Can You Run PWM Fan Without PWM Input?

[gomerpyle]

Excuse my lack of knowledge on PWM and fans, but can a fan configured with 4 wires (power, ground, tach. signal, and PWM input) be run off of a DC power supply without using the PWM to control its speed? As in, not using the PWM lead and just hooking up power/ground and controlling fan speed by adjusting voltage? Or is it required that the PWM be used if you wanted to be able to adjust the fan speed.

This is the fan I was looking at:

http://search.digikey.com/us/en/products/3122FT-D4W-B56-E50/P14778-ND/2647852

Don't have much of an electrical background so I was hoping someone with more knowledge could help me out.

 

[DragonPetter]

Could you set the PWM lead to 100% duty cycle, and then vary the DC power supply voltage?

 

[gomerpyle]

Could you set the PWM lead to 100% duty cycle, and then vary the DC power supply voltage?

Problem is I don't have a motor drive for PWM. All I have to work with is a DC psupply. That's what I'm wondering is if the lead wire for PWM just gives you the "option" to use that function if you want to, but otherwise you can just adjust fan speed directly with voltage if you hook up the other leads.

 

[DragonPetter]

Problem is I don't have a motor drive for PWM. All I have to work with is a DC psupply. That's what I'm wondering is if the lead wire for PWM just gives you the "option" to use that function if you want to, but otherwise you can just adjust fan speed directly with voltage if you hook up the other leads.

PWM at 100% just means its pulled up to logic high. Its PWM would be 0% if you grounded the lead.

 

[pantaz]

... run off of a DC power supply without using the PWM to control its speed? ...

Maybe. I remember reading a blog where the guy was experimenting with computer case fans. He found that the circuitry inside the fan varied by manufacturer. If I recall correctly, some fans will work without a PWM signal, others won't.

It might have been on Hack-a-Day or Instructables.

 

[gomerpyle]

PWM at 100% just means its pulled up to logic high. Its PWM would be 0% if you grounded the lead.

so to run the fan at 100% pwm I would just connect the pwm lead to the + terminal on the power supply?

 

[DragonPetter]

so to run the fan at 100% pwm I would just connect the pwm lead to the + terminal on the power supply?

Most likely. I didn't look at the datasheet. Assuming the power supply voltage is the same as the logic voltage, then yes. I would pull it up with a 10k-100k resistor, also maybe a .1uF decoupling capacitor from the PWM lead to ground so it has a stable voltage if the supply has transients from the motor.