- Jan 5, 2018
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I am just wondering what the difference is between a 3 pin fan and a 4 pin pwm fan i, because I am buying new fans for my computer.
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What's the difference between 3 pin fans and 4 pin pwm fans?
- Thread starter koenleung3
- Start date
FYI you can still control the fan speed of a 3 pin fan via mobo headers. I think the only advantage of PWM is that it's more efficient (not that fans use a lot of power regardless), and maybe have lower minimum speeds?
The two designs work differently and require different modes to control their speeds.
3-pin fans have these connections from mobo header to fan:
Pin #1 - BLACK - Ground
Pin #2 - RED - +DC voltage varying
Pin #3 = YELLOW - Speed pulse signal
The female (with 3 holes) connector on the end of the fan's wires has two ridges on one side just outside the Pin 1 and 3 locations, and these fit over a plastic tongue sticking up from the base of the male (with 3 pins) header on the mobo, so you can only plug the fan in one way - no errors.
The fan is powered by Pins 1 and 2. The mobo header varies the voltage on Pin #2 from 12 VDC (max speed) down to about 5 VDC. Any lower voltage may cause the fan to stall, and usually cannot start up a fan that is not turning. This Mode of control is called Voltage Control Mode or DC Mode, and it is the ONLY method of controlling the speed of 3-pin fans. The speed signal on Pin #3 is a series of pulses (2 per revolution) generated inside the fan motor and sent back to the mobo on this line to be counted. That's how the mobo can tell you the speed of a fan. But the mobo does NOT use that signal to control speed. It uses it for display to you for interest, AND to monitor the fan for possible failure. Automatic control of speed (if you allow that to be done, which is the default setting) is based on the actual TEMPERATURE measured by a sensor, and on control system targets pre-set in BIOS (or, in some cases, adjustable by you). There is always a temperature sensor built into the CPU chip itself, and it is used by the mobo CPU_FAN header to guide the speed of that CPU cooler. There is a separate sensor built into the mobo (sometimes more than one for you to choose) at a location decided by the mobo maker, and this should be used to guide the speeds of case ventilation fans connected to the SYS_FAN or CHA_FAN headers.
The newer 4-pin fan design is different mainly in the method of control. The new design includes some backwards compatibility features with the older 3-pin system, but they are not completely interchangeable. The uses of the wires is VERY similar, and the design of connectors is such that you CAN plug any 3- or 4-pin fan into any 3- or 4-pin header, and they will work to some extent (more later). The standard wire colors used for 4-pin systems are NOT the same as those for 3-pin ones above. The signals here are:
Pin #1 - BLACK - Ground (same)
Pin #2 - YELLOW - +12 VDC constant
Pin #3 - GREEN - Speed pulse signal (same)
Pin #4 - BLUE - PWM signal
The female connector for these fans is the same with those ridges on one side to fit over the header tongue. It just has an additional fourth hole beyond the ridge to contact the header's 4th pin.
4-pin fans have a new additional chip inside them that uses the PWM signal from Pin #4 to modify the flow of current from the fixed +12 VDC supply on Pin #2 through the motor windings, rapidly turning the current on and off at a frequency of about 25 kHz. This is called PWM Mode. That is how the motor speed is controlled, rather than changing the voltage. The advantages of this type of motor design are mainly at low speeds: the motor can be run at lower speeds without stalling, and it can be started up at slower speeds from a stalled condition.
What happens when you mix the systems? Two possibilities:
3-pin fan plugged onto a 4-pin header: the fan does not receive the PWM signal, and it could not use it anyway because it has no special chip. The fan receives on Pin #2 a constant +12 VDC, so it runs full speed all the time. You DO get good cooling, just no speed control.
4-pin fan plugged into a 3-pin header: the fan receives no PWM signal so its chip cannot modify the voltage from Pin #2. BUT that voltage is being altered by the mobo, so the fan's speed IS controlled by the mobo using the older Voltage Control Mode.
