided to the rotation sensor by the circuit of FIG.  1 . In this simplified example of the asynchronous relationship of the output pulse to the pulse width modulation, it can be seen that during various low states of the pulse width modulation, there are occasions where more than one output pulse should occur. For example, ticks  412 ,  414  and  416  are examples of a second output pulse that should be occurring within one low state. Because the microcontroller  120  can only count the low state as a single temporary reduction, the microcontroller  120  completely misses these second events indicated by ticks  412 ,  414 , and  416 , which results in the detected rotational speed being inaccurate. 
     As shown in FIG. 5, during the low state between the periods of high state for the pulse width modulation, the input signal is at the secondary level provided by the secondary voltage source  214  or other suitable voltage level, depending upon which alternative is providing the pull up voltage to the input  230 . This voltage on the input  230  continues to be seen as a logical high by the microcontroller  224  regardless of the state of the pulse width modulation, and regardless of whether the pulse width modulation is also connected to provide a pull up to the input  230  in the alternative discussed above (excess pull up of this alternative shown in shaded area although this is likely reduced by a voltage divider for most motors and controllers). This secondary voltage level during the low state of the pulse width modulation allows the temporary reductions in the input signal due to the output pulses to be detectable by the microcontroller  224 . 
     As shown in FIG. 5, temporary reduction  410 ′ which occurs during the high state remains detectable. Now, temporary reductions  412 ′ and  414 ′ are also detectable, in addition to the other temporary reductions during the same low state periods, such that the microcontroller  224  counts every temporary reduction corresponding to every output pulse. Every occurrence of an output pulse is being generated from the sensor  208  to create a temporary reduction at the input  230 , even during the low states of the pulse width modulation, since the secondary voltage continues to provide power to the sensor  208  during these low states while the input  230  continues to receive the pull up voltage that allows the temporary reductions to be detected. Furthermore, the secondary voltage source prevents the residual noise from producing false temporary reductions that further lead to inaccurate detection of speed, as illustrated in FIG. 7 for situations where the PWM has a higher frequency than the accurate frequency of temporary reductions that should be present at the controller input. Accordingly, the microcontroller  224  for the circuitry of FIG. 2 makes a more accurate detection of the rotational speed in these situations. 
     FIG. 3 illustrates a circuit that more accurately detects the rotational speed of a fan  302  and also detects the presence of the fan  302  where the fan  302  includes an internal pull up resistor  312 . The fan  302  includes a propeller  304  driven by a coil  306 . A rotational sensor  308  produces an output pulse to momentarily switch on a transistor  310 . The collector of the transistor  310  is connected to an input  332  of a microcontroller  324 , such that the pull up resistor  312  pulls up the collector and input  324  to the voltage being input to the fan  302  or a reduction of this voltage provided by a voltage divider (not shown). 
     So that the microcontroller  324  can detect a missing fan, a pull down resistor  328  is provided between the input  332  and a logical low voltage, such as system ground. When the fan  302  is removed, the pull up resistor  312  is no longer present to pull up the input  332  and the pull down resistor  328  pulls the input  332  down to a continuously logical low value enabling the microcontroller  324  to detect the missing fan  302 . The microcontroller  324  may then provide an indication of the missing fan, such as blinking an LED. 
     The fan  302 , when present, is provided an input voltage from two different sources. A pulse width modulated voltage is provided in parallel with a secondary voltage. The secondary voltage is provided from a secondary voltage source  314  through a diode  316 . The secondary voltage is small relative to the peak of the pulse width modulation voltage such that the secondary voltage does not significantly contribute to rotation of the fan  302  during the low state of the pulse width modulation. The secondary voltage is large enough to provide power to the sensor  308  to generate an adequate output pulse and avoid production of residual noise and is also large enough to provide a logical high pull up to the input  332  through the pull up resistor  312  (and voltage divider, if present) for periods when the pulse width modulation is in the low state. The diode  316  electrically isolates the secondary voltage source  314  from the pulse width modulation. 
     The pulse width modulation is provided from a FET  318  that receives power from a voltage source  320 . The FET  318  is switched on to provide a pulse form the voltage source  320  by current being drawn through a bias resistor  322  interconnected between the source and gate. The current is drawn through the bias resistor  322  by a transistor  326  being switched on. The microcontroller  324  provides a pulse width modulation output  330  to switch the transistor  326  on and off to thereby switch the FET  318  on and off to provide the pulse width modulated input voltage. 
     FIG. 6 illustrates the input signal to the input  322  of the microcontroller  324  to allow the microcontroller  324  to more accurately detect rotational speed and also detect the presence of the fan  302 . The signal to the input  322  includes pulse width modulation with high states  402 ″,  404 ″,  406 ″, and  408 ″. During these pulses when the input signal is varying between the high state and the secondary voltage, the microcontroller  332  detects that the fan  302  is present since the voltage is not continuously low. In this example, the fan  302  has been removed during the  408 ″ pulse such that the microcontroller  324  detects that the fan is missing due to the continuously low voltage that follows. 
     The input signal of FIG. 6 also illustrates that the temporary reductions in voltage at the input  322  that are being counted by the microcontroller  332  continue during the high state, such as at temporary reduction  410 ″, and during the low states when the secondary voltage is present at the input  322 , such as temporary reductions  412 ″ and  414 ″. Furthermore, false temporary reductions due to residual noise during the low states of the PWM where PWM frequency is higher than the frequency of the temporary reductions, such as shown in FIG. 7, is also eliminated since the rotational sensor is continuously powered. Accordingly, the microcontroller  332  counts the temporary reductions for all true output pulses to more accurately detect the rotational speed. 
     The various embodiments discussed herein have included a microcontroller. An example of such a microcontroller is the ATmega 128 manufactured by Atmel. However, it will be appreciated that such a control circuit may include other devices besides a microcontroller integrated circuit. Furthermore, various embodiments discussed herein have included a FET switched on and off by a bi-polar transistor to provide a pulse width modulation voltage. It will be appreciated that other circuitry for producing the pulse width modulation is also applicable. 
     Although the present invention has been described in connection with various illustrative embodiments, those of ordinary skill in the art will understand that many modifications can be made thereto within the scope of the claims that follow. Accordingly, it is not intended that the scope of the invention in any way be limited by the above description, but instead be determined entirely by reference to the claims that follow.