Homemade Iphone 3G Car charger

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MB Enthusiast
Sep 13, 2006
North East
Just to keep the Iphone fraternity happy ( tongue firmly in side of mouth )
heres how to make a car charger patch cable. Apple have changed the way these things charge and your old iphone or ipod or simple usb charger will probably not work, so heres how to make a patch cable.

Alternately £3.50 delivered from Play. :D

cut up a short USB-extension lead instead to make an
adapter cable that goes between the car adapter and the iPhone USB
If you have some electronics experience, follow the instructions below
to construct your own charging adapter cable.
If you never soldered before, then don't try. If you connect things
wrong there is the potential risk of damaging the phone.
As usual, if you do make your own cable, you do it entirely at your
own risk. If your phone dies, do not blame me. Mine is working fine,
so the procedure does work if done properly.
The USB has 4 wires:
Red-5V, White-Data, Green-Data, Black-Gnd.
They are numbered Pin1 to Pin4, in the same order, ie Red =Pin1.
1. Cut the USB EXTENSION lead, trim both pieces to a shorter size, but
make sure you leave enough length to work with !
2. Cut off the foil-shielding on both pieces, twist the braided wire
shielding into a single "wire" on each piece.
3. On the female piece of the lead solder resistors as follows:
R1=2K2; R2=680; R3=2K2
4. Trim resistor legs as short as possible. Make sure you put some
insulating tape between all the exposed bits of wire and resistor
legs. You want to pack the resistors as tight as you can, and you do
not want any metal bits touching!
5. On the male end of the lead, cut off the white and green wires; you
DO NOT want to connect them to anything! If they were connected it
would feed power back to the car adapter, which could conceivably have
those pins grounded as they're usually not used (hence the iPhone 3G
charging problem).
Even worse, you might one day accidentally plug this new DIY adapter
cable into a USB port on your PC, and it may take offense at having
power coming in on the data lines.
So cut the green and white wires on the male end.
6. Cut a piece of large-diameter heat-shrink and slide it over the
male piece of the lead. It should be long enough to later cover the
area where the resistors are. Hence the male end of the lead needs to
be at least as long as the length of heat-shrink you need!
7. Join the two pieces of lead by soldering the male red wire to the
female red, male black to female black, the shields to the blacks.
8. Carefully compact all the resistors together, ensuring there is
insulation between adjacent metal parts, and that nothing comes lose.
You might need to think about how you want to pack the resistors
Then slide the heat-shrink over the resistor assembly until it
overlaps the untouched part of the the USB cable on both sides of the
resistor assembly.
Heat up heat-shrink to make it grip.
* If the assembly is bigger than the heat-shrink you chose, remove
heat-shrink and tightly wrap the entire cable with electrical tape
instead. Done!
9. WAIT! DO NOT CONNECT iPhone, yet!!!!
10. Connect your new adapter cable to a car USB adapter.
With a multimeter (and thin probes, or bit of stiff wire soldered to
the probe tip) measure the voltages you get on the female end of your
new adapter cable.
Put the Gnd-Probe on the plug shield, and be extremely careful with
the other probe not to touch both the shield and a pin at the same
You should have ~5V on Pin1, ~2.8V Pin2, ~2V Pin3, 0V Pin4.
If you do, Congratulations, you now have a USB adaptor cable that will
turn any plain USB voltage adaptor into an iPhone 3G charger.
If you have no voltage, you might have forgotten to solder the shield
to the black. Put Gnd-probe on Gnd-Pin4, then measure all other pins
and shield. If the pins measure ok, and the shield is still 0V, you
are probably ok too.
All other results, and you need to re-do your cable!
11. IF YOUR CABLE MEASURES OK, at your own risk (!) you can now
connect your iPhone to the adapter cable, and then into the car
adapter. It should now charge.
Note: Apple states that the iPhone should only be connected to
authorised chargers,etc. Using the DIY adapter cable could potentially
void your warranty.
Many thanks for your 'How to'. i would not try personally but well done.

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