Cable Modem Active Fan Cooling




Introduction: Cable Modem Active Fan Cooling

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:

(one each)

Prototyping board

5V fan

LM317 Adjustable Output, Positive Voltage Regulator

270 Ohm and 820 Ohm resistors

1.0uF electrolytic capacitor

0.1uF capacitor

(optional) 500 Ohm (503) potentiometer

Some wire

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)

Screw driver

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!



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    12 Discussions

    I'm pretty sure there is 5V available on the modem pcb, if not, a simple resistor will make the job. I know it's not so clever and precise but it works fine enough.

    9 replies

    Soldering long wires to the pads of the surface mount parts of the modem would be tricky. In my case the MCU was 3.3V, so there were no 5v sources on the board.

    Using a resistor can work, but keep in mind that it will be dissipating heat proportional to the current you draw, which is proportional to the voltage drop across the resistor.

    If you wanted to, I suppose you would have to connect the fan to a potentiometer to the 12V supply with a voltmeter in parallel. You'd then adjust the pot until you had 5V with the fan operating. You could then use your multi-meter to measure the pot's resistance and then replace it with a similar valued resistor. Since it's unlikely an exact match will be available, the voltage won't quite be 5V. You'd also have to measure the amount of current (I) to ensure you used a resistor with ample heat dissipation rating. In this case we would be dissipating (7V*I) as heat, so you might end up needing a 1W or higher resistor.

    Linear reg's works the same ways as resistor do, they both dissipate heat propotional to current draw. The only difference between them is that lin. regs keep the voltage constant regardless of the current draw.

    And you get to slap a heatsink on a Linear reg to dissipate all that heat. I'd probably need a 3W rated resistor to do the same job.

    You know that most of the 5V fans will work with 3.3v but at a lower speed. It couldn't be enough for your app. Maybe a voltage converter from 3.3V to 5V would fit.

    You are absolutely right, If you have no Resistor with the right vallue and Wattage handy you can make your own by soldering smaller or larger ones in parallel and serial. Only the calculations are a bit time consuming, not to exceed the 250mW for small resistors. Are you aware that most smps like in your computer have high wattage resistors as dummy load. PC power supplys usually have ceramic 5Ohm 5Watt square ones(little white brick).

    And the dissipated heat is exact the same as with a linear regulater.

    I did go the other way with a dvb-s receiver i mounted a 24 volt very flat fan directly under the cover and connected it to 12 v. the loudest is the harddisk, and even with the hd shut down it is almost undetectable.

    I wonder why he used an 5 instead of a 12 V fan, when he already was willing to make a large hole in the housin. I have plenty of 12v laying around from old PC SMPS and CPU coolers. A12V is aso easier to slow down by a serial resistor because you nee much much less Wattage, think of it.

    Just keep in mind that the ratings on the power bricks that supply the modem aren't 100% exact. I have a bag of 'D-Link' branded wall-warts that say they are 7.5v 1000mA. But with no load they are 11.5V to 13V, and when loaded down they can source 860mA at best and the voltage drops down to 5.5V. I made a YouTube video about them:

    So if you are drawing 800mA without a fan, and expect to have the head room for a 170mA on a "1000mA" supply, you might not get the results you expect. The voltage might go lower and affect the operation of the modem or the fan won't spin as fast.

    or if there's no schematic diagram to show where it is located and does not have guts on playing around the board, might as well use a car
    USB charger board instead.. ;)


    That's pretty clever! They already do 12V to 5V!

    Why hack the case up if it already has air vent holes? I has CISCO
    modem is that has large 3x50mm vent opening, seems like it would be
    easier to mount an external fan or two in push pull setup. either
    powered from modem wall wart or externally from a 1A AC to 5V USB phone charger?

    1 reply

    Adding a fan was a last resort for me. When I originally hacked open the case, I wasn't considering adding a fan. I was hoping that switching the tiny ceramic heatsink with a slightly larger metal one would be enough to keep it passively cool. That wasn't the case, so I had to add a fan, and since I'm probably not the only one, I made it into an instructable about the LM317.