Abstract:
An apparatus and method for controlling a rotation speed of a fan. A comparator is used to detect an analog signal driving the fan. When the analog signal is detected abnormal, a voltage generation circuit generates a particular voltage to maintain a lowest operation voltage of the fan. A protection circuit is provided to let the fan driven by the analog signal again by means of receiving a signal from a system the fan is disposed at when the analog signal becomes normal.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS  
       [0001]     This application claims priority to Taiwan Patent Application No. 094137700 filed on Oct. 27, 2005.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to an apparatus and a method for controlling rotation speed of a fan; more particularly, relates to an apparatus and a method for providing a steady voltage to avoid a fan out of control when a voltage for controlling the fan is abnormal.  
         [0004]     2. Descriptions of the Related Art  
         [0005]     In recent years, because of the progress of information technique, the quality of the related products is highly improved. However, heat problem of these products also becomes serious. Therefore, highly-efficient cooling solutions are an important issue for new products. The most popular solution is to implant a system for controlling a rotation speed of a fan. The system may dynamically adjust the rotation speed of the fan to meet the need of cooling according to the environment temperature.  
         [0006]      FIG. 1  is a schematic view illustrating a rotation speed controlling system for a fan of the prior art. The system comprises a pulse width modulation (PWM) circuit  10  and a digital/analog converter  12 . The PWM circuit  10  is used to receive a digital control signal  11  for dynamically adjusting the rotation speed of a fan  14  via an Inter-Integrated Circuit ( 12 C) bus. The digital control signal  11  is converted to a modulation signal  13  by the PWM circuit  10 . The digital/analog converter  12  receives the modulation signal  13  and converts the modulation signal  13  into an analog signal  15 . The analog signal  15  has a voltage for controlling the rotation speed of the fan  14 . If the environment temperature gets high, the voltage of the analog signal  15  is then increased so that the rotation speed of the fan  14  becomes faster. If the environment temperature gets low, the voltage of the analog signal  15  is decreased so that the rotation speed of the fan  14  becomes slower.  
         [0007]     Since the products protected by such a rotation speed controlling system are usually electronic devices with high prices, e.g., an optical projector, a micro-processor, an image processing accelerator and a digital signal processor, the loss resulting from over-heat will be huge if either the PWM circuit  10  or the analog/digital converter  12  malfunctions to make the fan  14  stop running.  FIG. 2  illustrates the voltage of the analog signal  15  of the rotation speed controlling system, wherein V denotes voltage, T denotes time and Vout denotes the voltage of the analog signal  15 . Under normal circumstances, the analog signal  15  is between 7V and 12V. If an abnormal situation occurs, the analog signal  15  might go down to 0V. The fan  14  stops running when the analog signal  15  is 0V. The environment temperature, hence, becomes higher. It causes such important and high-price devices to malfunction once the environment temperature exceeds their operating temperature. Of course, the whole system will operate abnormal thereby. Consequently, a solution for maintaining normal operation of a fan even in abnormal circumstances is still desired.  
       SUMMARY OF THE INVENTION  
       [0008]     The primary object of this invention is to provide an apparatus for controlling a rotation speed of a fan according to an analog signal, wherein the analog signal is used to control the rotation speed of the fan. The apparatus of this invention can detect an abnormal condition according to the analog signal in order to control the fan maintaining operation when the abnormal condition causes the analog signal abnormal.  
         [0009]     The apparatus of this invention comprises a comparator and a voltage generation circuit. The analog signal has a voltage. The comparator is configured to compare the analog signal and a reference voltage to generate a comparison signal. The voltage generation circuit is configured to receive the comparison signal. The voltage generation circuit generates a particular voltage to control the rotation speed if the comparison signal indicates abnormality of the analog signal, wherein the particular voltage may be a lowest voltage for maintaining operation of the fan.  
         [0010]     The apparatus of this invention further comprises a PWM circuit, a digital/analog converter, and a protection circuit. The PWM circuit is configured to generate a modulation signal. The digital/analog converter is configured to receive the modulation signal to generate the analog signal. The protection circuit is configured to control the modulation signal according to the comparison signal; more particularly, is configured to restrain the digital/analog converter from receiving the modulation signal.  
         [0011]     Another object of this invention is to provide a method for controlling a rotation speed of a fan. The method comprises the following steps: receiving an analog signal, wherein the analog signal determines the rotation speed; receiving a reference voltage; determining whether the analog signal is lower than the reference voltage; and generating a particular voltage to control the rotation speed if the analog signal is determined lower than the reference voltage.  
         [0012]     The detailed technology and preferred embodiments implemented for the subject invention are described in the following paragraphs accompanying the appended drawings for people skilled in this field to well appreciate the features of the claimed invention. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]      FIG. 1  illustrates a rotation speed controlling system for a fan of the prior art;  
         [0014]      FIG. 2  illustrates variation of a voltage for controlling a fan of the prior art;  
         [0015]      FIG. 3  illustrates a first embodiment of the present invention;  
         [0016]      FIG. 4  illustrates a second embodiment of the present invention;  
         [0017]      FIG. 5  illustrates variation of a voltage for controlling a fan in accordance with the present invention; and  
         [0018]      FIG. 6  is a flow chart of a method in accordance with the present invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0019]     A first embodiment of the present invention shown in  FIG. 3  is an apparatus  30  for controlling a rotation speed of a fan  32  according to an analog signal. The apparatus  30  is embedded in a system, such as a projector. The fan  32  is configured to emit heat air out of the system to lower the environment temperature. The apparatus  30  comprises a PWM circuit  300 , a digital/analog converter  302 , a comparator  304 , a voltage generation circuit  306 , a protection circuit  308 , and a switch  310 . The PWM circuit  300  receives a digital signal  301  which is generated according to the environment temperature of the system. The digital signal  301  is converted to a modulation signal  303  by the PWM circuit  300 . The digital/analog converter  302  receives the modulation signal  303  via the switch  310  when the switch  310  is ON and converts the modulation signal  303  into an analog signal  305 . The analog signal  305  has a voltage, from 7V to 12V, for controlling the rotation speed of the fan  32  under a normal operation of the apparatus  30 . The rotation speed of the fan  32  becomes faster if the voltage of the analog signal  305  is increased. The rotation speed of the fan  32  becomes slower if the voltage of the analog signal  305  is decreased.  
