What is PWM?
What is high frequency PWM?
How do I set up and run PWM fans?
What is customizable on JMC's PWM fans?
What are the electrical parameters of PWM fans?

What is PWM?
Pulse Width Modulation (PWM) refers to the method of applying a signal to the fan that will vary the width of the fan's pulse. An 80% duty cycle means that the fan is "on" 80% of the time and "off" 20% of the time. A 50% duty cycle signal means that the fan is "on" 50% of the time and "off" 50% of the time (similar to a perfect square wave). On a PWM fan, the 4th wire consists of the PWM line and will be directly proportional to the speed of the fan. That is to say, a high duty cycle will produce high speeds and a low duty cycle will produce idle speeds..

What is high frequency PWM?
There is always a frequency associated with a PWM signal that is different from the PWM duty cycle and frequency will affect the acoustics of a fan. The clicking and whistling of the motor commutations per revolution cause the motor switching noise of a fan.

Let's imagine that that you apply a 50% duty cycle signal to a fan at a low frequency of 30 Hz. This results in 30 square waves per second. This low frequency of 30 Hz will result in an acoustically loud fan due to the motor switching noise of the fan.

Now let's imagine that you apply a 50% duty cycle signal to the fan at a high frequency of 25 KHz. This results in 25,000 square waves per second. A human can only hear frequencies up to 20 KHz, which means that the frequency of 25 KHz being applied to the PWM fan makes the motor switching noise so quiet that when the fan runs at low speed, it cannot be heard.

With JMC's high frequency PWM fans, the frequency applied to the fans is approximately 25 KHz.

How do I set up and run PWM fans?
You will need a function generator to apply the duty cycle and frequency PWM signal. You will also need a frequency counter to count the tachometer signal returned from the fan, or the actual revolutions of the fan. JMC uses an Agilent 34970A data acquisition unit with one 43901-switch module to measure the tachometer, voltage, current, and temperature. We then use an Agilent 33220A function generator to apply the PWM signal.

What is customizable on JMC's PWM fans?
The mechanical characteristics that can be readily changed on JMC's PWM fans are the idle speed and the maximum speed.

What are the electrical parameters of PWM fans?
Electrically the PWM signal is TTL/CMOS compatible where the PWM input impedance is greater than or equal to 10 Kohm. The PWM operating frequency is 25 KHz +/- 5 KHz. The maximum voltage for a low PWM signal is 0.8 Vdc and the minimum voltage for a high PWM signal is 2.0 Vdc

The Tachometer output signal from the fan is an open collector output that contains a 100-Ohm resistor in a series and can sink up to 10 ma. You will need a pull up resistor on your motherboard that will be pulled up to a maximum of 18Vdc.