The vast majority of PCs has at least one of them. They carry the heavy load to keep your PC cool and functional, either by providing fresh air in the box, or by forcing the hot air to leave a hot surface by pushing cool air. Read the following article to learn how the BLDC PC Fans operates...
What is inside a PC fan?
There are may types of PC fans that are assembled in different ways. In this article, i will explain the basic and most common fan type. The fan that i study is a 3-wire 4 coils 80mm fan rotating at 2200 rpm. Then i will explain some other common fans.
First of all i had to disassemble the fan. I am not the right person to disassemble something for the first time due to lack of patience. During the disassembling, i broke some parts of the housing and a fin. Still i did not find any way of easy disassembling. I suppose that the fan i chosen (and maybe many others) are NOT to be disassembled and re-assembled. Anyway, let's see what's inside a PC fan:
A victim in the name of science
Removing the fins from the housing, the controller is revealed
The rotor, the stator and the controller
It is more than obvious that the PC fan is not rotated from a simple DC motor. It has the permanent magnets fixed on the rotor, the stator carries the coils, there are no brushes, it has a controller... the sun is shining... it is of course a brushless motor. I have written a detailed theory about brushless motors. You can find it in the "Theory of operation of brushless motors" page.
Some different PC fan types
As long as the motor is concerned, i suppose that all PC fans use brushless motors. There are several reasons that a brushless motor should be used, among them is the reliability, the power efficiency and the rpm feedback. So the motor type would not be the proper way to categorize PC fans. Instead, i will categorize them with the most obvious characteristic: their connector.
There are actually 3 different types of PC fans. Those with a 2-pin connector, those with a 3-pin connector and those with a 4-pin connector. Let's see them one by one:
2-wire PC Fans
A 2-wire PC fan
These fans have usually a male-female molex 4-pins connector from where their power supply is drawn.
These are the oldest and most simple PC fans. Only two wires comes out out of the fan controller, the positive and the negative. Giving power to the fan, it will rotate at full speed. The internal diagram of a typical two-wire fan is as follows:
The connector of a 2-wire fan has a red and a black cable. The red cable goes to the positive of the power supply and the black to the negative. Usually, for more flexibility, they have a male-female 4-wire molex power connector. In one end of the connector the fan is connected in parallel with the 12V (YELLOW - BLACK). Therefore, the fan is powered normally and the cable of the PSU can be used to power another device.
3-wire PC Fans
A 3-wire PC fan
Yet another 3-wire PC fan with different wire colors
A very common type of PC fan. These fans introduced the "tacho" for the first time. The first two wires are the power supply of the fan. The third wire, comes directly from the output of the Hall sensor. This output generates 2 pulses per one revolution of a fan. The fan is then connected to the motherboard. From the third wire, the motherboard can "read" the tacho of the fan and see if the fan is running and with how many RPMs! It is a great innovation! If the motherboard sees no pulses or very low rpm, then the characteristic buzzer sounds to inform the operator that something is not ok. The internal diagram of a typical three-wire fan is as follows:
It seems that for once more, the manufacturers did not have the same wire provider, or their wire providers did not have the same colored-plastic provider... Two fans with 3-wire connectors may not have the same wire colors. Thus, instead of using the colors to distinguish the function, better go with the connector that is standard. No matter what color the cable has, it will be plugged in the same motherboard connector! So,as you look from the key-side of the connector, number 1 is the most left pin
1: Negative power supply
2: Positive power supply
BLACK: Negative RED: Positive YELLOW: Tacho
BLACK: Negative YELLOW: Positive GREEN: Tacho
4-wire PC Fan
This is the most modern type of PC fan. This fan is designed to be controlled with a PWM signal and increase or decrease its RPM. All fans actually can be controlled with PWM, but this particular type can also provide tacho feedback simultaneously, something that the 3-wire fan cannot do -under normal circumstances. The 3-wire fan powers the Hall sensor and the controller from the same line that the coils are powered. Thus, if someone tries to send PWM pulses to the coils of a 3-wire fan, the same pulses will arrive at the controller. The controller will then malfunction, because it needs constant current to operate. As a result, the third wire will not provide correct readings.
