PWM Signal, Splitters with Molex Power

NaThRo

Honorable
I understand how these kinds of splitters work,
http://www.advancetec.co.uk/media/catalog/product/cache/1/image/1250x1250/b65b7106e6199ae52d7f08f5a5033ae0/a/k/ak-cb002_-_akasa_4-pin_molex_to_3x4-pin_pwm_fan_1_connectoer_for_rpm_feedback_for_the_motherboard_.jpg

But I don't understand the type of signal sent to the fans by the PWM line.
Is the PWM signal a kind of percentage or the actual RPM rating?
"Run at 70%"
or
"Run at 1400RPM" (with the max fan RPM being 2000RPM)

If I were to connect 2 different RPM rated fans to the same PWM signal would they be OK?
I am assuming it's percentage based, but wanted to make sure. (and I'm not that great at interpreting some of the gargon I'm finding on Google...)

Solution

There are two separate...

Calculatron

Honorable
I believe it's voltage based, with 12 volts being at full power. So each fan will run at their own specifications.

I believe the main worry is if the fan header can actually take the juice; some motherboard manufacturer's don't recommend PWM splitters, and a lot of the new higher end boards have been providing another 4-pin fan header right-next-to/linked-with the CPU fan header.

Pinhedd

Champion

There are two separate mechanisms there. PWM (Pulse Width Modulation) to control the fan speed, and RPM feedback (not PWM) to report the fan's rotational speed.

PWM works by alternating a fixed frequency signal between on and off states. The speed of the fan is controlled by altering the fraction of each cycle that is spent in the on state rather than the off state. The percentage of time that the pulse spends in the "on" position is known as the "duty cycle" and the duty cycle is proportional to the real fan speed.

For example, assume that a PWM signal is sent at 1Khz, or 1,000 cycles per second. Each cycle is thus 1ms in duration. If the duty cycle is 50%, it will spend 500us in the on position, followed by 500us in the off position. If the duty cycle is 80%, it will spend 800us in the on position, followed by 200us in the off position.

The PWM signal is used to control a switching power regulator which drives the fan. Switching regulators are far more efficient than linear regulators. For the duration of the "on" pulse, the fan will run with a 12 volt DC supply, and for the duration of the "off" pulse it will run with no DC supply. For reasons that are beyond the scope of this explanation, a fan that runs at 12 volts half the time is more efficient than a fan that uses a linear regulator to reduce the voltage to 6 volts all the time. Of course a fan that runs at 12 volts all the time via PWM is just as efficient as a fan that runs at 12 volts all the time as a result of linear regulation.

In the picture that you posted, the PWM signal is sent over the blue wires.

The other mechanism at work here is the fan's RPM feedback. This is what allows the fans to report their own RPM to the system. Every half rotation of the fan creates a pulse of a very short but fixed duration on the feedback wire. For every full 360 degree rotation, 2 pulses are created. Controller hardware on the motherboard counts the number of pulses generated every second and divides them by two to get the fan's rotational speed over that one second period.

I hope that answered your question

NaThRo

Honorable

Thanks Pinhedd,

So, if I have two PWM fans connected to 1 PMW wire, one fan rated at 12VDC 2000RPM (we will ignore +-10%) and the second at 12VDC 1600RPM.
If the PWM duty cycle says 50%, the fans will run at 1000RPM and 800RPM respectively.
At duty cycle = 80%, 1600RPM and 1280RPM respectively.
At all times the fans would be working normally, not affecting each other.

The MB would report the speed of which ever one I connected the green wire to.

Sweet

That's correct!