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

Thread Starter

ThirtyWest

Joined Jul 15, 2017
114
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.
 

DickCappels

Joined Aug 21, 2008
6,312
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.
 

dl324

Joined Mar 30, 2015
10,527
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.
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:

DickCappels

Joined Aug 21, 2008
6,312
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.
 

MrChips

Joined Oct 2, 2009
20,904
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.
 

AnalogKid

Joined Aug 1, 2013
8,381
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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