Unlike the 3-wire fans, the 4-wire fans have a slight change that eliminates this problem. The controller and the Hall sensor are always powered with constant current. A transistor (fet) is placed before the coils. The base of the transistor is actually the fourth wire. So, the PWM pulses are driving the transistor. The coils receive these pulses through the transistor, but the controller along with the Hall sensor are not affected at all. This change can be seen in the internal diagram of a typical 4-wire fan:
Usually, the diagram is more complicated than this. This is to give you an idea about the principle of operation of the PC PWM Fans (as used to be called). The controller actually checks the PWM input pulses and sends pulses to the transistor accordingly. If the PWM Duty cycle is bellow a threshold value, then the fan either shuts down, or it remains in a stable 'LOW" rpm. There are also fans that even with 0% duty cycle, they keep on running at this 'LOW' speed. This is usually done in critical applications that even if the external controller fails to operate, the internal fan controller will bypass the signal and will keep the fan running.
As for the pinout... Just do not trust the colors. As you look from the key-side of the connector, number 1 is the most left:
1: Negative power supply
2: Positive power supply
4: PWM control
BLACK: Negative YELLOW: Positive GREEN: Tacho BLUE: PWM Control
BLACK: Negative RED: Positive YELLOW: Tacho BLUE: PWM Control
Can I connect a 3-wire fan to a 4-wire connector?
Yes you can. If you notice the pinout of the fans, the 3 first pins are the same for the 3 and 4 wire fans. Also, the keys are the same for both connectors. The 4-wire connector has smaller back-key to accept the smaller 3-wire fan connector keys. The fan will always run at full speed (as the control pin will not be used), but the rpm feedback (tacho) of the fan will operate normally and the motherboard will read the rpm normally.
Can I connect a 4-wire fan to a 3-wire connector?
A 4-wire fan connected to a 3-wire connector. No problem!
Yes (and no) and Yes. Although the connector is larger, the keys of the 4-wire fan have the same distance as the 3-wire connector. The fan will operate at full speed all the time, as the 4th wire from the PWM control will be on air. The motherboard will normally read the rpm feedback from the fan tacho.
Now, i am not quite sure about the internal connectivity of the 4-wire fans. No matter how may fans i tested, and no matter how many sites i visited, i found no clue that this configuration will fail. This means that the PWM control line must have an internal pull-up resistor, so that when the pin is unconnected, the control FET will be kept always ON. There could be a manufacturer though that maybe felt like making something different, and either he removed this resistor or replaced the FET with different channel than normal. I do not know why anyone would do this! But i have not find anywhere a norm that declares if the fan MUST operate or not with the 4th wire unconnected. A site-reader sent me a link from Intel (click) which definitely shows that a 4-wire fan can be connected to a 3-pin connector, so i suppose that this is the norm i was looking for. Thanks a lot Mysteron347.
Anyway, what you only have to do is to test it. Either plug it in and see what happens, or give power yourself to the first 2 pins (negative and positive). If it rotates, then no problem!
Hi, for typical computer fans, how many phase does the brushless dc motor have? Initially, I thought 3 phase (standard) but then was reading other
websites and said 3 phase brushless dc motors are big.
my knowledge of electrical problems is quite limited but I would like to hook up a couple of case fans to a PWM controlled CPU fan. I suppose if CPU fan is say 1500 rpm max. and the case fans as well, then they would all run at the same speed. But what happens if you mix fans with different specs?
For example if I used case fans of 1200 rpm max. (or any other max. speed lower than the CPU fan), would they run at 1500 rpm and presumably get overloaded or would they just run at their max. of 1200 rpm? Accordingly if the CPU fan runs at 50% of its max. speed would the case fans run at 50% of their max. speed?
I have a school project about computer fans. I am suppose to design a computer fan motor that controls the speed based on the temperature. I am using PSIM to simulate it. I chose a Brushless DC motor and as a replacement for the temperature values, I just used a step function as the input. My initial circuit is: Battery>step function (Temp)>(desired speed based on temp)>comparator> thyristor bridge>BLDC>Speed Sensor with feedback to comparator.
