iPad 2 DIY Charger

[graywolf]

How much power does the iPad require to charge?

5.1volts at 2.1amps

 

[timothyb1969]

Tim,
I am wondering if you can help me.

I finally built the circuit board. I have 6 USB ports connected to the circuit board. The problem I have is as soon as I connect two ipads to the system the power supply shuts down. This is completely confusing me since I purchased a 5 volt, 8 amps, 40 Watts Max power supply.

Any ideas?

Also my voltage on my pins are a little different than yours even with using the same resistors.

Pin 1 = 5.2 V
Pin 2 = 2.091 V
Pin 3 = 2.976 V
Pin 4 = 0V

Please note it does not happen right away but it does happen 15 to 20 min later.

Any thoughts?

The voltages will can vary depending on the accuracy of the resistors. In other words, the actual resistance as opposed to the stated resistance. It is a simple voltage divider circuit and using the same calculations I did you could select some different resistors based on what you may have in stock. You may want to confirm the voltages on the original apple iPad charger under full load conditions. That is when the iPad is connected and charging. Compare the voltages on each pin to the values that I measured.

To get pins 2 and 3 to match the apple iPad charger exactly, you could use a variable resistor (potentiometer) to get the resistance just right to provide fine adjustment on the pins 2 and 3 voltages to get them just right. I am pretty sure that all the power for charging the iPad comes from the 5v on pins 1 and 4 and the current draw on pins 2 and 3 should be very small. I am not sure what the allowable voltage tolerance that apple requires on pins 2 and 3 to allow the iPad to charge as apple does not publish this information as far as I am aware.

The bottom line, once charging begins, your iPad should charge at the maximum rate and your PS should not shut down and your charging rate should be the same as the OEM apple charger.

If you continue to have trouble after you try all this let me know and I will give it some more thought.

Good luck.

Tim

 

[timothyb1969]

Tim,
I am wondering if you can help me.

I finally built the circuit board. I have 6 USB ports connected to the circuit board. The problem I have is as soon as I connect two ipads to the system the power supply shuts down. This is completely confusing me since I purchased a 5 volt, 8 amps, 40 Watts Max power supply.

Any ideas?

Also my voltage on my pins are a little different than yours even with using the same resistors.

Pin 1 = 5.2 V
Pin 2 = 2.091 V
Pin 3 = 2.976 V
Pin 4 = 0V

Please note it does not happen right away but it does happen 15 to 20 min later.

Any thoughts?

Another comment- your power supply should have a trimming potentiometer or adjustment on it main board that will allow you to adjust the output to 5.0 as opposed to 5.2. This will also slightly change pins 2 and 3.

Another thought came to my mind. The iPad will only charge at when on under two conditions. 1> when connected to the apple wall charge that came with the iPad, 2> when connected to a high power USB port.
So how does the iPad know what type of USB port it is connected to? Figuring this out may be the answer to maximizing your charging rate.

Check out this link. They explain the different power charging capabilities of different USB ports. Sorry I only had time to skim this.
http://en.wikipedia.org/wiki/USB#Power

Also these guys have some good information that leads me to believe that the voltages on pins 2&3 tell the device how much current the USB port can supply.
http://voltaicsystems.com/blog/choo...s-and-ipads/n-voltages-for-iphones-and-ipads/

This appears to agree closely with the voltages that I measured. I would use the 2v and 2.75v as my target for the data pins and make sure that the voltage divider can supply enough current to maintain these voltages. Verifying the pin 2 and 3 voltages under full load and adding a potentiometer to adjust the resistances on the voltage divider to get the precise voltages on pins 2 & 3 is the way to go.

Also, check the current draw on the PS after hooking up 1 iPad. Then hook up the 2nd iPad. The current should approximately double. If not, you may have wires something wrong and you may have a short or a current leak some where that is causing your power supply to overheat and eventually go into thermal shut down or into over current mode which requires cycling the power to your PS to reset.

Good luck and let me know how you make out. My schedule prevent me from always answering promptly but I always try to check and answer my emails and posts as best I can.

Tim

 

[ricpaul]

Some more testing

After reading everything I could find and getting conflicting information, I mailed TimothyB about someone's suggestion the drawn current changes with supplyvoltage. He couldn't confirm this, so as a last resort, I decided to go and test for myself. Quite a feat for someone without any Apple products...

I'll first post the claim found on the net (possibly this thread) and then my measured result. Note that I used a switching powersupply which I used to read the current of off. The voltage was measured IN THE USB-plug, about 15cm/6" before the actual plug going into the iPad3. That rules out any voltage drop upto this point. I used 20k multiturn potmeters to adjust the voltages on D- and D+ (slider on D-, 20k between 5V and gnd, same with a second potmeter and D+). Please also note these voltages were set before loading the 5V voltage and were not adjusted during the test.

Claim: anything below 300k between D- and D+ will work.
Used nothing between D- and D+ (infinite resistance): iPad charged with 90mA.
Used 100k, the iPad3 charged with 1.36A
Used 10k, 5k, 2k and 0ohm: each time the iPad3 charged with 1.36A
Supply voltage did not make any difference (4.5-5.2V).

Claim: both D- and D+ at 2.66V is a good solution.
At 4.5V, the iPad charged with 1.86A
At 4.6V, the iPad charged with 2.09A
Every next increment of 100mV resulted each time in a charging current of 2.16 to 2.19A (fluctuated slightly, might e.g. be measurement inaccuracy of currentmeter, don't read anything into this).

Claim: 2.75V on D- and 2.00V on D+ results in 900mA charging current.
I used 2.725 and 2.000V and found 0.90A across 4.7-5.2V supply voltage.

Claim: 2.75 on D+ and 2.00V on D- results in 2A charging current.
I used 2.725 on D+ and 2.023 on D- and found 1.90A chargingcurrent from 4.8V to 5.2V.

Conclusions:
- Using a dead short between the datapins results in a worthwhile chargingcurrent of 1.36A. This may not be enough to keep it topped up during use, BUT it does make the charger compatible with HTC, Samsung and the rest of the world. One chargingtrick for all!
- Using 2.66V on both datapins results in the highest chargingcurrent. More than the (iirc) claimed 2.1A @ 5.1V of the 12W charger.
- Using 2.75 and 2.00V on the datapins results in 0.9 or 1.9A chargingcurrent, depending on which way you switch it. This opens up the possibility to make a simple switch to change between the two charge modes (e.g. on a solar-charger, I would need to draw this out, but I think a DPDT switch should do the trick).
- the claim the drawn current depends on the supplyvoltage was limited: they (link I cannot post: voltaicsystems) claim [email protected] and 2A@5V. I saw [email protected] and 2.2A@5V (different charging mode, but just to show my variation was not as extreme as theirs).

Hope this helps out!

Posted here as this discussion is one of the best out there.