Fan control circuit

1. A fan control circuit comprising:

a first resistor and a second resistor;

a fan comprising a control pin;

a hardware controller connected to the control pin of the fan;

a rectifier circuit connected to the hardware controller;

a first electronic switch comprising a first terminal connected to the rectifier circuit, a second terminal connected to a direct current (DC) power supply through the first resistor, and a third terminal grounded;

a second electronic switch comprising a first terminal connected to the second terminal of the first electronic switch, a second terminal connected to the DC power supply through the second resistor, and a third terminal grounded; and

a third electronic switch comprising a first terminal connected to the second terminal of the second electronic switch, a second terminal connected to the control pin of the fan, and a third terminal connected to the DC power supply;

wherein when the hardware controller outputs a pulse width modulation (PWM) signal to the rectifier circuit and the control pin of the fan, the rectifier circuit transforms the PWM signal into a DC voltage and outputs the DC voltage to the first electronic switch, the first electronic switch is turned on, the second and third electronic switches are turned off, a rotation speed of the fan is controlled by the PWM signal received from the hardware controller; and

wherein when the hardware controller does not output the PWM signal, the rectifier circuit does not output the DC voltage, the first electronic switch is turned off, the second and third electronic switches are turned on, the DC power supply supplies power to the control pin of the fan through the third electronic switch, and the fan rotates at full speed.

2. The fan control circuit of claim 1 , wherein the rectifier circuit comprises a diode, a capacitor and a third resistor, the diode comprises an anode connected to the hardware controller, and a cathode grounded through the third resistor and the capacitor in that order.

3. The fan control circuit of claim 2 , further comprising a fourth resistor, wherein the first terminal of the first electronic switch is connected to a node between the third resistor and the capacitor, and is grounded through the fourth resistor.

4. The fan control circuit of claim 3 , wherein the first electronic switch is an npn-type bipolar junction transistor (BJT), and the first terminal, the second terminal, and the third terminal of the first electronic switch respectively a base, a collector, and an emitter of the npn-type BJT.

5. The fan control circuit of claim 1 , further comprising a light emitting diode (LED) comprising an anode connected to the DC power supply through the second resistor, and a cathode connected to the second terminal of the second electronic switch, wherein when the second electronic switch is turned on, the LED is lit up.

6. The fan control circuit of claim 5 , further comprising a fifth resistor, wherein the first terminal of the third electronic switch is connected to the second terminal of the second electronic switch through the fifth resistor.

7. The fan control circuit of claim 6 , wherein the second electronic switch is an npn-type BJT, and the first terminal, the second terminal, and the third terminal of the first electronic switch respectively a base, a collector, and an emitter of the npn-type BJT.

8. The fan control circuit of claim 6 , wherein the third electronic switch is a pnp-type BJT, and the first terminal, the second terminal, and the third terminal of the third electronic switch respectively a base, a collector, and an emitter of the pnp-type BJT.