Introduction: Garmin & Samsung Charger Reverse-Engineering
Ever have an issue trying to charge a device that uses a proprietary charging cable? Especially one that has a standard USB port but won't work, or works poorly, with a standard cable? And you're stuck with paying Way Too Much for an OEM replacement? If you can find one at all? Yeah, me too, and I find it extremely annoying. Fortunately, most manufacturers who use USB connectors at all don't make it too hard to reverse-engineer their products. The two devices I have personally had experience with are some Samsung tablets (micro USB) and several Garmin Nuvi GPS units. I successfully made charging cables for both of these, using nothing more than one or two resistors and standard USB connectors.
Step 1: The Garmin Problem
When I plug a standard Mini-USB cable into my Garmin Nuvi, even if it's a charge-only cable (2 wires, no data), the unit goes into "PC Sync" mode and cannot be used for navigation when connected. Well, that's great if you just want to charge the battery for bicycle or non-vehicular use, but what if your original cable is lost or damaged, or you want a cable to plug into a USB port on your car, or you just want a different kind of cable than the manufacturer's proprietary cable? Like, one that's longer, or shorter, or not coiled like so many are? Are you out of luck?
No! With just a little over a dollar's worth of parts, plus one of those dozens (Hundreds?) of spare USB cables you've got lying around, you can easily make a custom power cable for your Garmin!
Step 2: The Garmin Solution
All you need to pull off this minor miracle is:
- A standard USB cable in your choice of lengths. It can be mini, micro, or standard, as long as it has the standard plug that fits a computer on one end. You will be cutting off the other end.
- An 18K (18,000 Ohm, brown-gray-orange color code) resistor, the smaller the better. 1/8 watt if you can get it, otherwise 1/4 watt.
- A Mini-USB connector shell like this one from Adafruit. Less than a dollar.
- Soldering equipment, wire cutters and strippers, and some (fairly small) degree of soldering skill.
- Optional but highly recommended: A mini-USB breakout board so you can probe your connector and be sure what wire goes where.
- Steady hands...
A little background: A standard USB cable has four wires: Two for power (Usually red and black), and two for data (Usually white and green). Unfortunately for us, the Mini-USB and Micro-USB standards allow for Five Wires! What's that extra wire for, you might ask? It's called an "ID" pin, and it is why we need that connector shell, instead of the slightly cheaper and easier method of cutting a USB cable in half and adding inline resistors. You see, the only difference between a "stock" USB cable and a Garmin cable is the Garmin cable has an 18K resistor between the "ID" pin and Ground.
Rather than go off on a fruitless search for a 5-wire USB cable, we will install our 18K resistor in the connector shell, and hopefully, if we did the job neatly enough, we might actually be able to close the shell around it.
Step 3: The Garmin Assemblage
The reason for the recommended breakout board will now become evident for two reasons: To verify which of those minuscule pads on that connector are which, AND to test your finished cable before risking letting the smoke out of your valuable gadget.
I have run across a few cheap Chinese cables that do not use the standard color coding of:
- Red - Vcc (+ power)
- Black - Ground (- power)
- Green - Data +
- White - Data -
If you just assume, as I did (once!), that the color coding is correct, and it's not, something will smoke, so unless you are sure of the pedigree of that cable, TEST! Fortunately, I only blew up a cheap power bank, and not my GPS or my phone...
The Mini-USB connector shell has five very small solder pads. Holding the connector with the pads up and the plug pointing away from you, the pads from left to right are: Ground, ID, Data +, Data -, and Vcc (+). If you have any doubts at all, plug that connector into your breakout board and use your meter or continuity tester to be sure.
Cut one lead of the 18K resistor very short - as in about 2mm or less than 1/8". solder this lead to the ID pin, which should be the leftmost pin in the top row. Bend the other lead 180 degrees to the Ground pin, which should be the leftmost pin in the bottom row. Don't solder it yet, but cut it to a length that will just reach the ground pin.
Cut the non-computer end of your USB cable off (The small or square end). Push the USB cable through the strain relief supplied with the new connector if it will fit (mine didn't, but there's a solution to that later). Peel off about 1/2" of the outer jacket, being very careful not to nick or cut the inner wires. If there is a foil or braid shield, cut that off close to the end of the jacket. You should now have four thin wires. If it's a charge-only cable, you'll only have two wires. Strip no more than 1/8" off each.
Solder the ground (black) wire first, after bending that other resistor lead so it's touching the leftmost bottom row pin. Then work your way across, soldering the D+ (green) wire to center bottom, D- (white) to top right, and Vcc (red) to bottom right.
Before closing things up, plug your new cable into a USB power port (I don't recommend plugging it into a computer at this point), plug your new connector into your breakout board, and use your multimeter to verify +5 volts between Vcc and ground. Double-check you've got the resistor on the ID pin and not one of the data pins. Unplug from your power source and use the Ohms setting to check. It should be 18K between ID and ground; all other resistances should be infinity.
Now you can close it up and test it on your GPS! If it doesn't work, try shorting the ID pin to Ground instead of using a resistor. All I know for sure is this works on a Nuvi 1250 and a Nuvi 660.
If your strain relief didn't fit because the cable was too fat, run a bead of hot-melt glue around the cable where it exits the connector, the slip a piece of heat-shrink tubing over connector and wire. When you shrink the tubing, the glue will re-melt, filling the space within. When glue starts to seep out the end of the tubing, you know you've got a good strain relief!
Step 4: The Samsung Problem
Some Samsung tablets (and maybe phones, too) - the later ones with the Micro-USB connector - when used with a standard Micro-USB cable, will not charge at their full rated charging current of 1.3 Amps. Instead, they will charge at a lower current of .4 to .5 Amps, doubling the charge time or worse, even if used with a power source capable of the full amperage. This is indicated on the battery icon of the tablet with a red "X" across the battery. If the tablet is being used, the battery may never catch up. This is caused by Samsung's use of proprietary charging cables. Luckily, this too can be fixed (or, "hacked," if you prefer).
Step 5: The Samsung Solution
For this you need:
- One 33K resistor (orange-orange-orange)
- One 10K resistor (brown-black-orange)
- One standard micro-USB cable.
All the preceding warnings about USB color-coding apply here also. Reference the schematic in this step. Cut the cable in half at whatever point along it's length suits you. Peel back about 1/2" of the jackets on both cut ends. Strip each of the internal wires. Most of the action here happens on the "tablet" end of the cable. Begin by connecting the Data+ and Data - (white and green on a standard cable) wires together. Twist one lead each of the 10K and 33K resistors together, twist onto the white and green wires, and solder this connection.
At this point, think a bit about how you will protect the finished splice. If you're using heat-shrink tubing, don't forget to slip it on now, before re-joining the two cable ends.
Now, take the red wire from the power end, the red wire from the tablet end, and the other lead of the 33K resistor, connect all three together, and solder. Do the same with the two black wires and the other lead of the 10K resistor. You're done, except for insulating and protecting the splice. For more ideas on splicing, see this Instructable.
Be sure to test your cable as noted in step 3; You'll need a Micro-USB breakout board for this one, of course.
Note: This cable has now become a charge-only cable, since you cut off the data wires on the power end. It's a good idea to label it as such, to avoid future frustration!
Note 2: This can probably also be done with a Micro-USB connector shell, but it'll probably be harder to fit two resistors into the shell.
Reverse-engineering is fun, and can be very profitable in terms of money saved. If you have any more reverse-engineering ideas, leave a comment and you may get a free premium membership!
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