IPhone 3G 3GS USB Cigarette Lighter Socket Charger

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I've a couple of Cigarette Lighter Socket USB Chargers which live quite happily in my glove box, these tend to come in useful for charging any sort of device which can be charged from a USB socket, the iPhone 3G (like some Blackberry devices) has been a notable exceptio and that wound me up!

Having googled about a bit I found out that there was a simple fix - this is how to do it. You'll need a couple of 100k resistors, soldering iron and solder and a few spare minutes.

The Problem


Most of these USB chargers only provide 5 volts on the power pins of a USB cable. There is a +5v and a Ground (GND) feed at pins 1 and 4 respectively. Pins 2 and 3 in a USB socket are for data, chargers tend to ignore these pins and have them either floating or tied to Ground.

For the iPhone 3G or 3GS to charge, they need to also sense a current on the 2 data pins. There are a number of manufacturers now making USB Cigarette Lighter Socket Adapters which also provide this current on the data pins - but they also tend to be charging a premium over the cheaper more basic variants.

Being tight and already having 2 knocking about I thought I'd have a go at fixing one. If you don't already have a charger like this you may be better off just purchasing a charger that's certified for use with the iPhone 3G, however, if you're tight like me and you happen to have some spare resistors knocking about and are handy with a soldering iron then maybe this is the hack for you!

You can pick up one of the basic USB chargers here at DealExtreme for just $1.99 with free worldwide shipping.

Pulling Things Apart

This is always my favourite part, unfortunately for me (and fortunately for you) this particular type of charger strips down in seconds - unscrew the plastic cap on the tip of the charger and remove the metal ring and the charger will fall apart in your hands. Click on the pictures for larger versions.

If the adapter you have is different it's probably still worth stripping it down as it's highly likely you'll find the same or a very similar circuit board on the one you have. The thing to look for is whether it has all 4 pins in the USB socket - it's highly likely it will have. You'll also need to pay attention to whether the middle two pins are connected to anything, on mine, they weren't which makes life easy for us.

The other thing to note is whether there's enough space underneath the circuit board for additional components to be packed in - in this case we need to add 2 resistors into the circuit. With this particular model there's plenty of space, so we're good to go!

Fire up the soldering iron!

The Solution

Having Googled a little I happened across this page which gives a very detailed write-up on the chip used to provide charging in the 3G and 3GS. According to the specs anything about 1.25V to both of the data pins should start the phone charging. The Apple USB Charger actually outputs 2.0V and 2.7V on pins 2 and 3 of the USB socket respectively, so it looks like there's a bit of variation and leeway in what we need to provide it. So having soaked all that up and having seen how much space we have to play with in the case, the solution is fairly simply - we're going to take two resistors and bridge pins 1 and 4 with them, creating around 2V on pins 2 and 3, this is basically what Steve is suggesting about half way down the page on here.

There's another suggestion on this page about using a USB extension cable and hacking into that - personally I think that's a bit messy, my version is much cleaner and keeps everything nicely housed inside the chargers housing, looking completely standard. The charger is still to USB spec so there's not much chance of it harming any other devices you may happen to plug in, but YMMV, please test your devices or check the specs before plugging anything in :)



For this recipe you will need:

  • 100k Ohm Resistor (x2)
  • Soldering Iron
  • Solder
  • Multimeter
  • About 20 minutes, max.

We need two 100k ohm resistors to get approximately the correct voltage, these should be coloured Brown, Black, Yellow - I used Gold ones which are +/- 5%, but it's not too important.

In the picture to the right (click for a close-up) you can see the first of the two resistors being soldered into position. You may want to experiment if your case is different, but I found this allowed me to sit the resistor nice and close to the board so it'd easily fit into the case. Keep the tip hot and solder quickly so as not to dislodge anything or cross the tracks as you solder.

Leave a small part of the legs on pins 2 and 3 longer - you're going to want to join pins 2 and 3 together using that as shown in the picture.

The finished article should have pins 1 and 2 bridged by a resistor, pins 2 and 3 joined by solder/wire and then pins 3 and 4 joined by another resistor - that's all there is to it.

We don't want the legs of the resistors to short out against any of the exposed metal on the back of the PCB, so use a piece of thick insulation tape and slide it between the resistors and the board, that'll stop them shorting out. The fuse in the socket should stop anything really nasty happening, but better to be safe than sorry :)

After checking your soldering is good it's worth testing what you can with a multimeter, make sure the pins are connected only where you expect them to be, check your solder joints are good and look nice and shiny.

Re-assemble the casing and hopefully everything should fit back together with no problems - time to put it into the car and test before plugging in the iPhone...


Make sure you test the voltage on all pins before attempting to plug in your iPhone - I accept no responsibility for any fried iPhones!

The voltages are quite low and the charger is fused, so we shouldn't have anything to worry about but it's worth thoroughly testing the socket before plugging in the iPhone. Put the charger into the cars cigarette lighter socket and firstly check to see if the LED illuminates on the charger, if it does and it stays lit then you're probably half way there. If it doesn't then you maybe need to start again or read some of the links for more background info.

With the LED lit it's worth taking your multimeter and checking the voltage on each pin, between pin 1 and 4 you should see somewhere around 5.5V. Between pin 1 and 2 or 1 and 3 you should see about 2.6V. It's up to you to decide whether to plug in your iPhone 3G or 3GS at this point - seeing 2.6V on pins 2 and 3 and 5.6V on pin 4 I went ahead and plugged in my phone which started to charge right away.

That's it - job done!


The iPhone 3G and 3GS have good protection for over and under current and I believe also for reverse polarity, so it's unlikely you'll do too much damage to your phone - but please check carefully before plugging the phone into the newly butchered charger, I accept no liability whatsoever if you manage to turn it all into a pile of smoke :)

If you have any problems then please don't hesitate to drop me a twitter or and email, my email address is John at the domain name of this website.

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