The fan is labeled:
DC BRUSHLESS
Model AFC0912DE
Dell P/N P2780
DC12V 2.50A
Delta Electronics, Inc.
I work with Dell PCs and I had a CPU fan laying around that came out of an older Optiplex PC. I had connected it up to a 12V power supply much earlier, and it acted very odd.
FIrst off, older fans had two wires: a red positive 12V and a black ground. Later, fans came with three wires: red for +12V, black for ground and a white or yellow wire for the tachometer to tell the MoBo that the fan was spinning, But this fan was different. Instead of three, it had four wires: red, black, white and blue. So I connected the 12V up to the red and black wires. It would start out slow, and gradually speed up to a high speed, then, Kerchunk! It would abruptly come to a near halt. then it would repeat the process. It was either putting out almost no air, or whining at high speed and drawing over 2 amps from the power supply. I kind of gave up and tossed it in the spare parts bin, where it sat for a long time.
I finally got around to try to get it to work. I went by the old maxim “An hour in the library is worth ten in the lab.” I went online and searched for information about the fan. I came across others who were trying to get another fan to replace their noisy Dell fan. Finally I found out that the red and black wires were the positive 12V and negative, and the white and blue wires were the tach and PWM speed control wires. But I did not find any information about what the PWM control was or how it worked.
So I started experimenting with the PWM control line. I tried a few different value resistors from +12V to the blue wire, and from negative to the blue wire. I found that if I put a 1k resistor from the blue wire to negative, the fan stopped acting weird and would run at a constant speed, not very fast but at least it was steady and quiet. Now I can connect the fan to a 12V wall wart, and plug it in, and set it on its edge on the table without having to worry that it was going to jump off and fall on the floor due to its kerchunking behavior.
Back to experimenting…