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Please can any1 help me to find out the RPM of a 120mm Fan via USB Idea ? without connecting the fan to the motherboard Answered

Hi folks,

i have a few PWM 120mm PC Fan's, but one is so cheap & nasty that it only had the power specs on it and i have quite a few fans like this but wanted to find out the RPM of it but without actually hooking it up to my motherboard of my PC as im severely mobility impaired and my pc is in such an area that it would take an able-bodied person a few days of removing the barricade-mountain of 'stuff', lol, from all around it to get to the PC, so...

ive got a bunch of usb cables just sitting around so is there a way i can use 3 USB cables to connect to 3 individual ports to supply the 12v needed by this 120mm fan (and somehow reducing the 15v in total from 3-added USB sockets' power added together to give 12v and my USB Hub is powered) and also a way of connecting the PWM wire of the fan to one of the USB ports so that a software like SpeedFan could detect it being connected so that i could find out the RPM of it (and write up an instructable about it too !!)

the only info on the fan is:
  • DC Brushless
  • 12V
  • 0.30A
this would be a great project i wouldn't mind sinking my teeth into and i simply cant afford such fancy devices like a digital stroboscope that measures fan RPM so kinda living on a shoe-string here and i would really love to make some kind of USB-type simple device that could detect any fan's RPM that plugged into it via 4-pin Molex or the 3-pin connector:
  • would i need extra circuitry for the software SpeedFan to use to enable it to detect a fan at the USB Port ?
  • would it be possible to make a simple schematic for me if circuitry would be needed (i can read-schematics (just-about, lol, only started learning electronics!) & make my own PCB's)
  • & how would i drop 15volts to 12volts so that the voltage doesn't fry my fan ?!?

any & all advice is greatly appreciated and i thank you all in advance !!


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8 years ago

You can get a strobe light that can tell you the RPM's but that is a little bit overkill. I don't know if LEDs can cycle fast enough to work as a strobe. So you would need to buy one rather than try and make one. At one time strobes were favorites for dances, but so where disco balls and lava lamps. Actually you can make a strobe by cutting slits in a disk and then rotating the disk at a known speed. You calculate how fast the disk turns and that tells you the time in between light pulses as the slit opens and closes. But then you have to know the speed of the disk in RPMS so you are back to where you started.


8 years ago

First of all USB only supplies 5V. Combining 3 USB cables from the same power source will not give you 15V. If you had 3 different USB power sources and lined them up in series you would get 15V. Then all you would need is a voltage regulator, like the 7812, that will drop the voltage to the 12V you need. The RPM of the fan doesn't matter as much as how much air it actually pushes. The reason the motherboard looks for the RPM is so you can set a limit. If the fan speed drops below that limit the motherboard can set off an alarm and throttle things back so the system doesn't overheat.

Secondly, if its not connected to the motherboard's fan port Speed Fan won't be able to see it.

You can buy PC accessories that will control and display the RPM of the fan. Just a matter of getting a 12V power source to it. You can set it up to run off your car's battery.


8 years ago

From your ibles I know You know about ;
  • LEDs
  • Arduino
  • Software
  • Breadboarding
So make a simple LED digital stroboscope that measures fan RPM.
You can calibrate it by counting instruction cycles or looking at your
other known fans, should make a killer ible for you.
Don't try big range responsive sadw  just add nop statements in your
off loop as a start :-)