Apple and other USB charger secrets

Everybody seems to be saying that you can’t charge Apple devices with normal USB power supplies. You need a special power supply from Apple or approved by Apple. I saw this kind of discussion at slashdot some time ago.

Usually, device makers need to sign a confidentially agreement with Apple if they want to say their charger ‘works with iPhone / iPod,’ and they’re not allowed to talk about how the insides work. I hate when manufacturers do crap like this to keep peripherals locked into a more profitable licensing agreement. Apples tendency toward total control is one of the things i don’t like about them. And many other manufacturers are just as bad. I wish companies would back off and be more open and/or use standard micro USB chargers.

The mysteries of Apple device charging article includes a 7-minute video we explore the mysteries of Apple device charging. The secret of Apple chargers is simple: just few resistors. If you don’t put these secret resistors on the data lines too, you get the dreaded Charging is not supported with this accessory. Those resistors like a way to signal to the iPhone that it can go ahead and “fast charge” by pulling 1A, or “slow charge” by pulling 0.5A. The iPhone needs to do a power negotiation to determine if the port is capable of providing 1000ma of power, because the upper-limit of a standard USB port is 500 mA. They just didn’t tell anyone about how to do that. I get why the resistors were initially added but I’m not understanding why it needs to be a trade secret.


There is nothing to stop them just drawing the 500mA if the right sort of charger is not detected. Refusing to charge at all unless the licensed parts are present is pure market control, nothing else. Here is the resistor configuration for 500 mA charging:


Resistance is Futile. The The mysteries of Apple device charging article demonstrates how anyone can make their own chargers that work with iPhone 4, 3Gs, etc. The pictures on this blog posting are from that article.

Apple devices are not the only one USB charged devices that can have some problems with USB chargers. So here are some resources on USB charging in general.

USB As A Power Source article gives an introduction USB Power Form.

European Commission has reached a voluntary agreement with some of the biggest names in the electronics industry to introduce a common charger for cell phones that fits all models. Information on this USB charging connector is available at USB Approved Class Specification Documents document directory. Read also Battery Charging v1.1 Spec and Adopters Agreement document.

Dealextreme USB charger discussion posting says that USB standard has 4 lines (+5V, ground and +/- data lines). Most USB chargers let the data lines float. Technically, the USB standard says that a USB charger should set the two data lines to specific voltages (~ 2V) to indicate how much power it can provide (I have not verified that from standards yet). The recent iPhones will not charge if the data lines are set incorrectly (i.e. not according to the USB standard).

USB Charging Guide comment: I believe having the data pins connected to each other is in the latest USB specification for charging. I had to interconnect the D+ and D- pins inside my USB AC charger to get it working with my Zune. Perfectly according specs but frustrating enough.

Wikipedia USB article: The USB 1.x and 2.0 specifications provide a 5 V supply on a single wire from which connected USB devices may draw power. The specification provides for no more than 5.25 V and no less than 4.75 V (5 V±5%) between the positive and negative bus power lines. For USB 2.0 the voltage supplied by low-powered hub ports is 4.4 V to 5.25 V.

A unit load is defined as 100 mA in USB 2.0, and was raised to 150 mA in USB 3.0. A maximum of 5 unit loads (500 mA) can be drawn from a port in USB 2.0, which was raised to 6 (900 mA) in USB 3.0.
All devices default as low-power but the device’s software may request high-power as long as the power is available on the providing bus.

In Battery Charging Specification, new powering modes are added to the USB specification. A host or hub Charging Downstream Port can supply a maximum of 1.5 A when communicating at low-bandwidth or full-bandwidth, a maximum of 900 mA when communicating at high-bandwidth, and as much current as the connector will safely handle when no communication is taking place (USB 2.0 standard-A connectors are rated at 1500 mA by default).

A Dedicated Charging Port can supply a maximum of 1.8 A of current at 5.25 V. A portable device can draw up to 1.8 A from a Dedicated Charging Port. The Dedicated Charging Port shorts the D+ and D- pins with a resistance of at most 200Ω. The short disables data transfer, but allows devices to detect the Dedicated Charging Port and allows very simple, high current chargers to be manufactured. The increased current (faster, 9 W charging) will occur once both the host/hub and devices support the new charging specification.

Without negotiation, the powered USB device is unable to inquire if it is allowed to draw 100 mA, 500 mA, or 1 A. Some non-standard USB devices use the 5 V power supply without participating in a proper USB network which negotiates power draws with the host interface

In most cases, these items contain no digital circuitry, and thus are not Standard compliant USB devices at all. This can theoretically cause problems with some computers; prior to the Battery Charging Specification, the USB specification required that devices connect in a low-power mode (100 mA maximum) and state how much current they need, before switching, with the host’s permission, into high-power mode.