Now comes the sources of confusion. Almost all mobos now are using 4-pin male headers for ALL fan headers, regardless of what signals they supply from them. So you can NOT tell from header pin count what it is doing to control the fans. There are four different ways they MAY be used.
1. Voltage Control Mode only - the signals on the first 3 pins are exactly what a standard 3-pin fan header would do, and the 4th pin simply is not used. This is Voltage Control Mode (aka DC Mode.) This CAN power and control the speed of either fan type because of the backwards compatibility feature of 4-pin fans, even though Voltage Control Mode is not quite as good (technically) as PWM Mode. (See note below for one problem.)
2. PWM Mode only - the header puts out exactly the signals for a standard 4-pin fan system header and is using PWM Mode. This can provide power to both fan types, but it can NOT control the speed of a 3-pin fan - that fan will run full speed all the time.
3. In BIOS Setup, for EACH fan header, you have a choice available to set manually whether the header will use Voltage Control Mode (aka DC Mode) or PWM Mode. Each such header can be configured for one fan type, and all the fans on it will be given the same signals.
4. Automatic Configuration - the header automatically detects which fan type has been plugged into it and adjusts itself to use the correct control Mode. In addition, in BIOS Setup for each such header you have the option to specify one of those modes, rather than allowing the automatic system make the choice.
Determining which method is used by the various fan headers on a mobo requires deciphering the details of the header's signals and the options available in BIOS Setup, according to the manual for the mobo. Some manuals are out of date and give incorrect info, but that's not too common.
For Type #4 above, there is a "cheat" method. Any fan header of type #1 - only Voltage Control Mode - will appear to be able to control the speed of any fan, so it can look like it is using some automatic configuration method when it is not.
A note about speed signals. A mobo header can count the pulses (to determine fan speed) coming back from only ONE fan. If two or more fans are connected to the header AND the pulse signals from both are fed into the header, it will become totally confused about the speeds and deliver wildly fluctuating readings. So any decent Splitter or Hub will send back to the host mobo header only ONE fan's speed signal, and simply ignore all the others.
A note about possible problems with Type #1. As I said, this type can control both fan types. The problem comes about when you try to use a 4-pin fan HUB.
A Hub is a device used to connect several fans to a single header. Back ground: most fan headers can supply power up to 1.0 amps total load to all the fans connected (by whatever means) to that header. A SPLITTER is a simple device that merely connects two or more fans in parallel to the fan header pins so they all share the same signals and power supply, so the fans in total are subject to this current limit. A HUB, on the other hand, does it differently. It has an extra connection cable that plugs into a power output (either 4-pin Molex or SATA) from the PSU to get power for all its fans, and it does NOT draw any power for fans from Pin #2 of the mobo header. The power available from the PSU directly is much more than a mobo fan header can provide, so may fans can be connected via a Hub to one header. This means that the only power the Hub can send to a fan is the full +12 VDC. Then it also sends out to all its fans the PWM signal from the mobo header (not subject to current limit) for speed control, and ALL of the fans on the Hub share exactly the same control signal. This means that ALL of the fans MUST be of the 4-pin design that can do their own control using the fixed 12 VDC supply and the PWM control signal. It also means that the Hub MUST have a PWM signal from mobo header Pin #4 to share out to its fans. This last point is how a Type #1 (Voltage Control Mode only) header can be a problem. That header CAN control one fan of any type, or even a couple if they are connected using a Splitter. But if you try to use a HUB with that type of header, there is no PWM signal to pass on to fans, and there is no way for the Hub to control its fans' speeds.
As a final note, I know of one Hub that is different (and there are one or two others similar). The Phanteks PWM Hub works only with a 4-pin mobo header that uses PWM Mode to supply the PWM control signal, and gets all fan power from the PSU directly, just like other Hubs. But internally it uses that signal to create its own group of six 3-pin fan ports that all use Voltage Control Mode based on the common PWM control signal. Since both 3- and 4-pin fans CAN be controlled for speed by this method, this Hub can be used with any mix of 3- and 4-pin fans connected to it, and all of them are based on the control signal from the one mobo fan header it is plugged into.
