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Need help with simple 12v fan cooling system!

  1. Jul 31, 2011 #1
    Alright so I have been bored this summer so decided to take apart my electronics such as my Xbox and AV receiver. I installed a nice 12vdc computer fan on the side of my Xbox so I moved onto my AV receiver and then ran into some problems. My current set up consists of:

    2x - 12vdc computer fans
    1x - regular light switch
    1x - 18vdc transformer (from old computer speakers or something)

    I understand that I will be running 9 volts to each fan which is not a problem and I have it wired all correctly, *I think*, but for some reason both of the 18vdc transformers I used overheated and stop working whenever I had them switched off. The fans ran fine for 10 or 15 minutes but once I turned the switch off with the plug still in the wall it overheated and would not turn the fans back on.

    I am guessing the problem is closing the circuit and backing up the electricity into the transformer causing it to overheat and fail but that is honestly my best guess. Please let me know if you need any other information.


    P.S. If anyone could help me rewire my circuit to run off extra power in the AV receiver so that powering the receiver off would turn the fans off that would be amazing! I wired my Xbox to the disk drive extra 12volts and would love to do the same with my receiver.
  2. jcsd
  3. Jul 31, 2011 #2
    You need to post a circuit diagram.
  4. Jul 31, 2011 #3
    Here is an attached circuit layout. to the best of my ability.

    Attached Files:

  5. Jul 31, 2011 #4
    omg! you have placed the switch the wrong. You need to place it in series with the load, not in parallel. Like this
  6. Jul 31, 2011 #5
    This is new stuff you put in there?

    Yeah, that switch is a disaster. It's in parallel with everything, so when you close it it shorts the transformer's secondary, causing massive amounts of current to flow, exceeding the rating of the transformer, overheating it, and likely destroying it or the switch.

    Rewire it like IAL said.
  7. Jul 31, 2011 #6
    Thank you guys. I just noticed that was a really dumb mistake! So in that picture is red positive or is black? And would any of you be able to help me wire it directly into the av receiver if I uploaded a few pictures of the interior?
  8. Jul 31, 2011 #7


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    Staff: Mentor

    In your OP you mention an 18V power supply, but in the diagram have 12V. Is it really 18V? If so, you're sending 18V to each of your 12V fans...
  9. Jul 31, 2011 #8
    Since you made that schematic, you must be knowing what is positive and what is negative.

    The switch can be placed on either the positive wire or the negative wire no problem.

    If you have a Multimeter, it may be possible to identify +12V supply from inside the Machine.
    Do the AV receiver has an external adapter or it is directly connected to the mains?
  10. Jul 31, 2011 #9
    Oh! I re-read the OP I now recognize that the OP seem to use 18V transformer to run 2 fans and he/she seems to assume each fan would get 9V (assumes they are in series).

    But the schematic is a total different. Whats the matter jheiges? If your transformer is really 18V then you are at the risk of damaging the fans with that circuit (both which you drew and which I posted).
  11. Jul 31, 2011 #10
    Okay first off thanks a lot for the help. I am obviously not the smartest when it comes to electronics and circuits.

    My transformers were 2 different volts and both burnt out for the same reason. The first was 18 volts and the second was 12. Sorry for the confusion on that. As for damaging the fans, I did assume that the way I had it would split the voltage in half for each fan but I am guessing from your post that is not the case. I will be hooking up a 12v dc using the circuit you provided.

    From the schematic I made I made black negative and red positive but when I saw the switch on the black wire in the new post I did not know it was perfectly fine to use either wire. As for the AV receiver I have found a cord similar to the one I hooked my Xbox fan through http://img.diytrade.com/cdimg/1243773/17807402/0/1292828089/XBOX360_DVD_DRIVE_Cable.jpg" but now that I am feeling even less than confident on my circuit abilities I don't know how I would run a correct circuit in order to not burn out my receiver in the same fashion I did my DC converter. Attached is what I think would work for the circuit but PLEASE let me know if I am doing it wrong. The 120vac main power cord is where I would be running the negative wires and that scares me. Also just because it says 12v+ does that mean its dc? Thanks again.