         [0020]     One input end of the comparator  304  receives the analog signal  305  and the other input end receives a reference voltage  313 . The reference voltage  313 , providing as a basis for comparison, is 7V in this embodiment. The comparator  304  compares the voltages of the analog signal  305  and the reference voltage  313 . If the analog signal  305  is higher than the reference voltage  313 , it means that the PWM circuit  300 , the digital/analog converter  302  or other apparatuses associated with controlling the rotation speed of the fan  32  in the system operate normally. An output signal  307  of the comparator  304  has, therefore, a high level. After receiving the high level signal  307 , an output end of the voltage generation circuit  306 , i.e., a linear voltage regulator becomes high impedance; this means a signal  309  does not exist. Accordingly, the analog signal  305  drives the fan  32  to rotate. Meanwhile, the protection circuit  308  also receives the high level signal  307  and outputs a high level signal  311  to the switch  310 . The switch  310  keeps ON in response to the signal  311 . The modulation signal  303 , hence, continues inputting to the digital/analog circuit  302 .  
         [0021]     If the analog signal  305  is lower than the reference voltage  313 , it means that the PWM circuit  300 , the digital/analog converter  302  or the other apparatuses associated with controlling the rotation speed of the fan  32  operate abnormally. The output signal  307  of the comparator  304  turns to a low level. After receiving the low level signal  307 , the output signal  309  of the voltage generation circuit  306  is set 6V. The protection circuit  308  also receives the low level signal  307  and outputs the low level signal  311  to the switch  310 . This is for restraining the digital/analog converter  302  from receiving the modulation signal  303 . The switch  310  is now OFF in response to the signal  311  so that the modulation signal  303  cannot be inputted to the digital/analog converter  302 . The analog signal  305 , hence, has no level. The 6V signal  309  drives the fan  32  to rotate. Once the analog signal  305  returns normal, the protection circuit  308 , comprising a switch, will release the restraint.  
         [0022]      FIG. 4  shows a circuit diagram of a second embodiment of the present invention. In contrast with the first embodiment, the analog signal  305  of the second embodiment is divided by series resistances R 1 , R 2  and then is inputted to the comparator  304 . The reference voltage  313  is derived from Vcc divided by series resistances R 3 , R 4 . The two divided voltages inputted into the comparator  304  are used to save power. They may be selected by determining the values of the resistances R 1 , R 2 , R 3  and R 4 .  
         [0023]     Moreover, a switch control end of the switch  310  of the second embodiment is connected to Vcc via a resistance R 5 . In other words, the switch control end of the switch  310  always stays with a high voltage so that the switch  310  is always ON. Therefore, the digital/analog converter  302  can receive the modulation signal  303  via the switch  310  all the time. The protection circuit  308  is a switch, which is controlled by a signal  41  generated from the system. More specifically, when the above-mentioned abnormal condition happens, the output signal  307  of the comparator  304  has a low level. The output signal  307  is transmitted to a processor (not shown) through a MCU end. The signal  41  is transmitted to the protection circuit  308  when the processor finds the abnormality of the analog signal  305 . The signal  41  having a low level turns off the protection circuit  308 . The protection circuit  308 , hence, has no output signal, and the switch  310  keeps ON. Meanwhile, the signal  309  outputted by the voltage generation circuit  306  pulls up the voltage transmitted to the fan  32  to 6V in order to maintain a minimum rotation speed of the fan  32 .  
         [0024]     The processor resets the digital signal  301  to recover the analog signal  305  when finding the abnormality of the analog signal  305 . If the analog signal  305  returns normal, then the output signal  307  of the comparator  304  comes back to high. After the processor receives the high level output signal  307 , the signal  41  is adjusted to a high level. The protection circuit  308  turns ON. The signal  311  is high in response to the signal  307  and outputted to the switch control end of the switch  310 .  
         [0025]     As shown in  FIG. 5 , the present invention maintains the voltage for controlling the rotation speed of the fan  32  in at least 6V. Even if the PWM circuit  300 , the digital/analog converter  302  or the other apparatuses associated with controlling the rotation speed of the fan  32  operate abnormally, the fan  32  may continue running. Accordingly, the environment temperature will not increase quickly. The object of cooling is achieved.  
         [0026]     A method for controlling a rotation speed of a fan is shown in  FIG. 6 . In step  601 , the fan receives an analog signal to determine the rotation speed of the fan. Then step  603  is executed wherein an apparatus of this invention receives a reference voltage. Then step  605  is executed wherein a comparator determines whether the analog signal is lower than the reference voltage. If not, it means the analog signal is normal so the method returns to step  601  to continue receiving an analog signal. If yes, it means the analog signal is abnormal so step  607  is executed wherein a voltage generation circuit generates a particular voltage to control the rotation speed of the fan. As mentioned above, the particular voltage may be a lowest voltage for maintaining operation of the fan.  
         [0027]     The above disclosure is related to the detailed technical contents and inventive features thereof. People skilled in this field may proceed with a variety of modifications and replacements based on the disclosures and suggestions of the invention as described without departing from the characteristics thereof. Nevertheless, although such modifications and replacements are not fully disclosed in the above descriptions, they have substantially been covered in the following claims as appended.