3-pin fans have these connections from mobo header to fan:
Pin #1 - BLACK - Ground
Pin #2 - RED - +DC voltage varying
Pin #3 = YELLOW - Speed pulse signal
The female (with 3 holes) connector on the end of the fan's wires has two ridges on one side just outside the Pin 1 and 3 locations, and these fit over a plastic tongue sticking up from the base of the male (with 3 pins) header on the mobo, so you can only plug the fan in one way - no errors.
The fan is powered by Pins 1 and 2. The mobo header varies the voltage on Pin #2 from 12 VDC (max speed) down to about 5 VDC. Any lower voltage may cause the fan to stall, and usually cannot start up a fan that is not turning. This Mode of control is called Voltage Control Mode or DC Mode, and it is the ONLY method of controlling the speed of 3-pin fans. The speed signal on Pin #3 is a series of pulses (2 per revolution) generated inside the fan motor and sent back to the mobo on this line to be counted. That's how the mobo can tell you the speed of a fan. But the mobo does NOT use that signal to control speed. It uses it for display to you for interest, AND to monitor the fan for possible failure. Automatic control of speed (if you allow that to be done, which is the default setting) is based on the actual TEMPERATURE measured by a sensor, and on control system targets pre-set in BIOS (or, in some cases, adjustable by you). There is always a temperature sensor built into the CPU chip itself, and it is used by the mobo CPU_FAN header to guide the speed of that CPU cooler. There is a separate sensor built into the mobo (sometimes more than one for you to choose) at a location decided by the mobo maker, and this should be used to guide the speeds of case ventilation fans connected to the SYS_FAN or CHA_FAN headers.
The newer 4-pin fan design is different mainly in the method of control. The new design includes some backwards compatibility features with the older 3-pin system, but they are not completely interchangeable. The uses of the wires is VERY similar, and the design of connectors is such that you CAN plug any 3- or 4-pin fan into any 3- or 4-pin header, and they will work to some extent (more later). The standard wire colors used for 4-pin systems are NOT the same as those for 3-pin ones above. The signals here are:
Pin #1 - BLACK - Ground (same)
Pin #2 - YELLOW - +12 VDC constant
Pin #3 - GREEN - Speed pulse signal (same)
Pin #4 - BLUE - PWM signal
The female connector for these fans is the same with those ridges on one side to fit over the header tongue. It just has an additional fourth hole beyond the ridge to contact the header's 4th pin.
4-pin fans have a new additional chip inside them that uses the PWM signal from Pin #4 to modify the flow of current from the fixed +12 VDC supply on Pin #2 through the motor windings, rapidly turning the current on and off at a frequency of about 25 kHz. This is called PWM Mode. That is how the motor speed is controlled, rather than changing the voltage. The advantages of this type of motor design are mainly at low speeds: the motor can be run at lower speeds without stalling, and it can be started up at slower speeds from a stalled condition.
What happens when you mix the systems? Two possibilities:
3-pin fan plugged onto a 4-pin header: the fan does not receive the PWM signal, and it could not use it anyway because it has no special chip. The fan receives on Pin #2 a constant +12 VDC, so it runs full speed all the time. You DO get good cooling, just no speed control.
4-pin fan plugged into a 3-pin header: the fan receives no PWM signal so its chip cannot modify the voltage from Pin #2. BUT that voltage is being altered by the mobo, so the fan's speed IS controlled by the mobo using the older Voltage Control Mode.
Now comes the sources of confusion. Almost all mobos now are using 4-pin male headers for ALL fan headers, regardless of what signals they supply from them. So you can NOT tell from header pin count what it is doing to control the fans. There are four different ways they MAY be used.
