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Stop a fan form being a generator

Everytime

Aug 2, 2012
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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!
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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Jan 21, 2010
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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.
 

Everytime

Aug 2, 2012
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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.

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.
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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I am sure that the issue is the fan pushing voltage back through the accessory wire

I think that's highly unlikely

because when I jam the fan with my finger before shutting off the ignition, everything is okay.

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.
 

Everytime

Aug 2, 2012
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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.

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.
 

Harald Kapp

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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.
 

Everytime

Aug 2, 2012
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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.

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.
 

Harald Kapp

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fan doesn't make a negative voltage so the diode will still let it the electricity through
I don't understand.
Consider this circuit:
attachment.php

Note: the diode parallel to the fan is optional, not necessarily required.

Harald
 

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GreenGiant

Feb 9, 2012
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I believe this happens because the fan is creating a current that shouldn't be there when the ignition is off.

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
 

CDRIVE

Hauling 10' pipe on a Trek Shift3
May 8, 2012
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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.
 

Electrobrains

Jan 2, 2012
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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!
 
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CDRIVE

Hauling 10' pipe on a Trek Shift3
May 8, 2012
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Sorry, but I don't believe so.
 
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