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PWM Voltage for a Fan? - Page 2

post #11 of 15
You want an open collector output from your microcontroller (ie. an NPN transistor with base to your uC through a resistor, the emitter to ground and the collector to the fan's PWM pin). The fan uses a pull-up to ~5V, you must pull it down to ground to create a low period for the PWM signal. Your microcontroller may well have built in open-collector outputs.

Useful links:
http://en.wikipedia.org/wiki/Pull-up_resistor
http://en.wikipedia.org/wiki/Open_collector
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post #12 of 15
Thread Starter 
Well the uC I am planning on using claims to have about 6 output pins that operate/can be used for PWM. I'll look at the board's schematic for a bit and look up some components so I'm certain what I'm dealing with

http://arduino.cc/en/uploads/Main/ar...-schematic.pdf

Also from their site


Each of the 14 digital pins on the Uno can be used as an input or output. They operate at 5 volts. Each pin can provide or receive a maximum of 40 mA and has an internal pull-up resistor (disconnected by default) of 20-50 kOhms.

PWM: 3, 5, 6, 9, 10, and 11. Provide 8-bit PWM output
Edited by Arinoth - 8/8/11 at 11:33am
post #13 of 15
A PWM output is not the same as an open collector output. You will need to connect a transistor to one of the PWM outputs as I described above.

The 40mA that the uC can source/sink is irrelevant to the PWM input of the fan. It is the current sink capability of the transistor that you should be looking at for that. But at these currents you don't need to worry about it anyway.

A generic 2N2222A or pretty much any other general purpose NPN transistor will do the job. A nominal 1K resistor in series with the base will be fine to ensure the transistor switches on fully whilst not drawing too much current through the base-emitter junction from the uC.
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post #14 of 15
Thread Starter 
Well i'll need to check out and see what transistors I have kicking around and we'll see
post #15 of 15

I know this thread is a week old, but this still warrants posting.

You can build a controller for PWM fans with as little as $10 in parts form RadioShack... I did. You can see it in action below...


You can find schematics and instructions here: PWM Controller

The whole thing is basically: 1 x TLC556, 2 Diodes, 1 x Potentiometer, a few resistors, a few capacitors and some wire...

With that setup; you simply feed the fans 12V all the time and by turning the Pot you change the frequency of the PWM signal output from the 556 which controls the speed of the fan.
Edited by xXSebaSXx - 8/21/11 at 4:37pm
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