Abstract:
A radiator includes a voltage regulator for providing a reference voltage, a fan including a power end connected to the reference voltage via a first resistor and a feedback end for outputting a pulse signal indicating the rotation speed of the fan, an integration circuit including an output end, and an input end connected to the feedback end of the fan for converting the pulse signal from the feedback end into a voltage signal, and a thermistor connected between the output end of the integration circuit and the reference voltage, for detecting temperature changes in order to adjust the rotation speed of the fan.

Description:
BACKGROUND OF INVENTION 
   1. Field of the Invention 
   The present invention relates to a radiator, and more specifically, to a radiator having a fan in variable rotation speed. 
   2. Description of the Prior Art 
   Common radiators include cooling fins and fans. Most of the cooling fins are composed of aluminum alloy while a small number uses other materials, but all of them have almost the same heat conductivity. Besides composing materials, the performance of a cooling fin depends also on its surface area. A cooling fin conducts heat to its surface so that the air can bring the heat away, thus the larger the surface area is, the better the performance of the cooling fin is. However, the cooling fin does not work well if the air flow is insufficient even if it has a large surface area, thus for a better performance, a fan promoting air flow is necessary. Generally, the higher the rotation speed, the better the performance of the fan. That is because the fan accelerates the air flow in high rotation speed so that the air can bring more heat away. The rotation speed of the fan can be known by its power consumption. A fan consuming more power rotates faster. 
   Please refer to  FIG. 1  showing a conventional radiator  10 . The radiator  10  includes a thermal sensor  12 , a microcontroller  14 , a driver circuit  16  and a fan  18 . The interconnection between devices of the radiator  10  is shown in  FIG. 10 . Generally the radiator  10  is installed in a system, the thermal sensor  12  is for sensing the temperature of the system, the microcontroller  14  compares the temperature sensed by the thermal sensor  12  with a predetermined temperature, and the driver circuit  16  turns on the fan when the temperature sensed by the thermal sensor  12  exceeds the predetermined temperature. The driver circuit  16  can output different voltages according to the requirements by the microcontroller  14  to control the rotation speed of the fan  18 , and the fan  18  has a signal line connected to the driver circuit  16  for outputting a speed signal of the fan  18 . Whenever the thermal sensor  12  senses a temperature raising, the microcontroller  14  requires the driver circuit  16  to speed up the fan  18 , so that the driver circuit  16  raises up the output voltage to the fan  18 , and when the speed of the fan  18  is raised up, the signal line transmits the speed signal back to the driver circuit  16 . And if the thermal sensor  12  senses a decrease in temperature, the speed of the fan  18  should be lowered down to conserve power, so that the microcontroller  14  requires the driver circuit  16  to lower down the output voltage to the fan  18 , and the driver circuit  16  knows the speed of the fan  18  by the signal line. 
   As mentioned above, the conventional radiator capable of controlling the rotation speed of the fan  18  uses the thermal sensor  12  to sense the environmental temperature, the microcontroller  14  compares the temperature sensed by the thermal sensor  12  with the predetermined temperature and requires the driver circuit  16  to control the speed of the fan, and the driver circuit  16  to compare the feedback speed signal of the fan  18  with the speed signal required by the microcontroller  14  in order to control the output voltage to the fan  18  to change its rotation speed, so that the speed of the fan  18  can be adjusted according to the environmental temperature. However, the radiator  10  requires the thermal sensor  12 , the microcontroller  14  and the driver circuit  16  which raise the cost. In addition, the temperature comparison by the microcontroller  14  and the rotation speed comparison by the driver circuit  16  also lower down the sensibility of the radiator  10 . 
   SUMMARY OF INVENTION 
   It is therefore a primary objective of the present invention to provide a radiator having a fan with variable rotation speed in order to solve the problems mentioned above. 
   Briefly summarized, a radiator includes a voltage regulator for providing a reference voltage, a fan including a power end connected to the reference voltage via a first resistor and a feedback end for outputting a pulse signal indicating the rotation speed of the fan, an integration circuit including an output end, and an input end connected to the feedback end of the fan for converting the pulse signal from the feedback end into a voltage signal, and a thermistor connected between the output end of the integration circuit and the reference voltage, for detecting temperature change in order to adjust the rotation speed of the fan. 
   These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings. 

