How to run a PWM controlled fan at 100% all of the time

Question

I've salvaged a Nidec G14T12BS2AF-56J14 fan from a broken Playstation 3 which I would like to use to make a dust blower for PCs.

As a test, I've connected the fan as follows:

• Brown: 12VDC
• Black: ground
• Grey: 12VDC

When connected like that, the fan spins at maximum speed (which is my desired result).

However, I'm not sure this is the correct way to connect this fan. First I suspected the grey wire to be a speed sensor, but the fan didn't spin when it wasn't connected. After touching the wire to 12VDC, it started to spin full throttle. I've found out that the grey wire is a PWM wire via http://eikimartinson.com/archives/104-Driving-a-Playstation-3-Fan.html, where it states the PWM should be a 25kHz signal. But I've also read comments from people who stated using 5VDC or 3.3VDC directly connected to the grey wire.

Can I connect the grey wire directly to 12VDC? Should I add a resistor (1k for example)? Should I use a voltage divider to get 5VDC or 3.3VDC? Or do I really need a PWM signal? Keep in mind that I do not want to control the fan's speed (it would be a bonus, but the fan should really be running full throttle when in use).

Thanks for the information!

Answer 1

Here's how you would use a resistor potential divider to generate a TTL level "high" of just under 5V to apply to the grey PWM control input of the fan:

^{simulate this circuit – Schematic created using CircuitLab}

Since the signal remains high 100% of the time, this is equivalent to a PWM duty cycle of 100%, and the fan should run full-beans.

Answer 2

A fan PWM input is "just" a logic signal that turns the fan motor on and off. You only need to send a PWM signal with a high frequency if you actually want a speed between fully on and fully off, so tying it "high" is fine.

However, the article you linked says that this fan takes a TTL logic level signal. TTL "high" is at most 5 V, so you should not use 12 V as it might damage the fan's input. (Or it might be fine, depending on how the circuit is constructed — it's not like the fan has a 5V rail for the input to be too high than, which would be the usual risk.)

So, to stay within the TTL spec, use a voltage divider, and make sure the voltage is between 2.4 and 5 volts.