    Attached Files:

    Last edited by a moderator: Apr 26, 2017
  12. Aug 1, 2011 #11
    Your latest diagram is a disaster, and potentially life threatening! Why in the world do you think you should tap into the main 120VAC power?!?!

    https://www.physicsforums.com/showpost.php?p=3430088&postcount=4" has a perfectly acceptable wiring schematic.

    I urge you, in the strongest possible terms, before you experiment any more with things that plug into the wall, STUDY ELECTRICITY! Here is a very good introduction -- http://www.faqs.org/docs/electric/index.htm. If nothing else, please read the section on electrical safety -- http://www.faqs.org/docs/electric/DC/DC_3.html.
    Last edited by a moderator: Apr 26, 2017
  13. Aug 1, 2011 #12
    Yeah, I repeat what pantaz just suggested.
    If you are sure you have 12V transformer follow the circuit at Post #4 for the moment. You will be just fine.
    Red is +ve, Black is -ve just as in the original circuit.

    Consider powering the fan from the AV reciever only after you have gained some knowledge and confidence.
  14. Aug 1, 2011 #13
    Alright thanks again for the help. I assumed it would work like my Xbox circuit and the 120vac negative cord would ground the negative wire but I guess that is wrong. I will study up before doing any more electrical circuits. Since I am way wrong about using the 120vac to ground the wire what exactly would I be looking for to ground that negative wire? Sorry for being more than dumb when it comes to electricity.
  15. Aug 1, 2011 #14
    This is the exact AV receiver I own. The wire that I marked 12v+ literally says "12v+" under one of the 6 wires in that cable. I marked the red circles to show spots I THOUGHT I could complete the circuit by plugging in the negative wire. One is the main power cord (this is bad from what you said), the other is the black wire running to the transformer, and the other is a cord that I think is a grounding wire because it ends right against that metal plating. If you do not feel like messing with this project anymore I would completely understand and will go with the schematic that was posted earlier. Thanks.

    Attached Files:

    • av.jpg
      File size:
      41.4 KB
    Last edited: Aug 1, 2011
  16. Aug 1, 2011 #15
    This should be the last picture. Thanks for the help.

    Attached Files:

  17. Aug 1, 2011 #16
    Can the existing 12V system handle the additional load the fans would put on the system?
    If it can't there is a distinct possibility you will sag the 12V rail and possibly cause other problems.
  18. Aug 1, 2011 #17
    Well I honestly have no clue... Would it hurt to try it and see what happens? I really don't know much so that is kind of why I am posting here. lol
  19. Aug 1, 2011 #18
    There is no positive or negative with Alternating Current (AC). The ground wire for household AC power literally connects to the ground -- Earth. There is a long metal rod buried in the ground outside your home with a wire connected to it.

    Direct Current (DC) circuits often list a "ground", but this is more correctly called a "chassis ground". The chassis ground is almost universally connected to the negative side of the DC power supply, BUT, there are exceptions! Also, do not count on the DC chassis ground being connected to the household ground, especially on devices that do not directly plug into a wall outlet for power.

    It is best not to use the term "ground" for DC negative in circuits such as your fans. Using "Positive" and "Negative" prevents misunderstandings and errors.

    Now... I really want to see how you wired your XBox. I'm very nervous about what you might have done. Seriously.
  20. Aug 1, 2011 #19
    "Bad" is correct. Leave it alone.

    I don't know what the black wire might be. The color of the wire means nothing -- Unless a wiring diagram specifically identifies particular wire colors for particular purposes, DO NOT ASSUME!

    Maybe, maybe not! Without a schematic, or at least carefully studying the circuit and measuring voltages, you cannot be certain.

    Regarding the notes in the photo, the best I can say is "maybe". The wires labeled "GND" are probably the DC negative, but without a volt meter, it's just a guess.

    As mentioned in post https://www.physicsforums.com/showpost.php?p=3431261&postcount=16", there is a good chance that the power supply can't handle the extra load of the fans. Even if it can, it's possible that the systems circuitry will behave erratically with the extra load, or detect it as a fault.
    Last edited by a moderator: Apr 26, 2017
  21. Aug 1, 2011 #20
    My xbox has been running for about 1 months now no problem. I ran it just like you would run the 12v fan mod for the rear fans except I used my own fan. My Xbox is not the problem now but I think I am going to give up on using the internal 12v on my receiver and stick with running a dc power supply from the wall. Thanks.
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