This Instructible is a simple application of a linear voltage regulator to power a 5V fan from a 12V cable modem. (UPDATE: As people have been pointing out in the comments, you could use a 12V fan without the need for anything else.) In this case I had the adjustable voltage LM317 linear regulator and a 5V fan handy, so I chose to use them, but a fixed 5V linear regulator would have worked just as well.
Here's what you'll need:
LM317 Adjustable Output, Positive Voltage Regulator
270 Ohm and 820 Ohm resistors
1.0uF electrolytic capacitor
(optional) 500 Ohm (503) potentiometer
mounting screws (I used 2 wood screws for the fan and two random little ones I had handy)
Soldering iron (and supplies)
Power drill / drill press
Drill bit (for drilling pilot holes for the mounting screws)
Problem: My cable modem kept overheating and I would lose internet connection while streaming Netflix. I tried improving the air flow by chopping open the top of the modem with a Dremmel Plastic cutting disk and replacing the heatsink with a slightly larger metal one. NOTE: (If you decide to replace your heatshink, make sure you use thermal plaster and not thermal paste. The difference is that thermal plaster hardens into an adhesive bond after a few hours, ensuring the heatsink will stay on the chip without the need for any screws or fancy mounting clips. Everything I use I purchased from ebay.)
The cable modem runs off of a 12V supply but the only fans I had available were only 5V fans.
Solution: A simple linear regulator circuit will drop the voltage down to 5V and provide a nice stable 5V source.
I tapped the cable modem's barrel jack internally, which required opening the cable modem. (*WARNING*) If you rent your cable modem opening it will void the warrantee and if you break the modem you will be out $100-$200. I own my modem and took the risk.
(Never drill through the lid while it is above your cable modem board, remove the lid entirely and work on it separately so you don't slip and break your modem)
Then I drilled a hole in the lid and fed the positive and negative wires out. (I recommend doing this last)
Step 1: Tap the Barrel Plug for a Power Source
As shown in the picture (in most cases) the centre pin is positive. You can determine this by looking at the glyph below the letters "DC" in "DCM476".
In the other pictures I show that I've already chopped a slightly bigger than necessary hole in the top lid of my modem. The other photo shows (in the bottom left) where the barrel plug is and which pins to solder your wires to.
NOTE: Not all modems use a 12V supply, this is why it is preferable to have a voltage regulator and a 5V fan since connecting a 12V fan would provide mixed results.
Step 2: Prototype Your Circuit (recommended)
Using a breadboard I played around with various resistor combinations to find the correct voltage output.
The first two pictures are of my test setup.
Although the sample circuit recommends R1 to be 220 Ohm, I found better results using a 270 Ohm resistor. With R1=270 Ohm and R2 = 820 Ohm I had a Vout = 5.04V
(Optional) Replace R2 with a 270 Ohm resistor and the 500 Ohm Potentiometer in series to make fan speed adjustable. The fixed resistor is there to make sure the potentiometer doesn't short the LM317 to ground. The potentiometer has 3 pins, the middle one is the wiper, short it to either of the other two pins to make it an adjustable resistor.
This is also a good chance to make sure your fan won't be annoyingly loud! Choose your fan carefully!
Step 3: Put It All Together
During the actual assembly and soldering of the circuit it is important to pay close attention to the connections you are making. Frequently check your circuit diagram to ensure you are connecting the correct things to the correct pins. Some diligence now will save you a lot of frustration (and replacement parts) later.
Keep in mind where you will connect the power wires you attached to the cable modem, by not attaching them at the start, you can work on this while having the internet available to check on any information you need.
As you can see on my board, I used a pair of wires to move the connections for the fan away from the rest of the circuit, I did this so that if I had to change out the fan, it would be easier. As it turns out, this fan was way too loud once attached to my cable modem! I think it developed a ballberring problem shortly after being turned on (but hey, it was $0.99 on ebay!) Luckily I had another 5V fan that is a lower profile (10mm) and it was much quieter. So luckily I had made this provision to make changing the fans easier.
Step 4: Mount It on the Cable Modem
To make maintenance easy, I used screws to attach the protoboard to the top of the cable modem. To make this work you need a power drill or drill press and a drill bit between 5/64th and 1/8th to drill pilot holes where you want the screws to go. (Don't do this over the circuit board, remove the lid, then do the work).
Once you're ready to attach the board to the modem, solder the power wires you attached earlier to the protoboard for ground and Vin. Now use a screw driver (or power drill) to put the screws through the prototype board/fan and into the cable modem. As shown in the picture. As you can see, I cut back a little bit too much of the lid of my modem, so the fan doesn't sit quite over top of the heatsink, but it works fine.
I say, use a screwdriver for the screws because the plastic is pretty soft and if you use a drill you run the risk of putting a screw through your circuit board. Ideally, use screws that are so short they can't reach the circuit board.
THAT'S IT! Hopefully you've learned something about the application of linear regulators like the LM317 and you now have a nice and cool modem ready for some Netflix and Chill!