1
$\begingroup$

This may be a basic question, but I am not sure about the exact reason, so I'm asking this

I've noticed quite a few times that when you plug an iPad charger to a socket and then switch it on, then take the charger out and connect the USB cable to iPad, it still detects current for a moment. Why does this happen so? Is there some residual charge left on the cable? Why are chargers made so,then? How does current flow when there is no complete path?

Why does it not happen with all the devices that we use?

$\endgroup$
5
  • $\begingroup$ This sounds very much like just a strange behaviour of iPads and not like there's really any current flowing.... $\endgroup$
    – Anedar
    Feb 8, 2016 at 2:30
  • $\begingroup$ Almost every power supply has a capacitor somewhere... $\endgroup$
    – Jon Custer
    Feb 8, 2016 at 2:30
  • $\begingroup$ @JonCuster Are we talking about the capacitor of the rectifier? $\endgroup$
    – Quark
    Feb 8, 2016 at 2:38
  • $\begingroup$ There likely is a cap across the output of the rectifier. The iPad will sense the voltage, but when trying to draw current will, unsurprisingly, fail if it is unplugged. $\endgroup$
    – Jon Custer
    Feb 8, 2016 at 2:40
  • $\begingroup$ What about other devices? $\endgroup$
    – Quark
    Feb 8, 2016 at 2:41

1 Answer 1

2
$\begingroup$

I don't know the details of Apple's charging software, but this is almost certainly an artifact of the OS sensing the plug pulled out and taking its time to update the screen.

Remember, when the device is charging a lot of processes happen differently (power saving mode controls everything from volume to brightness to which apps get priority), and I don't imagine the little notification has too high of an importance for updating compared to the processes keeping the phone running.

Is there some residual charge left on the cable?... How does current flow when there is no complete path?

Nope, none of these things happen. The characteristic time it takes for current to stop flowing from a (nearly) instantaneous unplugging event is much shorter than what the API for the batter indicator applet checks the hardware.

$\endgroup$
1
  • $\begingroup$ I should reiterate that this answer is all speculation; I have never worked with Apple's OS code nor do I know anything about the rate their kernel scans ports for connectivity. $\endgroup$ Feb 8, 2016 at 13:22

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.