1. Voltage Control Mode only - the signals on the first 3 pins are exactly what a standard 3-pin fan header would do, and the 4th pin simply is not used. This is Voltage Control Mode (aka DC Mode.) This CAN power and control the speed of either fan type because of the backwards compatibility feature of 4-pin fans, even though Voltage Control Mode is not quite as good (technically) as PWM Mode. (See note below for one problem.)
2. PWM Mode only - the header puts out exactly the signals for a standard 4-pin fan system header and is using PWM Mode. This can provide power to both fan types, but it can NOT control the speed of a 3-pin fan - that fan will run full speed all the time.
3. In BIOS Setup, for EACH fan header, you have a choice available to set manually whether the header will use Voltage Control Mode (aka DC Mode) or PWM Mode. Each such header can be configured for one fan type, and all the fans on it will be given the same signals.
4. Automatic Configuration - the header automatically detects which fan type has been plugged into it and adjusts itself to use the correct control Mode. In addition, in BIOS Setup for each such header you have the option to specify one of those modes, rather than allowing the automatic system make the choice.
Determining which method is used by the various fan headers on a mobo requires deciphering the details of the header's signals and the options available in BIOS Setup, according to the manual for the mobo. Some manuals are out of date and give incorrect info, but that's not too common.
For Type #4 above, there is a "cheat" method. Any fan header of type #1 - only Voltage Control Mode - will appear to be able to control the speed of any fan, so it can look like it is using some automatic configuration method when it is not.
A note about speed signals. A mobo header can count the pulses (to determine fan speed) coming back from only ONE fan. If two or more fans are connected to the header AND the pulse signals from both are fed into the header, it will become totally confused about the speeds and deliver wildly fluctuating readings. So any decent Splitter or Hub will send back to the host mobo header only ONE fan's speed signal, and simply ignore all the others.
A note about possible problems with Type #1. As I said, this type can control both fan types. The problem comes about when you try to use a 4-pin fan HUB.
A Hub is a device used to connect several fans to a single header. Back ground: most fan headers can supply power up to 1.0 amps total load to all the fans connected (by whatever means) to that header. A SPLITTER is a simple device that merely connects two or more fans in parallel to the fan header pins so they all share the same signals and power supply, so the fans in total are subject to this current limit. A HUB, on the other hand, does it differently. It has an extra connection cable that plugs into a power output (either 4-pin Molex or SATA) from the PSU to get power for all its fans, and it does NOT draw any power for fans from Pin #2 of the mobo header. The power available from the PSU directly is much more than a mobo fan header can provide, so may fans can be connected via a Hub to one header. This means that the only power the Hub can send to a fan is the full +12 VDC. Then it also sends out to all its fans the PWM signal from the mobo header (not subject to current limit) for speed control, and ALL of the fans on the Hub share exactly the same control signal. This means that ALL of the fans MUST be of the 4-pin design that can do their own control using the fixed 12 VDC supply and the PWM control signal. It also means that the Hub MUST have a PWM signal from mobo header Pin #4 to share out to its fans. This last point is how a Type #1 (Voltage Control Mode only) header can be a problem. That header CAN control one fan of any type, or even a couple if they are connected using a Splitter. But if you try to use a HUB with that type of header, there is no PWM signal to pass on to fans, and there is no way for the Hub to control its fans' speeds.
As a final note, I know of one Hub that is different (and there are one or two others similar). The Phanteks PWM Hub works only with a 4-pin mobo header that uses PWM Mode to supply the PWM control signal, and gets all fan power from the PSU directly, just like other Hubs. But internally it uses that signal to create its own group of six 3-pin fan ports that all use Voltage Control Mode based on the common PWM control signal. Since both 3- and 4-pin fans CAN be controlled for speed by this method, this Hub can be used with any mix of 3- and 4-pin fans connected to it, and all of them are based on the control signal from the one mobo fan header it is plugged into.
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