   
     BRIEF DESCRIPTION OF DRAWINGS 
       FIG. 1  illustrates a conventional radiator. 
       FIG. 2  is a circuit diagram of a radiator according to the present invention. 
       FIG. 3  illustrates the relationship between Vo and Vx. 
       FIG. 4  illustrates the relationship between Vo and Rt. 
   

   DETAILED DESCRIPTION 
   Please refer to  FIG. 2  showing a circuit diagram of a radiator  20  according to the present invention. The radiator  20  includes a voltage regulator  22 , a fan  24 , a first resistor  26 , a second resistor  28 , a third resistor  30 , a capacitor  32  and a thermistor  34 . The interconnection of these devices is shown in  FIG. 2 . An output end of the voltage regulator  22  provides a stable reference voltage, first ends of the first resistor  26 , the second resistor  28  and the thermistor  34  are connected to the output end of the voltage regulator  22 , and a second end of the second resistor  28  is grounded for providing a stable current. The fan  24  has a power end, a ground end and a feed back end, a second end of the first resistor  26  is connected to the power end of the fan  24  for providing operational voltage to the fan  24 . A first end of the third resistor  30  is connected to a first end of the capacitor  32 , and a second end of the capacitor  32  is grounded to form an integration circuit  36 . An output end of the integration circuit  36  is the first end of the third resistor  30  connected to a second end of the thermistor  34 , and an input end of the integration circuit  36  is a second end of the third resistor  30  connected to the feedback end of the fan  24 . A speed pulse signal of the fan  24  outputted by the integration circuit  36  becomes direct current (DC) voltage. On node r in  FIG. 2 , a formula can be obtained according to Kirchhoff s current law (KCL):
 
( Vo−Vr )/ R   1 +( Vx−Vr )/ Rt−Vr/R   2 =0  formula (1)
 
   Vo, Vr, Vx are voltages of node o, r and x. Vo is an input voltage of the fan  24 , Vr is an output voltage of the voltage regulator  22 , Vx is a feedback voltage output by the integration circuit  36 . R 1 , Rt, R 2  are resistances of the first resistor  26 , the thermistor  34  and the second resistor  28 . Under a fixed temperature, Rt is also fixed so that formula (1) can be simplified as follows:
 
 Vo =(1 +R   1 / Rt+R   1   /R   2 ) Vr −( R   1 / Rt ) Vx   formula (2)
 
   If the rotation speed of the fan  24  is fixed, Vx is also fixed so that formula (1) can be simplified as follows:
 
 Vo =(1 +R   1   /R   2 ) Vr −( R   1 / Rt )( Vx−Vr )  formula (3)
 
