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Joined: Sun May 05, 2019 12:09 pm

Using a PC fan

Sun May 05, 2019 12:15 pm

Hello world!

I want to know if I can use a PC fan I found in my old PC...it is rated at 12V 0.16A. will I be able to use it in GPIO @5v?
Also, should I unplug the fan before turning off power to the pins as the fan spins for a few seconds after it is off

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Re: Using a PC fan

Sun May 05, 2019 12:51 pm

I don't know about plugging it in to gpio however the resen it spins is inertia or momentum
I do strange things and am sometimes the techhead stereotype.
deal with it!

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Re: Using a PC fan

Sun May 05, 2019 10:36 pm

It should work provided you only connect the positive and negative wires. You may be able to use the PWM wire if present. (don't confuse it with the tachometer wire!)

Should you unplug it? Well... There is a chance to damage the Pi with back emf if you don't, but I'd recommend instead of unplugging it you simply add a reverse-biased fast diode and a small (~1uf) tantalum capacitor across the supply wires at the fan end.

Chances are, being a PC fan, this protection is already in place.

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Re: Using a PC fan

Mon May 06, 2019 10:36 am

A 12v fan will probably work off 5v, but there're no guarantees

Personally, I'm against unplugging anything if you don't have to - two reasons.
1) There'll come a time when you forget.
2) There's a chance you might replug it wrongly, each time you plug it back in.

Another thing - the 5v pins on the GPIO header aren't GPIOs, and the GPIOs aren't 5v.
Just a matter of nomenclature, and unimportant in the grand scheme of things, but getting it right could avoid someone else getting confused.

And finally, while a motor (in a fan or elsewhere) is inductive, and so can create a reverse voltage when you stop the current through it, it also has mass, which acts like a capacitor.
When you remove the power source it'll keep spinning and provide 5v for a while. That shouldn't hurt anything - it'll just keep the Pi alive for longer than you expect.

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