So I was wondering how would you start this using power electronics to simulate a computer fan or what advice would you have?
@GaryC If you inject PWM to a 3-wires fan, you will get PWM at its feedback as well making it impossible to measure. I've tried some methods to solve this problem, the best was this one:
Hi, nice article,
I want to use 3 wire fans (not connected to a Mobo) in PWM but although you state all fans can work that way, you then state that the controller (in the fan) gets screwed up. Will it work or not? I wanted to use the speed feedback too, but your article saws no. That alerts me to possible work to see if I can make it workaround. I plan to use PIC or PICAXE as controller with necessary buffering/interfacing.
There is another 4 wire type not mentioned here. They're call 'speed stable' or 'speed regulated' in contrast to 'free running'. A PWM signal is fed into the motor controller to set the speed and the motor controller will do the best it can to make the fan run at the set speed until it reaches it maximum rated power. As a colleague of mine put it: these fans do not stop for a pencil or a childs finger.
This in contrast where the external PWM signal is directly used to control the motor. A speed stable fan does not require a PWM frequency above 20kHz to avoid whistling as the actual motor controller does the hard work.
With speed stable fans, the tacho output can be changed into a trip point alarm. Nidec by default activates this wire when the actual speed is 70% or worse from the set speed.
@Radovan Horak Actually, i really cannot tell. I can only suspect that no one would have embedded such a "security" functionality for the mad man that wants to bypass the feedback. I strongly believe that the 555 phantom fan will work.
I come back to the post written by Dan Canham 3 April 2012 regards a fantom PWM fan tach output.
For about a week I have new HP Z420 Workstation. This otherwise beatiful computer is fitted with the PWM fans which are whatever but not quiet. Of course, when you plug-off them the BIOS starts protest. Furthermore when you disconnect the PWM fan built in PSU the computer stops working at all.
To solve the noisy problem I desided to tune it up by liquid cooling. But I will need to replace physical fans by let say "fantom" ones. I have the same idea - to drive motherboard sensor pin by fantom tach signal generated by 555 based circuit... But not sure whether there is some BIOS feedback check between PWM sent to the fan and tach signal returned. If yes the most complicated circuit which is able to mimics all behavior aspects of a PWM has to be developed.
Now I have information that may help me deal with a problem that has arisen with my old (circa 2003) Samsung HLN5065wx DLP HDTV. A couple of weeks ago an error message suddenly appeared on the screen indicating a problem with "Fan 2." After much searching, I found a page (http://www.fixyourdlp.com/forum/viewtopic.php?f=10&t=596&p=2047#p2047) indicating that a failing fan could be replaced by an ordinary 3-wire PC fan. Good to hear since an official replacement fan is $23 S&H and I have old PC fans lying around. I haven't actually made the replacement yet because I revived the TV fan with blast of compressed air, but if it does fail I will be able to match the wires correctly (they are different colors).
I have a question, just got a motherboard with 3 case fan connectors, two are 4 pins and one is 3 pins.
I want to buy 3 fans with 4 connectors, only that there's going to be one fan that I wouldn't be able to control...
Thinking on connecting just the PWM control cable of the 3rd fan to the 2nd fan, controlling those fans with one controller but still monitoring the fans independently. Is this possible?
@Dan Canham my man! i like the way you think!
Yes it is possible. Make a circuit with the 555 timer as astable oscillator at 50 Hz (typical frequency for 1500 rpm) and then drive the pulses to an open collector transistor.
The collector can be connected to the feedback pin of the motherboard directly. The emitter to the ground. The base through an 1K resistor to the pin 3 of the 555. This emulates a fan rotating at 1500 rpm.
Is it possible to fool a motherboard into thinking a 4 pin fan is running?
My atom desktop switches off if the laptop style 4 pin fan is unplugged or if it isn't spinning. The bios is heavily restricted. This is a pain as the desktop runs within acceptable temperatures passive. (I would rather risk damaging the desktop than listen to the buzzy noise during my music and films)
@Tanay I'm afraid that only the manufacturer can answer this to you. I can only take an educated guess and tell you that the tacho output is an open-collector transistor and that the voltage depends on the pull-up resistor voltage that you will use.
