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Stop a fan form being a generator

Discussion in 'General Electronics Discussion' started by Everytime, Aug 2, 2012.

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  1. Everytime

    Everytime

    4
    0
    Aug 2, 2012
    Hi guys, I'm new here and I need some help with a fan circuit.

    I have recently installed a new JVC deck/head unit in my car, and it was constantly overheating so I decided to MacGyver a heatsink from an old motherboard onto the back of it. The heatsink works great and gets very hot so its doing it's job but I need to get airflow through there.

    I picked up a run of the mill 80mm 12v computer case fan and wired it behind my deck to get cool air circulating. I hooked up the fan's positive wire to my car's red "Accessory" wire that only provides 12v when the ignition is turned on. I also hooked up the fan's negative wire to my car's ground.

    When I turn my ignition on, all is fine and the fan spins up normally. However, when I go to shut my ignition off it makes an unnerving clicking sound behind the steering wheel and the check engine light went on. I have deducted this is because when the ignition is shut off, the fan will continue to spin until it runs out of kinetic energy, acting as a generator and pushing current back through the car's 12v Accessory wire. When I manually stop the fan with my finger before shutting the ignition off, no clicking sound and everything is good.

    I have a basic understanding in electronics, from resistors, diodes, and transistors and I am more than comfortable soldering. I just need to find a way to stop the fan from acting as a generator when the power going to it is shut off.

    Any help is greatly appreciated. If you need any more information please let me know.

    Thanks very much!
     
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    It sounds more likely to me that you don't hear the fan's noise above the engine.

    A small PC fan isn't going to generate much power (if any at all) as it spins down.
     
  3. Everytime

    Everytime

    4
    0
    Aug 2, 2012
    Thanks for the reply. When I was doing this in my driveway, the car's engine wasn't even on, I was just turning the key to the ON position so the accessory wire would get 12v. I know for a fact the fan was spinning, as I saw it.

    And I did hook up a multimeter to the PC fan in question, and when it is spun (by me blowing into it) it does create about 3.5 volts.

    I am sure that the issue is the fan pushing voltage back through the accessory wire because when I jam the fan with my finger before shutting off the ignition, everything is okay.
     
  4. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,401
    2,777
    Jan 21, 2010
    I think that's highly unlikely

    So you say the fan is less noisy when you've stopped it.

    Try stopping the fan by unplugging it and see if it still makes a noise.

    If so, it's noisy because... it's noisy.
     
  5. Everytime

    Everytime

    4
    0
    Aug 2, 2012
    The fan's noise is not the problem. The noise I am referring to is a distinct clicking behind the steering wheel, what I presume to be in the ignition module, that happens when the fan is left to spin when shutting the ignition off. I believe this happens because the fan is creating a current that shouldn't be there when the ignition is off.

    I just tried to unplug the fan before shutting the ignition off, and there is no clicking noise from the ignition. These are the same results that happens when I jam my finger into the fan and completely stop it before shutting the ignition off. This furthers my belief that the fan being left to spin for 2 seconds is the issue.
     
  6. Harald Kapp

    Harald Kapp Moderator Moderator

    10,071
    2,152
    Nov 17, 2011
    If it is truly the fan generating a voltage, use a simple diode i series with the +-wire: Anode to red accessory wire, cathode to fan's plus.
     
  7. Everytime

    Everytime

    4
    0
    Aug 2, 2012
    Thank you for the reply. I had initially thought to this as well, but when I tested it with my multimeter and a power supply, it didn't work. When you think about it, the fan doesn't make a negative voltage so the diode will still let it the electricity through. The fan would be generating in the same polarity that it was spinning in. I will test this in the car tomorrow to be sure though and post the results. Thanks again.
     
  8. Harald Kapp

    Harald Kapp Moderator Moderator

    10,071
    2,152
    Nov 17, 2011
    I don't understand.
    Consider this circuit:[​IMG]
    Note: the diode parallel to the fan is optional, not necessarily required.

    Harald
     

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  9. GreenGiant

    GreenGiant

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    Feb 9, 2012
    No plain and simple.

    Yes under no load it may generate a small voltage, but the current will be insubstantial, and if it was truly under any sort of load (a.k.a. doing something) the fan would stop in less than 2 seconds.

    Also if you do have it tied into the ignition that is just a switch, and the left over voltage would go (depending on where its tied to) the starter (which needs somewhere around 50-100 or more amps to even attempt to do anything) or the radio (which could potentially click I guess) or the fan system, which if shut off wouldnt do anything, and even if on there would not be enough voltage/current to drive anything.
    My only other thought is ignition coil but again not enough voltage/current to do anything
     
  10. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    May 8, 2012
    I'm guessing that the fan is not wired to a true switched (+) rail or neg (far less likely) is not connected to GND. Nothing else make sense. These fans are milliwatters. It wouldn't surprise me at all if the + lead is connected to a sensor circuit of some sort, yet it still runs. Try wiring directly to the ACCY fuse.
     
  11. Electrobrains

    Electrobrains

    259
    5
    Jan 2, 2012
    I could imagine, the source of the problem is not the fan playing generator, but rather some unclamped inductance running crazy at the switch-off moment (the high voltage sparkle on the supply line will trigger some other circuit to do what it shouldn't).

    Either it's the electromagnetic coils of the fan motor or a built-in serial inductor preventing RF noise shooting back into the supply system.

    I am sure the simple two-diode circuit that Harald Kapp posted will work.
    My prediction is that even the parallel diode alone will do the job - test it!
     
    Last edited: Aug 4, 2012
  12. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    648
    May 8, 2012
    Sorry, but I don't believe so.
     
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