Computer Fan Controlled By MB Fan Header Using PSU Power

Discussion in 'General Electronics Chat' started by nissan20det, Jul 22, 2016.

  1. nissan20det

    Thread Starter New Member

    Apr 3, 2015
    19
    0
    Hey there!
    To start here's my setup if needed.

    Mobo- Crosshair Formula V
    CPU- AMD FX 8320
    GPU- Zotac GTX950
    Cooling- Swiftech H240x with additional 240
    Fans - 7*120's, 3*80's,1*60
    Power- Crosair HX850

    So I just got these new "Works" fans from Micro Center. There very very loud lol, 240CFM @ 5200RPM at a whopping 64dbs lol. Now I got these cuz if i want the cooling power, I have it. I want to run these off a fan header so there controlled by FanXpert(Asus control software). The problem is, these fans pull 36 watts!! My headers can only do 12W a piece. My thought is to use the fan header to control a Mosfet, So those fans can get power directly from the PSU. I need some help on the circuit. I made a couple schematics but bare with my I may be way off I'm not an EG. Let me know if you see somthing I need to change or which I route I should go.

    The first three ar ones I saw online and I forgot all 12V and GND points going to fan will be directly from psu not from header like in diagrams. Sorry

    The fourth I made with an isolation optocoupler in the circuit to protect my PC. I only have NPN Optos and N channel Mosfets. So I'm not sure how to do that. How the circuits drawn is the only way I see the opto working. I do like it better but what do you guys think?

    Thanks

    Fan_Con_1.JPG
    Fan_Con_2.JPG
    Fan_Con_3.JPG
    Fan_Contr_1.JPG
     
  2. shteii01

    AAC Fanatic!

    Feb 19, 2010
    3,377
    494
  3. #12

    Expert

    Nov 30, 2010
    16,248
    6,745
    The, "simple mosfet motor controller" pretty far down the page from shteii looks like the right one. Just assume the ttl driver is in the MB, and it supplies a 12 volt PWM signal. You have my permission to be sure it is really a 12V pwm, because if I'm wrong, you might have problems driving the mosfet gate. Other than that, give that fan a personal 12V power wire and do a redundant, wired ground for the mosfet so you don't melt any MB traces.

    Don't worry about D1. It's already inside most mosfets and the diode across the fan motor takes care of that job, anyway.
    I believe common, 1 amp diodes, can handle a one time shut-off pulse, but this seems to be a PWM activity, so I would use at least a 3 amp rated diode just to be sure I won't have problems with it.

    I have seen 5 amp and 10 amp diodes on eBay during another Thread, and they're dead cheap, like 10 for $4.
     
  4. nissan20det

    Thread Starter New Member

    Apr 3, 2015
    19
    0
    Sweet Thanks Guys!!!

    Okay that seems simple but slightly i'm confused, Ill get to that. I have plenty of diodes from 1-30amp I think. Okay well one problem here. I put my scope on the PC today and I cant get a wave to show at all. I tried all the wires. 12V wire shows 12.2vRms, RPM wire(No motor connected) Shows 3.84vRms and the PWM shows a flat 5.08vRms. I tried adjusting the chassis fan headers speed and my other two fans change in speed but all pins on CHA1 stay the same.(I have 3 headers for chassis). Maybe Ill try a separate header tomorrow cuz that's wiggin me out.

    Okay so now to the controller you suggested. I'm pretty sure the PWM is 5v I actually dont think I have even heard of a PC's PWM signal being 12v. I dont know for sure yet since I cant even see it on the scope.(Must be a bad header). Now if my PWM signal is only 5v will I be okay to use an opamp and supply it with 12v-GND? Their for making my 5v PWM a 12v, being enough to make the Mosfet clip? and speaking of clipping the voltage ladder thats made (RG-R2) would'nt this be lowering my needed 12v signal? Isn't R3 just a pull down, So why not have it between RG and logic out to avoid a unwanted voltage drop? If I cant do that do I just make the value of R3 greater to make a minimal drop?

    Secondly the diode you say to use, is that the back EMF one? I've only seen a schottky diode in this position for back EMF, never a diode in series with a resistor. Could any of you elaborate on this? I have some N5821 3amp 50V Schottky's, If I use those should I still use a resistor? If so what value/wattage?

    Thanks guys really appreciate it!!
     
  5. #12

    Expert

    Nov 30, 2010
    16,248
    6,745
    No, you just make sure you use a, "logic level" mosfet, usually with an "L" in the suffix. The gate drive graph on the datasheet will show good amperage at about 3 volts.

    Do not put a resistor in series with the diode across the motor. Yes, schottky is good. Yes, it's called the "back EMF" diode.

    As for resistors: If the drive to the mosfet gate is DC, you have few worries. Just hang a few k to ground to make sure the gate shuts off when the drive signal drops out. A series resistor to the gate is usually very small ohms, just to stop it from ringing. Sometimes you don't need one, sometimes they are as small as 10 ohms. It depends on the switching speed and the gate capacitance of the mosfet. So, 10 ohms in series with 10k? That's not going to knock your drive voltage down very much!

    Connect the "usual" fan to the header and scope the drive in that condition. I expect the MB is refusing to try to drive a fan that is not there. After you know if it's PWM and how fast the PWM is switching, you can choose a logic level mosfet and decide the resistor values to allow the MB to drive the mosfet gate without slowing it down with too much resistance in series with the drive voltage.

    For a first try, I would just connect the drive pin to the mosfet gate, add 10k to ground, and connect the fan to the mosfet.
    Then scope the gate and see if the drive is square, sharp, and not ringing.
     
  6. shteii01

    AAC Fanatic!

    Feb 19, 2010
    3,377
    494
    As far as fan headers and speed signals, I have similar problem. I have Intel DP45SG mobo with:
    1. CPU fan header
    2. Front fan header
    3. Rear fan header
    4. Auxiliary fan header

    CPU, Front, Rear I can adjust fan speed. Auxiliary fan header, I have no control of speed, it just runs full speed.
     
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