Noob question

Discussion in 'General Electronics Chat' started by robertn78, Jun 14, 2013.

  1. robertn78

    Thread Starter New Member

    Jun 14, 2013
    4
    0
    Hello,

    I have a PC that had a fan go out. The fan is 5V. I cannot find a replacement (Foxconn proprietary). I found a 12V Fan & Heat-sink that work better than the original. My issue is, there is no 12V in the box at all. It is laptop components and does not require 12V. The best I have is 5V and 3.3V. Coming in from the power supply is 19.8V which is promising. I figured I would just use a votage regulator to lower it to 12V. I tried a 7812 but on futher inspection, I think it requires 35V. I was hoping 35V was the max and it would output 12V anyway. There is nothing else in the circuit. It just goes 19.8V to the input and no caps or resistors. Sorry, I'm a Noob:rolleyes:.
    So, I am turning to you guys for help:). I think the best route would be to voltage drop or regulate it, but I don't know how. I am good at following instructions and soldering, but my knowledge is not there. Keep in mind the space inside the case is at a premium. The heatsink for the TO-220 casing had to be squished to fit.
     
  2. tubeguy

    Well-Known Member

    Nov 3, 2012
    1,157
    197
    The 7812 does not require 35 volts if that's what you're thinking. It will work fine with ~ 19 volts DC input. Why not post a drawing of how you have it wired.
     
  3. robertn78

    Thread Starter New Member

    Jun 14, 2013
    4
    0
    TubeGuy - Ok, so that is good. I was hoping that it did not need 35V. But I have it running direct in and out. I have a feeling I need capacitors on the Vi and Vo to ground like the 2nd pic below.

    What it is now...
    [​IMG]


    Probably what it should be
    [​IMG]
     
  4. #12

    Expert

    Nov 30, 2010
    16,247
    6,739
    Yes. Figure 7 is the right thing to do.
    Your idea is simple and easy for people that do this all the time.
    You did not say the 19 volts is DC. That is a requirement for the 7812 chip to work correctly. Check that. If it is AC, we can fix that, too.

    Personally, I would not try to stuff extra parts in a laptop computer, but I have a strict rule that says your choice is none of my business. If you can physically fit it together, I can only congratulate you.
     
  5. Shagas

    Active Member

    May 13, 2013
    802
    74
    Just keep in mind that you will have a fan blasting at 100% all the time . Won't that bother you ? Sure as hell would bother me :)
     
  6. robertn78

    Thread Starter New Member

    Jun 14, 2013
    4
    0
  7. robertn78

    Thread Starter New Member

    Jun 14, 2013
    4
    0
    I built a circuit simular to figure 7. The only change is, I used a .22uF and a .1uF in parallel to make the effective .32uF instead of .33uF in schematic at Ci. I used the correct .1uF at Co. I used Polyester-Film Capacitors and all are rated at 50V max. I get exactly 12V across ground and Vo. But, the fan seams to over speed. Any ideas why?
     
Loading...