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:
@Mark Main 21KHz is the frequency for the PWM pulses. The other frequencies are for the fan speed (yellow wire). You can ground pin 5 through a 10pf capacitor if you want, not directly.
For more info on 555 read my theory
http://pcbheaven.com/wikipages/555_Theory
You wrote that the frequency \"will never fall bellow 21KHz thus you will not hear a thing\" and later cite \"17 Hz (510 rpm) is the minimum rpm predefined by the manufacturer\" and \"64 Hz (1920 rpm) is the maximum RPM\" in the pictures. I\'m confused by this point.
Also, would it be ok to Ground pin 5 on this design?
How do we calculate what the frequency will be? Thanks for a nice job on this by the way.
@milkshakes the 555 should not overheat, not even get hot. There is a problem in the connection, or the fan you use sinks lot of current in the 4th pin. Increase R1 to 1.5K and put an additional resistor between pin 7 and r1, about 150 to 220 ohms. If still overheats you have bad connection somewhere
ok,so i made the circuit.it controls the fans speed (i tested it with 4 fans) but i noticed that the 555 timer gets very hot.do i need a different 555 or should i put a heat-sink on it?
thanks.
@jon you can directly connect R1 to 5v instead of 12, but i do not think that this is the case. All the fans i've used work pretty fine.
At 29 September 2011, 21:57:13 user jon wrote: [reply @ jon]
I don't have the oscilloscope to monitor the frequency but pretty much sure that it works coz the voltage varies and base on the online calculator will achieve the required frequency. thanks
At 29 September 2011, 21:47:28 user jon wrote: [reply @ jon]
I'm using a 4 wires TM-CF06 cpu fan, substitute for LGA CPU Cooler. the same color code in http://pcbheaven.com/wikipages/How_PC_Fans_Work/
(black, red, yellow, blue
thanks for the help
At 29 September 2011, 0:39:02 user jon wrote: [reply @ jon]
hi,
i followed your 1st circuit but couldn\'t control the speed. I also learned from your link (4 wire pwm spec) that the max voltage capacity pwm output signal is 5.25v. pin 7 exceeded to its max voltage requirement. did I destroyed my fan\'s control input? it is still function directly using ground and 12v.
@cuelebre oh, i see. No you cannot do it that simple. For this reason, i have design this:
http://pcbheaven.com/circuitpages/Voltage_Controlled_PWM_Generator
You will replace the 1.5K resistor from the dc voltage input with your thermistor.
@p00chie i suppose that you use circuit #2, with the IRF520. The IRF has a 0.3ohms resistance, so to have a 1.3 volts drop, the motors have to draw 4.3 amperes (2.1 ampere each motor). If this is the case, then you can do nothing else than to increase the supply voltage up to 13.3 volts.
If your motors do not draw that much current, then i suppose that the circuit never reaches 100% duty cycle. There is one simple test to do: remove the wire from pin7 of the 555. What happens? You should hear the fans run at full speed. Is this correct?
I\'m using this circuit to control 2 fans. Works great, however, I\'ve noticed the fans do not run at full speed... they go I bit faster when connected to the 12vdc power supply bypassing the circuit. I measured the output to the fans when using the circuit at max speed and I get 10.7/.8 volts.. is there a way I can avoid that 1.3 drop?
@milkshakes 8 fans will not be a problem. You have to make a small change though. You have to add a pull-up resistor for each fan PWM control wire (usually it is blue). In the schematic i have only one pull-up, that will be R1 (1K). But you have to use one pull-up for each separate fan. This resistor must be 4.7K and not 1K (as i have now).
the fans that i want to use are the 4 wire type.
how do i find the minimum current,do i have to rip the fan to pieces or is there a safer way (for the fan) to find it?
i want to control between 4 and 8 coolermater xtraflow fans.
i havent decided on a 555 yet,havent even had a look,could you give me a max current that one can output?
@milkshakes for 4-wire fans, i cannot answer, because i have to know the minimum current required for each fan for the PWM input, as well as the maximum current the 555 can sink through the discharrge pin (7).
For 3-wire fans, it can control as much as the Q1 can handle. I use a 9A mosfet. Average, a PC fan draws some 400mA, so i could run 20-22 fans.
@Jerrywaz yes it can, but a triac will not operate normally as you expect. if you want to make a dimmer for AC circuit, better see my other circuits that i have an AC dimmer. Triacs are meant to work mainly with AC
You can go as high as you like, but the higher you go, the less rpm band control you have. It is up to you. Of course, you need to go high enough for the fan not to stall! This is important. Regarding the capacitor, you can add the small capacitor as well with no problem. It will filter higher frequency. And yes, it is good for the power supply of the PC. But the power supply (and everything that is connected to it) has its own filters, so this is not absolutely necessary.
I'm using the second circuit to control a 120mm, 2-wire, ball bearing, brushless fan in my computer.
When I turn the pot all the way down, and then stop the fan with my fingers, it doesn't start back up, even if I change R4 to 2500 Ohms. Is it ok to go higher? Is there a limit, or should I change something else?
Also, I read that it is good to put a 0.1uF poly metal or ceramic capacitor parallel to C2 (supply to the 555). Is that ok to do?
Am I right that C2 (and the 0.1uF ceramic - if I add it) relieve the stress on my computers' power supply when the 555 is changing from HIGH to LOW?
Rainer, you do not increase or decrease the resistance. What you need to do is test first before you wire permanently.
At 29 January 2011, 12:39:23 user Rainer wrote: [reply @ Rainer]
Hi,
As far as I understood the circuit R2 is used for changing the Fan speed.
Question: If I increase the resistance what does the fan do ? Increase or decrease the speed ? I would like to know it so I can wire the Potentiometer in that way that by turning CW the fan speed increases and CCW the fan speed decreases.
I built the first circuit and killed a fan with it. PWM fans are rated with a max of 5.25 volts on the control line, so 11.6 volts at 100% dutycycle kills the internals of the fan.
Also it didn´t worked for 4 of the 9 fans i´ve tested.
And I´ll get some really annoying high pitch noise...
If you read the 2 last paragraphs you will see why. The lower duty cycle is about 40%, so to prevent the fan from stalling. Read the paragraphs and you will see how you can change this value to almost 0.
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