LaFonera Hardware Power from USB

From DD-WRT Wiki

Jump to: navigation, search

You are here: DD-WRT wiki mainpage / LaFonera / Hardware / Power from USB

The LaFonera is a nice piece of hardware, but it is somewhat limited by its power supply which needs to be plugged into a wall power socket. Wouldn’t it be nice to be able to take your LaFonera with you, to use as a portable access point, or to provide wireless access for a laptop? I think so. :)

As we know from looking at the hardware specifications for the LaFonera, it takes DC power at 5v. Just like USB ports provide. Only one problem - the LaFonera's transformer is rated at for 2000 milliamps at 5v. USB ports can only provide 500 milliamps per port. If the load gets to high on the LaFonera, it may become unstable and crash. However, in my initial tests it seems to perform fine running only off a single USB port on a laptop on its own batteries.

This mod is quite simple; it requires no modification to the LaFonera, just a custom power cable which you will need to make.


[edit] Parts Needed

These are the parts and tools I used.

[edit] Parts

  • An old (no longer needed) AC-DC transformer with the same plug that the LaFonera uses. Or you can buy the plug from an electronics shop. Maplin in the UK sells (I think) the right connector here: 2.5mm DC Power Plug.
  • An old USB plug with a few cm of cable left on it. Or you can buy a USB plug from an electronics shop. Maplin in the UK sell them here.
  • Some 2 core cable if you are not scavenging it from the old transformer.
  • Electrical tape, or something else to insulate the joints from each other.


  • Heat Shrink Tubing

[edit] Tools

  • Soldering Iron Solder
  • Sharp knife
  • Wire cutters


  • Lighter to shrink heat shrink tubing
  • A Multimeter - very useful for checking which cable is connected to what etc.

[edit] Creating the Cable

[edit] Preparing the Cables

Cut the plug off the transformer cable leaving enough cable for your power cable. I used about 1 meter, as my Ethernet cable was that long. The length of this cable is the final length of the cable, as

Strip about 1 cm of the insulation from the wires, and split them apart a little way. Twist the ends to stop them fraying after stripping.

This is the USB cable I got the plug from. Cut it off something like the second photo.

Carefully strip off the outer black insulation being careful not to cut into the inner wires. Use your knife to carefully cut most of the way through the black insulation, then bend it a bit at the cut to break the last part before pulling it off.

Generally you will have a black, red, white and green wire. The black is ground and the red is 5v DC (be careful with this, the green can be the 5v and the red ground in some cables!). The others are for USB signals and can be cut off.

The inner wires are very fine compared to the power cable. They will break very easily, so be very careful when stripping the insulation off them. Drag the wires over the blade of the knife rather than pushing the knife against them to avoid cutting too far. A sharp knife will easily cut through the copper core, and if you score the core it will break more easily at that point.

[edit] Assemble Cable

Remember! - If you are going to use heat shrink tubing, make sure you put enough onto the cable to support and cover the connection before joining the two cables together!

I used this much heat shrink tube.
I used this much heat shrink tube.

Before you connect up the wires you need to confirm which cable should be ground and which is 5v. The easiest way to do this is with a multimeter using it to test resistance between one end of the cable and either the outside or the inside of the power connector. The outside of the connector needs to be ground, and the inside 5v. For me the cable with the white stripes was 5v but this may not be the case for all transformers, and some may use other styles of cables. If you don't have a multimeter then you will have to improvise some way to check which wire goes where. E.g. you could hook up a computer fan through the cable.

Once you know which wire is which, twist together the wires to hold them together while you solder them.

I am no soldering expert, so to avoid potentially teaching people bad habits I won't write any instructions for soldering. Instead you can read this page if you need help with soldering.

my cable looked like this after soldering.
my cable looked like this after soldering.

I actually had to take it apart after that photo, because I forgot to put the heat shrink tube onto the cable before I joined the two halves.

After you have a good solder join you need to insulate the exposed metal. I wrapped it in electrical tape.

Now you have to finish up the cable to make it look tidier, and provide strain relief to the connection. The USB cable's wires are so thin that they will break if even a small amount of force is put on them. To avoid this you need to fix the thicker power cable to the remains of the thick part of the USB cable, with the thin internal USB cables under no tension. This means any force on the cable is put on the thick wire rather than the thin wire. You also need to support the connection so it can't bend easily at this point.

To achieve this I used 3 layers of heat shrink tube, which seems to have worked well.

I bent the two yellow insulated parts towards the USB connector, then shrunk a small piece of heat shrink tube over them to hold them in place. To shrink the heat shrink tube just use a lighter, and move the flame back and forth under the tube. You shouldn’t need to actually touch the tube with the flame. Be careful not to heat the connection to much, or things will start to melt...

...after the first piece is shrunk
...after the first piece is shrunk

I then added a longer piece of heat shrink to completely cover the area.

Finally I added an even longer piece to cover everything up.

The completed cable.
The completed cable.

The result is a lumpy, but strong connection. You can now connect your LaFonera to a USB port and power it up. No special software configuration is required before powering up the first time. Enjoy your portable LaFonera!

- Hal 02:37, 10 Apr 2007 (CEST)

Dual USB power cable for 1000 mA
Dual USB power cable for 1000 mA

[edit] I need more power!

Some 2.5" USB harddrives use a special cable with 2 USB connectors on the PC end to provide up to 1000mA.

[edit] Update on two-head USB cable

Confirmed. The two head USB cable (typically for external 2.5" hard drives) will work to power a device that is rated as 12 volts, 1000ma. I'm sure there are dangers in doing this as the device will be underpowered. I suspect radio range will suffer and you could potentially brick your router if you tried to write out nvram settings while underpowered -- if, for example, the laptop was operating off of battery and went into hibernate right when you tried to save. I'm sure there are other dangers. You have been warned. It's been working for a few hours now. In fact, I can even unplug one USB and the router stays on. - Ascott Feb 23rd 2010.