Thank you for your time. I've ordered the parts and I'll tell you how it worked (ordered the 1k resistor with 1/2W because like you said I don't need that much current). Should I watch out for the transistors heat?
@Mike The transistor base is to save the transistor itself from over-current situations. If you are absolutely sure that no overcurrent will occur, then yes, you can directly connect the transistor without base resistor. But the TIP142 is a darlington pair, which means that it has big current amplification (if i remember well it is nearly 1000). So a 470 ohms resistor would not make any difference in operation.
The circuit that you have to build is called Emitter Follower. It amplifies the current of the base by a factor of hfe (1000 in your case) leaving the voltage 0.7 V(1.2 V for darlington) smaller than the input.
You need 2A, which means that the input current must be at least 2/1000 = 2mA. At 7 volts (worst case scenario) a 470 ohm resistor will allow i=u/r = 14mAmp... So you see, you have enough current for your fans.
@Giorgos Lazaridis Thank you for your quick answer. That guy knows what he's doing I don't. But from your answer plus that topic got the idea. One simple question and I leave you alone: because I only need to increase from 0.82A to 2A can I use only one TIP142 without resistor on the transistors base, because I have enough of fans acting as resistors?
Again, thanks for your patience and help.
I have a NZXT Sentry LXE and love the looks.
The problem is it can only feed up to 10w per channel (for 3 pin fans only not 4), 5 channels all together, but I have 19 PWM fans. It's possible to power the fans from power source directly and use the rpm wire connected to NZXT to control them (I have 2 sets of 6, 1 set of 4, 1 back of the case and 1 set of 2 in front). I don't want to use the mobo because I like the fan controller. Please, ANY IDEA?
Hi, I have done something really stupid and want to turn a 4 wired CPU fan into a desktop fan by connecting the positive and negative to the positive and negative of a 12Volts adapter.
Is this actually possible? I have tried by connecting both positive to positive and negative to negative (the positive for the adapter is a black and white wire and the negative for the adapter is a solid black wire) but it still didn't work.
I even tried to switch it around hoping that I got the charges wrong but it didn't work still :( I feel like I've just wasted £20! Is there any way I can do this? I can't seem to find a molex that goes from 4 pin to 2 pin... since that would have been my alternative. Any help would be great thank you everyone in advance.
Looking for some advice/help????? I have a 220/240V cooling fan, its not for a computer though (so they say), it has 2 identicle black wires but nothing to connect it to, hav I been stiffed??????? Thanx in advance
Hi- I need to mount a fan in an enclosure where there is no conventional PC, but rather a microcontroller. Still want to keep it cool, but there is no motherboard to attach a fan to. Seems like I need a fan controller and a fan. Then supply the power and those two together are a self-contained unit. Is that correct? Anyone have any product recommendations? Just need something that I can mount, supply power to and it does the rest, depending on what the temp is. Thanks!
@navi you will connect the positive wire (red or yellow if there is no red) to the 12V of the PSU and the negative (black wire) to the 0V... That;s all it takes. If it does not work, then connect the PWM wire (usually blue) through a 1K resistor to the supply- but it should work already.
i have a question i hope it dose not take too long to respond i have a 4 pin cpu fan but I'm trying to connect it to a power supply how would i accomplish this task safe you blog already helped me to identify the live wires
I suppose I am a bit confused as from what I see, the power supply was feeding the board to 3 pin mount. The feed from the old PSU was only 2 wires. Perhaps It was for monitering the PSU? Two wires, blue & black. one on each end of the 3 pin board mounting..
Is what you are saying (as from other posts here), take a 4 wire molex from the new PSU and connect to the 3 pin on this board?
I recently replaced my old Antec SP-450 450Watt Power Supply Unit with a new OCZ ModXStream-Pro 600Watt. I have an ASUS P5LD2-VM Motherboard. When I removed the old PSU I removed a 2 wire from existing PSU to the 3 pin labeled PWR FAN on the MOBO.
Upon installation of the new, no 2 or 3 wire for this.. is it that the new PSU supplies a 4 pin for installation and I need a converter from 4pin to 3 pin? Any help appreciated as I don't want to screw something up.