   Please refer to  FIG. 3  showing the relationship between Vo and Vx, and  FIG. 4  showing the relationship between Vo and Rt. Under a fixed temperature, Rt is also fixed and formula (2) has only two variables, which are Vo and Vx, while other parameters can be regarded as constants. Define a=(1+R 1 /Rt+R 1 /R 2 )Vr, b=(R 1 /Rt), and formula (2) can be simplified as Vo=a−bVx. The relationship between Vo and Vx is shown in  FIG. 3 , when Vo increases, Vx decreases, that means when the fan  24  rotates fast, the feedback end of the fan  24  will output pulse signals in longer period and a smaller voltage will output the integration circuit  36 , and when the fan  24  rotates slowly, the feedback end of the fan  24  will output pulse signals in shorter period and a larger voltage will output the integration circuit  36 . In such a manner the relationship between the rotation speed of the fan  24  and the output signal from the feedback end can be known. And if the rotation speed of the fan  24  is fixed, Vx is also fixed so that formula (3) has only two variables, which are Vo and Rt, while other parameters can be regarded as constants. Define c=(1+R 1 /R 2 )Vr, d=R 1 (Vx−Vr), and formula (3) can be simplified as Vo=c−d/Rt. The relationship between Vo and Rt is shown in  FIG. 4 , when Rt increases, Vo also increases, that means the resistance of the thermistor  34  increases according to the temperature, because the fan  24  speeds up when Vo increases. In such a manner the relationship between the thermistor  34  and the temperature can be known.  FIG. 3  and  FIG. 4  indicate the characteristics of the fan  24  and the thermistor  34  of the radiator  20 . First, the pulse signals from the feedback end of the fan  24  decreases when the rotation speed increases. Second, the resistance of the thermistor  34  increases according to the temperature. 
   The operation of the radiator  20  is described as follows. The radiator  20  is installed in a system in order to keep the temperature T of the system in a reasonable range. When the radiator is activated, the voltage regulator  22  provides the reference voltage Vr, and the input voltage Vo 1  of the fan  24  is generated. The speed signal Vx 1  of the fan  24  can be obtained by formula (2), and the initial temperature T 0  of the system determines the resistance Rt 0  of the thermistor  34 . The input voltage Vo 2  of the fan  24  can be obtained by formula (3), and under the initial temperature T 0 . The speed signal Vx 2  of the fan  24  can be obtained by formula (2), and the input voltage Vo 2  of the fan  24  keeps the fan  24  rotate in a fixed speed. When the system operates, the temperature rises from T 0  to T 1 , and accordingly, the resistance of the thermistor  34  rises from Rt 0  to Rt 1 . By formula (3) we can know Vo 2 &gt;Vo 1 , so that the input voltage of the fan  24  rises from Vo 1  to Vo 2 , that means the fan  24  rotates faster, and by formula (2) we know Vx 2 &lt;Vx 1 . After the fan  24  is accelerated for a while, the temperature of the system falls down from T 1  to T 0 , and accordingly the resistance of the thermistor  34  falls down from Rt 2  to Rt 1 , and the input voltage of the fan  24  falls down to Vo 1 , the speed signal of the fan  24  returns to Vx 1 . After the fan  24  lowers down, since the system keeps on operating, the temperature rises again after a period of time. With such kind of operation, the system can be prevented from overheating and the efficiency of the fan  24  is also increased. As mentioned above, the flow of the operation is as follows: T increases=&gt;Rt increases=&gt;Vo increases=&gt;Vx decreases=&gt;T decreases=&gt;Rt decreases=&gt;Vo decreases=&gt;Vx increases=&gt;T increases 
   The resistance increase of the thermistor  34  according to the temperature is analog. Whenever the resistance rises up or falls down, the input voltage of the fan  24  will changes accordingly so that the rotation speed of the fan  24  changes precisely according to the temperature. However, if the thermistor  34  reacts only when a larger temperature change occurs, the input voltage of the fan  24  and the speed signal will keep balance by formula (2). 
   As described above, the radiator  20  uses the thermistor  34  for sensing the temperature, and since the thermistor  34  changes its resistance according to the temperature, the input voltage of the fan  24  can be changed according to the temperature in order to have the fan  24  rotate in different speeds according to different temperatures. In the present invention, the radiator  20  uses the thermistor  34  with its resistance increasing according to the temperature, and the fan  24  with the feedback end. The feedback end of the fan  24  lowers the pulse signal down when the rotation speed increases, and the integration circuit  36  puts the pulse signal as the feedback voltage out. By the first resistor  26  and the second resistor  28 , the input voltage of the fan  24  changes according to the feedback voltage. When the temperature rises up, the input voltage of the fan  24  also rises up so that the fan  24  rotates faster for better heat dissipation. When the temperature goes down, the input voltage of the fan  24  lowers down so that the fan  24  rotates slower in order to conserve power. 
   In contrast to the prior art, the radiator, according to the present invention, utilizes the thermistor with resistance changing according to the temperature to change the input voltage of the fan according to the temperature, so that the fan rotates faster as the temperature increases. On the other hand, the conventional radiator requires the thermal sensor, the microcontroller and the driver circuit and also changes the rotation speed by comparing the temperature with the rotation speed. These active devices not only increase the cost, but also raise the probability of misjudgment. The radiator, according to the present invention, uses low cost passive devices such as the resistor and the capacitor. Furthermore, the thermistor changes its resistance according to the temperature by its own material characteristics, so that misjudgment may not be done. 
   Those skilled in the art will readily observe that numerous modifications and alterations of the method may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.