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3 April 2010
Author: Giorgos Lazaridis
High Frequency PWM Fan Controller

The circuit on a breadboard for testing

The PWM fan controllers have many advantages against other rpm fan controllers. Yet, they tend to be difficult in construction when high frequency is required. The PWM controllers usually generate acoustic noises, when the PWM frequency is within the acoustic spectrum (20Hz to 20KHz). A high frequency PWM controller usually operates above the 20KHz, thus the human ear cannot hear this sound. Moreover, PWM controllers can achieve very stable and low speeds without the possibility of a fan stall.

The circuit that i will present to you has all the advantages of the high frequency PWM controllers, but it uses only a 555 timer to generate the pulses. Although the frequency is not very stable (come on it is just a 555 for crying out loud), it will never fall bellow 21KHz thus you will not hear a thing

The circuit

The circuit is very easy to follow. The output (pin 3) of the 555 timer will control the charging and the discharging of the capacitor. This is the only point that needs of your attention. Usually the capacitor is charged through a resistor directly connected to the power supply, and discharged through pin 7 (discharging capacitor). Now the capacitor is charged from the output of the 555 when it is HIGH, and discharge the same way when the output is LOW. The frequency is calculated from the capacitor itself ( C1 ) as well as from the total resistance of the potentiometer parallel to R3.

Here is the circuit:

The PWM pulses

The duty cycle is changed according to the position of the potentiometer. For the complete period calculation of a pulse, the total resistance of the potentiometer (in parallel to R3 of course) is calculated. But, during the charging of the capacitor, the current goes only through D1, and during discharging, the current goes only through D2. This, according to the position of the potentiometer, the charge period and the discharge period will change, but the complete period of the pulse will always be the charge period plus the discharge period! That is the whole idea of the PWM generation.

Unlike a normal 555 astable multivibrator circuit, the output is taken from pin 7. If you see the internal diagram of a 555 timer, you will see that pin 7 is connected to the discharging capacitor. But the discharging is taken over by the output of the 555, as described above. Thus this pin is free to be used. A resistor is connected to this pin and the it turns the internal transistor into a switching transistor (inverting amplifier). As a result, during the discharging time, pin 7 is logic LOW, while during the charging time pin 7 is logic HIGH. This is driven directly to the control wire of the 4-wire fan. No driving transistor is needed, as we use the internal 555 transistor for this. The rest is taken over by the fan controller itself, as the 4-wire fan carries internally the switching FET (see page "How PC Fans work").

 The 4-wire fans have a very clear RPM feedback even if they are driven with PWM pulses 17 Hz (510 rpm) is the minimum rpm predefined by the manufacturer. 64 Hz (1920 rpm) is the maximum RPM

Bill Of Materials (First circuit)
 Resistors R1 Resistor 1 KOhm 1/4 Watt 5% Carbon Film R2 1 KOhm potentiometer R3 Resistor 4.7 KOhm 1/4 Watt 5% Carbon Film Capacitors C1 0.1 uF ceramic capacitor C2 1 uF 16 Volts electrolytic capacitor Integrated Circuits IC1 555 Timer Diodes D1 1N4148 Switching Diode D2 1N4148 Switching Diode

What about the 3-wire and the 2-wire fans?

A 3-wire fan connected to the circuit on a breadboard for test

No problem... almost... You see, 4-wire fans have internally a switching FET, something that 3-wire and 2-wire fans lack of (see page "How PC Fans work"). Thus, we need to add this FET externally. The circuit is the following:

You may notice that i have add another resistor to the circuit, the R4. The R4 will prevent the fan from running at very low speeds with the possibility to stall. Actually, the circuit will now generate a minimum duty cycle of around 40%. You may change this resistor to get lower or higher initial speeds.

You need to have in mind also, that, in case you use a 3-wire fan, the feedback from the third wire cannot be used. Instead of the revolution pulses, the third wire will return the PWM pulses. If you want badly to have rpm feedback from a 3-wire fan, you should consider using other circuits:

• Switching power supply using PWM to control a 3-wire fan with rpm feedback
• PWM 3-wire fan controller with rpm feedback (pulse stretching method)
• Simple fan linear rpm controller with a potentiometer and a transistor

•  You can see the adjustments. The 330 resistor to increase the lower duty cycle and the switching FET. Due to the 330 ohm resistor, the lower duty cycle is now around 40% Using 3-wire fans along with this circuit, the feedback from the tacho cannot be useful!

This is the circuit with the modifications to operate with 2 and 3-wire fans in operation:

Bill Of Materials (Second circuit)
 Resistors R1 Resistor 1 KOhm 1/4 Watt 5% Carbon Film R2 1 KOhm potentiometer R3 Resistor 1 KOhm 1/4 Watt 5% Carbon Film R4 Resistor 330 Ohm 1/4 Watt 5% Carbon Film Capacitors C1 0.1 uF ceramic capacitor C2 1 uF 16 Volts electrolytic capacitor Integrated Circuits IC1 555 Timer Transistors Q1 IRF520 9.2A, 100V, 0.270 Ohm, N-Channel Power MOSFET Diodes D1 1N4148 Switching Diode D2 1N4148 Switching Diode