Alarm circuit

Abstract

An alarm circuit includes a fan controller with a rotation speed control pin and a rotation speed detection pin, a first rectifying circuit connected to the rotation speed control pin, a switch circuit connected to the first rectifying circuit, a first indication circuit for indicating whether the fan fails to operate based on a switch signal from the switch circuit. The alarm circuit further includes a second rectifying circuit connected to the rotation speed detection pin, a comparison circuit for outputting a comparison signal after comparing a direct current signal from the second rectifying circuit with a first reference voltage and a second reference voltage, and an indication circuit for indicating whether the fan fails to operate based on the comparison signal.

1 . An alarm circuit for a fan, the alarm circuit comprising:
 a fan controller connected to the fan to control the fan to operate, the fan controller comprising a rotation speed control pin for outputting a control signal, and a rotation speed detection pin for outputting a detection signal;   a first rectifying circuit for converting the control signal to a first direct current (DC) signal;   a switch circuit for outputting a switch signal based on the first DC signal;   a first indication circuit for indicating whether the fan fails to operate based on the switch signal;   a second rectifying circuit for converting the detection signal to a second DC signal;   a comparison circuit for outputting a comparison signal after comparing the second DC signal with a first reference voltage and a second reference voltage; and   an indication circuit for indicating whether the fan fails to operate based on the comparison signal.   
     
     
         2 . The alarm circuit of  claim 1 , wherein the first rectifying circuit comprises a first resistor, a first diode, and a first capacitor, a first terminal of the first resistor is connected to the rotation speed control pin, a second terminal of the first resistor is connected to an anode of the first diode, a cathode of the first diode is grounded through the first capacitor. 
     
     
         3 . The alarm circuit of  claim 2 , wherein the switch circuit comprises a first and a second electronic switches, and a second and a third resistors, a first terminal of the first electronic switch is connected to the cathode of the first diode, a second terminal of the first electronic switch is connected to a DC power supply through the second resistor, a third terminal of the first electronic switch is grounded, a first terminal of the second electronic switch is connected to the second terminal of the first electronic switch, a second terminal of the second electronic switch is connected to the DC power supply through the third resistor, a third terminal of the second electronic switch is grounded; wherein the first and second electronic switches are turned on in response to the voltages at the first terminals being high-level, and wherein the first and second electronic switches are turned off in response to the voltages at the first terminals being low-level. 
     
     
         4 . The alarm circuit of  claim 3 , wherein the first electronic switch is a negative-positive-negative (npn) bipolar junction transistor, the first to the third terminals of the first electronic switch are a base, a collector, and an emitter of the transistor, respectively. 
     
     
         5 . The alarm circuit of  claim 3 , wherein the second electronic switch is a negative-positive-negative (npn) bipolar junction transistor, the first to the third terminals of the second electronic switch are a base, a collector, and an emitter of the transistor, respectively. 
     
     
         6 . The alarm circuit of  claim 3 , wherein the first indication circuit comprises a fourth resistor, a second diode, a second capacitor, and a first buzzer, a cathode of the second diode is connected to the DC power supply, an anode of the second diode is grounded through the second capacitor, and is further connected to the second terminal of the second electronic switch, a first terminal of the first buzzer is connected to the DC power supply, a second terminal of the first buzzer is connected to the anode of the second diode through the fourth resistor. 
     
     
         7 . The alarm circuit of  claim 6 , wherein the second rectifying circuit comprises a fifth resistor and a third capacitor, the rotation speed detection pin is grounded through the fifth resistor and the third capacitor connected in series. 
     
     
         8 . The alarm circuit of  claim 7 , wherein the comparison circuit comprises a first and a second comparators, and a sixth to a ninth resistors, a non-inverting input of the first comparator is connected to the DC power supply through the sixth resistor, and is further grounded through the seventh resistor, an inverting input of the first comparator and a non-inverting input of the second comparator are connected to a node between the fifth resistor and the third capacitor. 
     
     
         9 . The alarm circuit of  claim 8 , wherein the second indication circuit comprises a tenth resistor, a third diode, a fourth capacitor, and a second buzzer, a cathode of the third diode is connected to the DC power supply, an anode of the third diode is grounded through the fourth capacitor, and is connected to outputs of the first and the second comparators, a first terminal of the second buzzer is connected to the DC power supply, a second terminal of the second buzzer is connected to the anode of the third diode through the tenth resistor.