Speed control circuit of brushless DC fan motor

Abstract

There is provided a brushless DC fan motor which can eliminate an oscillator for PWM, control the motor speed at a low cost with high accuracy, and efficiently adjust the temperature in a housing for electronic appliances when radiating the heat in the housing. The brushless DC fan motor with the rotational speed thereof controlled by controlling the voltage of a control input terminal of a drive circuit comprises a differential amplifier in which the voltage signal for controlling the speed is inputted in a first input terminal and the reference voltage signal is inputted in a second input terminal. The differential amplifier is linear in the input-output characteristic, and can set the rise characteristic of a desired gradient, and control the motor speed with high accuracy without using the PWM signal for the input signal. The voltage signal from the output terminal of the differential amplifier is given to the control input terminal of the drive circuit.

What is claimed is:  
     
         1 . A speed control circuit of a brushless DC fan motor which is driven by a drive circuit, and controlled in the rotational speed thereof by controlling the voltage of a control input terminal of the drive circuit comprising: 
 a differential amplifier in which the voltage signal for controlling the speed is inputted in a first input terminal and the reference voltage signal is inputted in a second input terminal,    wherein the differential amplifier is linear in the input-output characteristic, and can set a rise characteristic of a desired gradient, and give the voltage signal from the output terminal of the differential amplifier to the control input terminal of the drive circuit.    
     
     
         2 . A speed control circuit of a brushless DC fan motor according to  claim 1 , wherein the reference voltage signal of the differential amplifier is given from a resistance voltage division point of a resistance voltage divider connected between a power source for driving the circuit and the ground, and the resistance of the resistance voltage divider is set to be an appropriate value in order to obtain a desired minimum rotational speed.  
     
     
         3 . A speed control circuit of a brushless DC fan motor according to  claim 1  or  claim 2 , wherein the voltage signal for controlling the speed is inputted in the first input terminal of the differential amplifier via a first resistor for setting the amplification degree, a second resistor for setting the amplification degree is connected between the first input terminal and the output terminal of the differential amplifier, a desired amplification degree is obtained for the differential amplifier by suitably setting the resistance of the first and second resistors for setting the amplification degree, and a desired rate of change in the rotational speed is set.  
     
     
         4 . A speed control circuit of a brushless DC fan motor according to any one of  claims 1  to  3 , wherein the first input terminal and the second input terminal of the differential amplifier are grounded individually via a capacitor.  
     
     
         5 . A speed control circuit of a brushless DC fan motor according to any one of  claims 1  to  4 , wherein the voltage signal from the output terminal of the differential amplifier is given to the control input terminal of the drive circuit via a resistor for finely adjusting the rotational speed.  
     
     
         6 . A speed control circuit of a brushless DC fan motor according to any one of  claims 1  to  5 , wherein a voltage signal transmission line from the output terminal of the differential amplifier to the control input terminal of the drive circuit is grounded at a desired point via a capacitor.