@Joska Keunekamp the simplest answer for this is "NO", but if you plan to spent some time on this, the answer is YES. What you have to do is you need to totally remove the built-in controller of the fan and make one yourself. There is no direct way to change rotation on a brushless motor that has a built-in controller.
There is also a three wire connector attached that i want to use.
And yes, i know that the fan will not work optimal when spinning the other way, but i think it will be enough for me.
I am working on a mod, and I rather don 't want to look to the ugly backside of the fan.
My mod will be watercooled.
Hello, very interesting article, congratulation for it. I have a question about a 4 wire dc at 24.5 V.I bought this fan and it doesn't work by connecting the yellow and black wire to molex,it should start with 12V even if it is for 24V. The problem is that when i connect the negative and positive wires it tries to move a little - very little ( this little move confirmed that is not broken). i don't know what to do with it.can you help me with an advice?
@Neil You can use all fans, because you only need to have power and rotate the fan. So, the simpler and cheaper solution is a 2-wire fan. Now, if you plan in the future to have a digital read-out of the speed of the fan, or you want to have a fan failure alarm when a fan fails, then you need at least 3-wire fan.
Finally, if you want to have BOTH failure alarm (or digital readout of speed) AND you want also speed control on the fan, then you need 4-wire fan.
So, the number of wires has only to do with the application you plan to make.
im planning of constructing a USB cooler for my NB
planning on using 3fans
but im confused on choosing btween a 4pin,3pin etc...
what should i say to the shopkeeper while purchacing the fan?
and that will all 3 fans work on a single USB connection?
I am designing a BLDC motor and for that i need some help.
Is there any good source/book that explains the effect of the winding, number of armatures, magnets and other parameter that govern the final output of the motor.
it will certainly work, but i do not know if the fan has internal limiting resistor to protect the gate of the fet. I mean, i should have, but i am not 100% sure. I have never seen one that has not, but this doesnt mean that it does not exits. You can take your chances and test it. Or just add a resistor, like 470 to 1500 ohms...
im modding an xbox by puting more fans in it and all i have is four pin ones but every time i hook up my 12v fan to a 12v source nothing happens is there another thing all i have hooked up is the positive and some negitives ?
Dont worry Clayton, just leave the 3rd wire (tacho) unconnected and the fan will operate as if it was a 2 wire fan. The power of the fan depends absolutely on the supply that it takes. If your supply has PWM or variable voltage (which i doubt), then the fan will change speed as well. Replace with no problem.
I am trying to replace a fan in my car stereo amp. The original fan is a 2-wire PC fan but I am having problems finding one. If I used a 3 wire fan and didn't hook up the third wire what would happen? Does the fan run at max output at all times? Thanks in advance!
Depends Jack. If the motors are BLDCs, then you may manage somehow to get and amplify the signal from the hall sensor. Read the BLDC theory to see what i mean:
That would be your 3rd wire for the rpm feedback. If this is not possible, then you can add an IR light beam cut detector (like this one http://pcbheaven.com/circuitpages/IR_Short_Distance_Beam_Cut_Detector) that will detect the fins of the fan while rotating.
As for the PWM wire, that will be the most difficult. If it is a BLDC, you need to add a mosfet right before the coils (as seen in theory). If the motor is not BLDC, then a simple PWM controller (like this one http://pcbheaven.com/circuitpages/High_Frequency_PWM_Fan_Controller/) will do the job as well. You need to check and experiment yourself. Strictly depends on the motor type.
the "key" is a piece of plastic on the connector, that does not allow it to be inserted on the wrong side. So, the key side, is the side that these pieces are. For example, look the photos that i have for the 3-wire fans. You can see clearly the "keys" on the molex.
Yes you are right. It must be a BLDC, because the receiver of the tranceiver is very sensitive for electric noise. The motor is powered by 2 wires from +13.8V supply through 100+120 ohms resistors wich can be partly eliminated by thermo-switch when the temperature of the power amplifier of the transmitter is too high. The current is about 30mA.I will have to do some dismantling to see whether there is a control circuit outside the motor. The motor looks from its front like a small classical cylindrical motor. Using a standard PC BLDC is a final solution because it needs some annoying embedying.
