Does the iPhone power adapter hold a charge due to capacitors?

Discussion in 'Test & Measurement Forum' started by ThirtyWest, Sep 6, 2018.

  1. ThirtyWest

    Thread Starter Member

    Jul 15, 2017
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    Does anyone know if the iPhone cube power adapter holds a charge? I was testing mine for voltage for a different project and found well after unplugging it still trickled slowl down to zero volts.

    I know it has two steps of AC to DC for control and smoothing. Just never considered it could hold on that long.


    The 12W one for the iPad took about 30 seconds to deplete.
     
  2. panic mode

    Senior Member

    Oct 10, 2011
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    What exactly is the “that long time”?
     
  3. ThirtyWest

    Thread Starter Member

    Jul 15, 2017
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    The iPhone took about 8 seconds to deplete. The iPad 12W wart took around 20
     
  4. DickCappels

    Moderator

    Aug 21, 2008
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    There is almost certainly a capacitor on the output of those supplies to keep the outputs clean. Given the size of the cube and the low voltage it is doubtful that it stores much energy after being unplugged from the AC line.
     
  5. ThirtyWest

    Thread Starter Member

    Jul 15, 2017
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    I found this article that breaks down the internals.

    I suppose I could use the 'cube' to power an LED and see; but it did show voltage on my DMM for quite as while as noted above.

    http://www.righto.com/2012/05/apple-iphone-charger-teardown-quality.html

    It makes me wonder if that's why they recommend using those things--the iPhone/ipad require really specifically pampered current.
     
  6. dl324

    AAC Fanatic!

    Mar 30, 2015
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    The input impedance of a DMM is in the gigaohm Megaohm range.

    An LED will draw several orders of magnitude higher current.

    EDIT: corrected input impedance value.
     
    Last edited: Sep 7, 2018
  7. ThirtyWest

    Thread Starter Member

    Jul 15, 2017
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    right.
     
  8. DickCappels

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    Most of my DMMS have a 10 Meg input resistance. The really cheap one that I wish I had not bought has a 1 meg ohm input resistance.

    With the high voltage probe the Fluke has an input of a gigaohm.
     
  9. dl324

    AAC Fanatic!

    Mar 30, 2015
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    10M is more common. Corrected my post...
     
  10. MrChips

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    Oct 2, 2009
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    The current through an LED diminishes as the voltage falls. Hence the amount of light emitted diminishes but can still be visible down to less than 10μA. A 4700μF aluminum capacitor charged to 3V can keep a blue LED visible for about 20 seconds.
     
  11. AnalogKid

    AAC Fanatic!

    Aug 1, 2013
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    The schematic shows 430 uF of output capacitance in parallel with 2 K to GND, plus other parallel resistors of higher values. That time constant is less than 1 second, so the output holdup must be due to the AC input stage bulk filter capacitors keeping the converter going after AC goes away. IOW, no undervoltage lockout (UVLO) on the input.

    Input holdup calculates out to a little over 0.072 w-s at low line voltage, which would be 3.6 mw average over 20 seconds. That is less than 1 mA at 5 V. Doesn't smell right.

    ak
     
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