Recommended transistor for PWM signal on 4 wire fan

Thread Starter

DJ_AA

Joined Aug 6, 2021
95
Hi All

I have a 24V FAN, which has four wires. VCC,GND, PWM and TACT.

The FAN requires a voltage between 0-4.5-5V on PWM to vary its speed.

The FAN has an internal pullup on the PWM line to 4.5V, whereby if the PWM is not connected to anything, the FAN runs at the maximum speed as the voltage on PWM is about 4.5V.

My microcontroller is working on 3.3V. Therefore I would need to use either a MOSFET or transistor.

Is there a certain parameter I should keep in mind when selecting MOSFET or transistor for PWM?

At the moment, we are using the 2N3904 but found that we had to use an additional pull-up to 5V; otherwise, when the PWM varied, it was not adjusting FAN proportionally. It was as if the transistor was drawing more current from the PWM than supplied by the fan internal pullup. Maybe I am wrong.

Is there a 3 pin transistor or Mosfet which would be recommended?
 

Thread Starter

DJ_AA

Joined Aug 6, 2021
95
Thanks

I want to avoid have 5V power rail, as this is only necessary for the PWM signal, which already has a pullup.

With those modules, you still need to add a 5V line.
 

DickCappels

Joined Aug 21, 2008
8,169
How did you have your transistor connected? That should have worked, generally speaking. Also, how much current can the pull-up supply?
 

Thread Starter

DJ_AA

Joined Aug 6, 2021
95
There is 10K internal pullup, as shown in the diagram below. Transistor collector pin was connected to wire



fan.jpg
 

DickCappels

Joined Aug 21, 2008
8,169
Do you want to use a transistor that is controlled by the ESP8266 to control your fan? If so, how much current does the fan draw?

From what you are saying, we will be ignoring the tachometer signals and running the fan at full speed when on.
 

Thread Starter

DJ_AA

Joined Aug 6, 2021
95
I have simple AVR outputting a PWM signal from its PIN. The fan draws about 0.5A at full speed.

At moment I am finding that I need to add an additional pullup on my transistor, other wise its does not work with internal pull up alone.
 

Thread Starter

DJ_AA

Joined Aug 6, 2021
95
This is my current design.

The Collector is connected to the PWM wire.

My PCB is board is powered with 12V and I am using Zener diode to create a 5V for the pull up. Its working well for now. But I have found that a 5V pullup is needed even though that Fan has it own internal pullup.

If I can get it working with only internal pull up it would be great.

With out the additional pullup, the PWM signal seems to be a little distorted and speed of FAN is not proportional to duty cycle as specified in the datasheet.

I am thinking can it be my choice of transistor, which could causing the issue.



pwm.png
 

ericgibbs

Joined Jan 29, 2010
14,712
The FAN has an internal pullup on the PWM line to 4.5V, whereby if the PWM is not connected to anything, the FAN runs at the maximum speed as the voltage on PWM is about 4.5V.
hi.
No, that 5V is the voltage that you see on that wire from the fan.

E
 
Top