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New Ipad & Iphone 4S charging voltages
July 31, 2012 1:26 PM   Subscribe

Hi, I have a 3rd party iphone charger which supplies 5.2 volts. It charges the iphone 4s alright but cannot charge (charging symbol is not shown on screen) on the new ipad. With a multimeter I measured the original chargers' outputs. They vary between 5.12-5.15 voltages for both of the devices. So, my first question is whether you know which smart apple technician decided that so little deviation from the standard is not acceptable Secondly, do you know what are the extra voltage tolerances for both of the devices? (Apparently, they are less than 2 percent) Cheers!
posted by raphael19 to Technology (14 answers total)
 
Have you left the charger for any length of time? I've found that even with the "not charging" warning on the iPad 1, 2, 3 scattered among my friends and family, a trickle charge occurs.
posted by tilde at 1:32 PM on July 31, 2012


The issues is most likely not voltage, it's current. The iPad charge can supply more current. If you could measure the voltage of the chargers when they are charging, you would likely see that the 3rd party charge voltage was much lower than the iPad charger.

The current specs of each charger are probably on the labels.
posted by Long Way To Go at 1:32 PM on July 31, 2012 [3 favorites]


As tilde implied, it may actually be charging, albeit at a slower rate.
posted by alms at 1:32 PM on July 31, 2012


1) Original Iphone 4S charger supplies 5.12-5.15 volts and 1.0 amperes. It works on the Ipad. (I know it will take more time to charge with respect to a 10W charger)
2) Third party charger supplies 5.2 volts and 1.0 amperes. It does not work on the Ipad.
posted by raphael19 at 1:36 PM on July 31, 2012


The iPad charger I have sitting here is rated at 5.1V, 2.1A.
posted by primethyme at 1:37 PM on July 31, 2012


I recently needed a charger for an iPhone. I found that the reviews for aftermarket chargers for iDevices were typically negative, sometimes blaming the chargers for damaging the device. Might be worth considering buying an OEM charger, those are expensive devices.
posted by HuronBob at 1:43 PM on July 31, 2012


There's no way that little of voltage deviation would cause the problem. Most likely your third party charger is supplying less than the rated current, especially if it's a cheap knockoff.
posted by ReadEvalPost at 1:46 PM on July 31, 2012


USB devices that are intended for charging are built to send "hints" to the devices that are plugged into them. The V+ and GND lines are 0 and 5VDC, respectively, but the D+ and D- lines are set to specific voltages. Those voltages can be measured by smart devices to allow the device to determine whether the charger is capable of supplying the required current, in lieu of requiring every charger to have a microcontroller that can negotiate with the device.

Your charger does not or cannot supply the required current, and tells the iPad this. The iPad then refuses to charge. It can, however, supply enough current to charge an iPhone 4S. Likewise, the iPhone knows this, and will charge.
posted by TheNewWazoo at 1:46 PM on July 31, 2012 [3 favorites]


It's not the volts it's the amps. iOS devices have a signaling protocol that tells the charger to dial up the current, and not all chargers support it. You will always get at least the USB minimum of half an amp unless the charger is not to spec. With its own charger the iPad draws 2.1 amps.
posted by w0mbat at 1:47 PM on July 31, 2012


For more information, see this overview.
posted by TheNewWazoo at 1:47 PM on July 31, 2012


iPad's have always demanded higher current charging. Chargers need to signal to the iPad that they can deliver this 2A charge. Most 3rd party chargers don't do that even if they can handle 2A.

Macs have been supplying 2A on USB for ages so iPads charge fine on integrated Mac USB ports. This tends to make people assume they can charge their iPad on any ol' USB port but it isn't true.
posted by chairface at 4:35 PM on July 31, 2012


Measure the resistance between D+ and D-. If it is more than 200 ohms, then the iPad is only permitted to draw a maximum of 100 mA from the charger. 100 mA might be enough to charge you phone but not enough for the iPad.

If the charger does not have connections for D+ or D- or high resistance between those pins, you might be able to get your iPad to charge by connecting a 200 ohm resistor between those two pins. The iPad then sends a small voltage into D+ (0.7 volts), through the 200 ohm resistor and then detects it on D-. This tells the iPad that it is connected to a high-power charger, allowing it to draw up to 1.8 A.
posted by JackFlash at 5:20 PM on July 31, 2012


Macs have been supplying 2A on USB for ages so iPads charge fine on integrated Mac USB ports. This tends to make people assume they can charge their iPad on any ol' USB port but it isn't true.

iPads should charge on any PC USB port because it will provide up to 500 mA charging current. You may need to turn off the screen and charging may be slow, but it will charge on any PC USB port. It will not charge on an unpowered hub that can only supply 100 mA.
posted by JackFlash at 5:52 PM on July 31, 2012


This page from LadyAda has a pretty detailed analysis of iPhone/iPad charging specifications.
posted by neckro23 at 6:13 PM on July 31, 2012 [1 favorite]


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