Yes it is most likely to be a BLDC. BLDCs have the gift of reliability, for they do not carry brushes, and thus they are used in such applications where reliability is critical. How many wires goes to the fan? If the wires are 2, then i suggest you remove the fan for a while and measure the voltage. If it is for example 12V, i think you should replace it with a normal PC fan 12V... Cheap and reliable solution. To be sure that the fan is a BLDC, you should look for the controller. It should be on the fan, a small PCB.
is it possible that the fan motor of the amateur tranceiver ICOM 751A, looking like a classical cilindrical DC motor, is a brushless one, with the internal control circuit? Sometimes it hapenes that the motor cant´t start and its DC resistance is only about 4 ohms. It needs some manual pushing (without power), than it´s resistance gradually grows to 17 ohms and it starts moving again. Thanks for help.
Hi Frank. I did not say that it will work. Frankly, i believe that it will not work this way, unless the M/B has such a feature. You can try it though. I would use an 1 KOhm carbon film resistor 5% tolerance 1/4 Watt. You get each for a cent. Put it in pins positive and tacho (look above to see which fan you have and which color you need to use) to pull the feedback up. Good luck!
Yes I gues I can make the resistor-connector myself.
If I get your point I should extract the pins from the old 4 pins molex-connector the old PSU and replace the pins (wires) with resistors.
What type of resistors should I buy and of how many ohm (is it still of 1k ohm?)?
With your tutorial removing the pins is no problem.
How should I go by installing the resistors in the empty molex 4 pins? I never did that and I never mingled with electricity and electronics.
So the end result with be a molex-resistor that fouls the motherbord bios in believing there is a working fan.
This is the original PSU model http://store.cwc-group.com/436956.html that came with the Hp dc5750 sff. I replaced it with this very quiet and performing new sff PSU form silverstone http://www.silverstonetek.com/products/p_contents.php?pno=ST45SF. So the new one does not have any connector for the motherboard PSU fan sensor.
Actually, there is no such thing as 1K resistor molex. It is yet very easy to make it yourself. Either get the molex from a store, or you can reuse the molex from an old fan. You need of NO soldering at all. I suggest you provide yourself some heatshrink tubes to make a better job. I have an article that might help http://pcbheaven.com/exppages/Reuse_and-or_extend_the_Molex_connectors/
As for the resistor, you will get 10 for a cent.I suppose that the resistor goes to positive (12V) and feedback pin is that right?
I have a Hp dc5750 sff computer and I chaged the original power supply for a more powerfull one in wattage. The problem is the motherboard and bios (not removable in bios) has PSU fan detection on boot.
So no I always have to manually continue the boot by pressing f1 key.
I read on a forum that a dude was able to BYPASS this detection by adding a 1k ohm resistor on the 4 pin molex where the fan cable usally goes.
The problem is he did not mention if he did welding using the resistors.
I took it for granting that he added RESISTORS that are female like and goes in the 4 pin molex on the motherboard to trick the presence of a fan.
Question is were can I find a four pin female resistor of 1 k that will work?
The motor and controllers are the same. They have different fin geometry that allows to have more air flow with less rpm. Also they have better mechanical bearings and staff. This way they make less noise. The motor is the same.
Using 4-wire fans on 3-wire connector. Yes, per Intel 4-wire fan standard, the PWM pin (pin 4) has a weak internal pull-up to 3.3 Volts (5 volts for old standard). The motherboard has a weak pull-down resistor on the PWM pin. This has two purposes. For a three-wire fan plugged into a four-wire connector, the motherboard can look at the PWM pin and detect if a four wire fan is present (>0.7 volts=fan present, <0.7 volts=fan absent) and act accordingly. On the other hand, if one were to plug in a 4-wire fan into a three-wire socket, the weak pull-up internal to the fan would force the fan into constant-on operation, emulating a three-wire fan.
The used CoolerMaster 4-wire fan I just bought will run at full speed with just +12v (Red), Neg (Black) applied so you are correct that a pull-up resistor is on the control line. I also noted that it takes about a half a second for the fan to start.