Abstract:
A circuit for controlling a rotation speed of a fan of an electronic device according to a temperature of the electronic device. The circuit senses the temperature of the electronic device, and outputs a voltage changing with the sensed temperature. The rotation speed of the fan changes with the voltage. The circuit slows the rotation speed of the fan down when the sensed temperature of the electronic device is decreased.

Description:
BACKGROUND 
     1. Technical Field 
     The present disclosure relates to controlling circuits, and more particularly to a circuit for controlling rotation speed of a fan of an electronic device. 
     2. Description of Related Art 
     Various electronic devices, such as computers, game players, etc., generate heat when operating. These electronic devices may be damaged if the heat is not dissipated in a timely fashion. Generally, fans are used to facilitate removal of heat to keep the temperature of the electronic devices within safe temperature ranges. The temperatures of the electronic devices may be changeable. It is not energy efficient if the fans speed cannot be adjusted according to the temperatures of the electronic device. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a first embodiment of a circuit for controlling a rotation speed of a fan of an electronic device. 
         FIG. 2  is a second embodiment of a circuit for controlling the rotation speed of the fan of the electronic device. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIG. 1 , a first embodiment of a circuit  1  is to control a rotation speed of a fan  10  of an electronic device. The circuit  1  includes a temperature sensor  100 , and a rotation speed adjusting circuit  200 . 
     The temperature sensor  100  includes a thermistor RT, and a voltage divider connected between the thermistor RT and ground. The voltage divider includes two resisters R 1  and R 2  connected in series between the thermistor RT and ground. The thermistor RT is a negative temperature coefficient (NTC) thermistor. 
     The rotation speed adjusting circuit  200  includes an electronic switch Q 1 , a resistor R 3 , and a zenzer diode ZD 1 . In this embodiment, the electronic switch Q 1  is a bipolar junction transistor (BJT) having a base connected to a node between the two resistors R 1  and R 2 , a collector connected to a negative terminal of the fan  10 , and an emitter grounded. A positive terminal of the fan is connected to a power supply Vcc. An anode of the zenzer diode ZD 1  is grounded. A cathode of the zenzer diode ZD 1  is connected to the collector of the electronic switch Q 1  via the resistor R 3 . In other embodiments, the electronic switch Q 1  may be a metal oxide semiconductor field effect transistor (MOSFET). 
     The thermistor RT senses a temperature of the electronic device. The base of the electronic switch Q 1  receives a first voltage Vb from the voltage divider. The first voltage Vb is determined according to the equation: Vb=Vcc*r 2 /(rt+r 1 +r 2 ), wherein rt, r 1 , and r 2  are resistances of the thermistor RT, the resistor R 1 , and the resistor R 2  respectively. The resistance rt of the thermistor RT decreases with increasing temperature of the electronic device. When the resistance rt of the thermistor RT decreases, the first voltage Vb increases, which makes a base current of the electronic switch Q 1  increase. It can be determined from the output characteristic of BJTs that a collector current of the electronic switch Q 1  increases with increasing of the base current. Current flowing through the fan  10  increases since the current flowing through the fan  10  is equal to the collector current of the electronic switch Q 1 . Therefore, the fan  10  is driven to rotate faster. 
     On the contrary, when the temperature of the electronic device decreases, the resistance rt of the thermistor RT increases. The first voltage Vb decreases to decrease the base current of the electronic switch Q 1 . The current flowing through the fan  10  decreases to slow down the rotation speed of the fan  10 . 
     Referring to  FIG. 2 , a second embodiment of a circuit  2  is to detect and control the rotation speed of the fan  10  of the electronic device. The circuit  2  includes the temperature sensor  100 , the rotation speed adjusting circuit  200 , and further includes a rotation speed detector  300 , and a processor  400 . 
     The rotation speed detector  300  includes a rotation speed sensor  20 , a resistor R 4 , an electronic switch Q 2 , and a zener diode ZD 2 . A first input of the rotation speed sensor  20  is connected to the positive terminal of the fan  10 . A second input of the rotation speed sensor  20  is connected to the negative terminal of the fan  10 . The electronic switch Q 2  is a BJT having a base connected to an output of the rotation speed sensor  20 , a collector connected to the first input of the rotation speed sensor  20  via the resistor R 4 , and an emitter grounded. An anode of the zener diode ZD 2  is connected to the emitter of the electronic switch Q 2 , and a cathode of the zener diode ZD 2  is connected to the collector of the electronic switch Q 2 . The collector of the electronic device Q 2  is connected to the processor  400 . In other embodiments, the electronic switch Q 2  may be a MOSFET. 
     The rotation speed detector  20  monitors the rotation speed of the fan  10  by detecting the current flowing through the fan  10 , and outputs a second voltage to the base of the electronic switch Q 2 . The electronic switch Q 2  outputs a monitoring signal according to the second voltage. The monitoring signal indicates changes in the rotation speed of the fan  10 . For example, when the rotation speed of the fan  10  increases, the second voltage increases. A base current of the electronic switch Q 2  increases. The collector of the electronic switch Q 2  then outputs the monitoring signal to the processor  400 , indicating that the rotation speed of the fan  10  increases. In this embodiment, the zener diode ZD 2  is used to protect the electronic switch Q 2  from being damaged by an over voltage between the collector and the emitter of the electronic switch Q 2 . 
     The processor  400  receives and processes the monitoring signal. For example, the processor  400  may convert the monitoring signal to a display signal to indicate the changes in the rotation speed of the fan  10  on a monitor of the electronic device. 
     The foregoing description of the exemplary embodiments of the disclosure has been presented only for the purposes of illustration and description and is not intended to be exhaustive or to limit the disclosure to the precise forms disclosed. Many modifications and variations are possible in light of the above everything. The embodiments were chosen and described in order to explain the principles of the disclosure and their practical application so as to enable others of ordinary skill in the art to utilize the disclosure and various embodiments and with various modifications as are suited to the particular use contemplated. Alternative embodiments will become apparent to those of ordinary skills in the art to which the present disclosure pertains without departing from its spirit and scope. Accordingly, the scope of the present disclosure is defined by the appended claims rather than the foregoing description and the exemplary embodiments described therein.