How to make your own iPad 2 USB Charger
(DIY iPad 2 USB Charger)
I decided to build a car usb port that could charge the iPad 2 while in use. (5vdc/10W/2A)
Warning - I have not tested this design beyond my own usage and anyone that uses this design does so at their own risk. I take no responsibility and this information is offered as is and is offered as a reference only. Make sure you verify all details independently before plugging in your device.
Step 1: First I took voltage reading on the 10W USB charger that comes with the iPad 2. Here are the voltages that I measured in the no-load condition. So the main power used to charge is 5vdc applied to Pins 1 & 4. Pins 2 & 3 are needed to fool the iphone /iPad into thinking it is connected to a real usb port otherwise charging will not occur as apple wants to see these voltages on these pins before the iphone or ipad will allow charging to begin. (Original Charger is 10W=5vdc/2A).
Step 2: Pin 2 and 3 probably do not need to be precise but approximate as USB uses TTL logic so these pins will go from 0vdc to 5vdc during communications. I picked out a circuit using standard components. Using a voltage regulator to maintain the 5vdc, I then use two voltage dividers to get close to the 2.02vdc and 2.73vdc values for Pins 2 and 3 on the usb port. See my worksheet for designing these two voltage dividers below:
Step 5: Board Layout
The board can be laid out as shown below or you can design your own board and layout. Just make sure you have the pins on all components connected properly and test the voltages on each pin prior to hooking up any equipment.
Note Below: You can use this as a template. Be sure the solder pads are on the back side of the board where you will solder. Picture below shows the the component side for mounting. I cut out this section on a band saw using the 0.100” spaced holes as my reference. You will need to add 2 holes to the side of the board to accommodate the two pins on the usb connector to give it proper strength. If you have more room then I would suggest that you spread out the connecting terminal as soldering is a bit tricky in the current layout unless you are a good solderer with the right equipment. Not to intimate that my soldering is good as it has a lot to be desired.
Step 3: Select Components and design simple circuit.
See final circuit below. Even though I show 12vdc for the input, the dc-dc converter will accept any voltage between 7-18vdc.
Step 4: Parts List:
1X DC-DC Converter / Digi-Key / 102-2188-ND / $10.15
1X Capacitor, 22uF, 16v / Digi-Key / 445-4825-ND / $1.24
1X Capacitor, 10uF, 25v / Digi-Key / 445-2898-ND / $1.14
1X Fuse, 3A / Digi-Key / F2438-ND / $0.91
1X Prototype PC Board / Digi-Key / V2010-ND / $7.33 (Enough to make 4 units)
1X USB Connector, Dual Port, PCB mount / Digi-Key / MUSB-C111-30-ND / $9.34
1X Resistor, 10KOhm, 1/4W Watt / Digi-Key / CF14JT10K0CT-ND / $0.08
2X Resistor, 15KOhm, 1/4W / Digi-Key / CF14JT15K0CT-ND / $0.08ea
1X Resistor, 20KOhm, 1/4W / Digi-Key / CF14JT20K0CT-ND / $0.08
Grand Total: ~$30 + Tax + SH
* Data Sheets and Drawings can be found on the digi-key website for each of the parts listed above.
Step 6: Put It All Together:
First two pictures show the circuit board without the USB connector installed as I am still waiting for them to arrive for the 2nd and 3rd module that I am building. The external wires are to connect to input power. In my application, I panel mounted the usb connector next to the cigarette lighter in my car and soldered the input wires to the correct pins on the cig. lighter that is right next to it. I will update these pictures later. The last two pictures show the first unit I built installed and working completely.
Tools Needed for this job:
>Soldering Iron + Solder
>USB Cable (To create test cable shown above)
>Volt Meter (To verify voltages when done)
>Band Saw (To cut the circuit board to smaller size if needed. Can use dremel or comparible method or leave full board is space is not an issue)
>Milling Machine (to add the two holes to the circuit board for the usb connector. (Could layout by hand and use a drill if you are good with hands/tools)
> Insulated and non insulated wire (for board jumpers and input wires)
> Components listed on the Bill of Material
> Time to do it right.
Step 7: TEST BEFORE YOU PLUG ANYTHING IN:
This usb charger works for both my iphone 3gs and my new iPad 2. Both have been tested on the first module I built and installed. Use an existing USB cable, carefully remove the metal shell on the correct end so you can probe the voltages. Apply voltage to the input cables of the module, plug your test cable into the either usb port and probe the voltage on each pin. Compare voltages with the calculated values listed at the top of this page. Make sure you have the pin numbers correct.
Panel Mount the USB Connector:
To cutout for the usb Connector and panel mount, I removed the section of the dashboard, clamped it in a milling machine, and then machined out the opening and the clearance holes for the two mount screws. You may be able to do this by hand but my skills are not good enough to do this neatly so I used a machine for accuracy. See below for reference.
GOOD LUCK AND LET ME KNOW IF YOU RUN ACROSS ANY MISTAKES OR IF YOU HAVE ANY INFORMATION TO IMPROVE THIS DESIGN.
January 14th, 2012 Update:
Section under Charging Ports:
Before the battery charging specification was defined, there was no standardized way for the portable device to inquire how much current was available. For example, Apple's iPod and iPhone chargers indicate the available current by voltages on the D- and D+ lines. When D+ = D- = 2V, the device may pull up to 500mA. When D+ = 2.0V and D- = 2.8V, the device may pull up to 1000mA of current.
It is now my belief that the iPad 2 knows when it is connected to the apple 10W usb wall charger thus allowing the full 1.5A current draw to realize maximum charging rate. I would also believe that this is determined through pins 2 and 3. One way is the voltage on these pins. Based on the usb standard, the voltages of D+=2v and D-=2.8 volts equate to 1000mA (1A) charging capacity. Based on my initial design, I confirmed that the voltages on pins 2 and 3 (D+ & D-) are in fact 2v and 2.8 volts respectively which would indicate a charging rate of 1000mA.
So it is my educated guess that the ipad perform some short communication over the D+ and D- pins enough identify itself as the apple iPad usb wall charger and thus allow the iPad to demand a full 1500mA.
So this explains why the charger that I designed does not charge as quickly as the apple usb wall charger. My next step will be to do some research and find out if there is a way to fool the iPad into thinking it is connected to the apple usb wall charger so it will draw the full 1500mA (1.5A) for maximum charging rate.
More to come.