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I have an iPhone 4 and an iPad. Since the iPad really charges best off the wall charger I keep it plugged in, but I usually just charge the iPhone off the computer. Although frequently if the iPad is charged I will charge the iPhone with the wall charger.

Now the Apple sales rep said he heard from someone who heard from someone that that would burn out the phone because the iPad charger is 10 watt instead of 5 watt. It is true that it is twice as many watts. I said that I thought it shouldn't burn out the phone because a device should only pull as many watts as it needs, so the watts were irrelevant. He said that made sense, but he didn't really know.

So I thought I would ask the experts. Has anyone had their iPad charger fry an iPhone or iPod? Am I correct in my understanding of watts in chargers? Is there an official word about the iPad charger causing trouble with other devices? Have I already voided my warranty?

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up vote 23 down vote accepted

Your assumption is correct. Your iPhone has an intelligent charge controller on-board and this controller will only draw as much as it needs.

Sure, the iPad power supply can supply twice as much power as the iPhone needs, but it should not be dangerous by any means.

I don't know if there is an "official" word on this, but I would be genuinely surprised that it voids your warranty. Apple engineers are very smart and know that users will interchange powersupplies if they recognize that the connectors are physically compatible (as is the case here.)

Long story, short: you should be fine.

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however the IPod charger may have a higher running cost then a iPhone charger. – Ian Ringrose Sep 13 '10 at 9:53

The UK Apple store says the iPad charger is fine with iphones / nano

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but only with dock "It also charges iPhone and all iPod models with a dock connector." – Metal Gear Jul 4 '11 at 5:08
By "dock connector" they mean the 30 pin connection. No "dock" required. – AMomchilov Jun 10 '14 at 2:31

TL;DR: Yes, you can use an iPad charger to charge an iPhone.

Basic electrical laws apply. The USB specification is a nominal 5Volts. That applies to ALL USB devices and chargers. It has to be so for a UNIVERSAL (U) SERIAL (S) BUS (B) to be UNIVERSAL!! (Duh sounds obvious and it IS).

So your charger may deliver 2Amps at 5Volts => 10Watts (W=V*A) Your charger may be capable of delivering 3A. At 5V that implies it is capable of handling a (3A * 5V = 15W) load.

Ohms Law states that Current = Volts/Ohms. Therefore for a given resistance (Ohms) and a given Voltage there WILL be a given current.

So let's say your phone has a terminal resistance of 10 Ohms. the USB specification says that the USB supply voltage is 5V therefore the current your phone will draw is 5V divide by 5 Ohms = 1A

Your 10W charger CAN (ideally) supply 2A at 5V which is 2 times what your phone needs, BUT the phone will ONLY draw a maximum of 1A BECAUSE that's all it needs and can draw due to it's internal resistance and the charging circuitry which will limit it even more as the battery charges up.

So for the guy who's charger wires burned; the ONLY way that this could have happened is firstly if the charger wires were severely under-rated and the current being drawn was more than they were capable of. This could only happen if the resistance at the end where you'd plug into your device got shorted out. The charger will not be at fault. UNLESS the charger was faulty - then it would have damaged any USB device you plugged it into.

As regards the phone that supposedly burned black. Again the same thing. The charger can ONLY supply current depending on it's capability and the resistance of the load (phone/ ipad, etc). The load device has a given idle resistance and charging circuitry to control and limit the charge current. The charger cannot supply more current than Ohm's law allows and also than what it is capable of delivering.

So the conclusion is that either your charger was faulty and it's output voltage was greater than 5V which would cause (push) a greater current to flow than the load device was capable of; OR your load device (phone etc) charging circuitry was faulty which caused it to draw (pull) more current than it was supposed to.

Bottom line is that when all devices are functionally sound and non-faulty, it is impossible for you to burn it out by using a charger that has a higher rated wattage.

Using a charger with a lower rating can cause the charger to overheat and maybe fail as the load device will attempt to draw the charge current that it needs but the charger can only supply the maximum it is designed to supply.

So for example if you have a charger that can only supply 500mA @ 5V (ie: 2.5W or power) then if your iphone, ipad, etc.... tries to draw 1A or 2A, etc... the charger can ONLY supply max of 0.5A so Ohm's Law applies again.

Let's once again assume that your load device is 5 Ohms. Remember Ohm's Law V = I*R therefore if I=0.5A and R=5 Ohms then the charger voltage will fall to V = 5 * 0.5 = 2.5V because the charger can only deliver max of 500mA so if you draw too much current the voltage will drop.

Take the extreme: if you shorted out the charger terminals the short circuit voltage is 0V and the short circuit current is 0.5A because the short circuit resistance is 0 Ohms.

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Loads, such as a phone, draw as much current as required for their resistance (see Ohm's Law). Having a power supply that can supply more current than a device can use is not only possible, but basically universal. E.g. A MacBook charger can supply 65W of power max (at a constant voltage, this implies constant max current), but most of the time the MacBook will be drawing FAR less power (and thus less current).

TL;DR: Yes, you can use an iPad charger to charge an iPhone.

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protected by bmike Jan 7 '15 at 16:44

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