USB Charging Guide tells some more details on mini-USB plug: the mini-USB plug actually has 5 pins in it. This can be important as the extra pin (Pin 4) USB_ID is usually either connected to ground or left floating. Sometimes a pull up resistor needs to be added to from the USB_ID to Pin 1 (VDD) to select “Device Mode” rather than “Host Mode”. This resistor is in the device side plug as the USB_ID pin is not wired through to the PC side connector. The good news is that quite a few USB cables have this. So sometimes you can get round the not charging problem simply by trying out different leads and one may work rather than buying the manufacturers “special” cable. On some Creative players you can also solve this by pulling down both data lines (with 2x15k resistors) at the source to emulate what the host (PC) does when setting line speed. This is not so common.

So the current state of USB charging is a little bit of mess…


  1. Tomi Engdahl says:

    This is the world’s thinnest charger

    Israeli Kado describes well the current mobile phone chargers: no one has ever thought of designing them. According to the company

    The company has introduced the world’s thinnest charger, which is only 5 millimeters thick. Still, it downloads all the USB devices that you want with it.

    Charger plugs fold into the enclosure. Removable, the charging cable inside the device is 85 centimeters long and at its end is a lightning connector suitable for microUSB, USBC or Apple devices. The charging power is 10 watts and supports fast charging.


  2. Tomi Engdahl says:

    How to Power Nearly Anything Off a USB Port

    USB runs at 5v. The max current you can draw is 500ma. Therefore the max load is 5v x 0.5A=2.5. Watts. (W=VxI) If you try and draw more than 500mA, you may overload the port which will cause it to break

    Okay so your going to need to get any USB cable this can be a printer cable or a MP3 player cable

    now you will want to cut it to the length you want it , you only need the Male end so the other bit can be discarded

    Okay you will want to strip the wire to reveal the wire underneath once you have done this there should be 4 wires , you only need the red and black ones as they are the power the green and white are data so they can go :)

    So you will now want to solder the red wire ( positive ) and the Black Wire ( negative ) to the power in to your circuit or appliance

  3. Tomi Engdahl says:

    Ride Bike, Charge Phone

    If you’ve ever used a motor as a generator, then you can see where this is going. That’s the underlying principle behind [Creativity Buzz]’s bike-powered phone charger. As the bike wheel turns, the rim comes in contact with a small wheel attached to the output shaft of a DC motor. Cranking the output shaft of a motor with permanent magnets inside will induce a small voltage, and here it is amplified with a DC-DC boost converter and output to a USB jack.

    Bicycle Mobile Charger

  4. Tomi Engdahl says:

    Weird USB battery charger circuit.

    This is the circuitry inside a cheap USB two cell charger.

  5. Tomi Engdahl says:

    Teardown of an illuminated wire USB charging lead.

    Flowing light USB cable teardown and schematic.

  6. Tomi Engdahl says:

    Samsung USB Charger Failure and Repair

    In this video I’m examining and repairing my faulty USB phone charger (5V 1A switching power supply). It failed after 2 years of seldom use. I’ve drawn a partial schematic in the video. Full reverse engineered schematic of this charger is here:

  7. Tomi Engdahl says:

    USB Adaptive Charger (2.7A per port) with Wattmeter

    A 10.8A, 4 port USB charger with a wattmeter and adaptive intelligent charging.

    We built the Pirl Charger because we were unhappy with the quality of existing chargers. Few consumers know they gradually ruin their expensive devices by using poor quality chargers that deliver high noise, high ripple energy. Every consumer device comes with a charger, and those are usually well-built, but they usually have only one port, and if you lose it, you usually buy an aftermarket charger. However, replacement chargers range widely in quality, and most people don’t know.

  8. Tomi Engdahl says:

    Hacking Qualcomm (Quick Charge) QC 2.0/3.0 with ATtiny85

    Get not only 5 volts but also 9, 12 volts (18 Watts max) out of any QC-compatible charger/power bank for supplying power hungry projects.

    Many USB powered consumer electronic devices comes with Qualcomm QC solution for rapid charging at different voltages rather than usual 5.0 volts. This enables the opportunity to use QC compatible Power Banks for projects require more power or higher voltages like 9/12 volts.

    The purpose of this project is to develop a device that can hack the QC protocol and allow hobbyists, developer, engineers use their power banks for more voltage/power for their next power hungry project.

  9. Tomi Engdahl says:

    The truth inside lightning cable original – XFix

    The truth inside lightning cable original, this is what i found inside a original iphone cable right from apple. I did my best to open it up to show everybody how it looks.

  10. Tomi Engdahl says:

    DC Input, USB Fast Charger

    50W, 10.8A, multiport USB charger with wattmeter that intelligently detects and adapts to device charging speed

    The Pirl Charger is a high capacity USB charger for consumer devices.

    Current USB chargers restrict you to wall outlets and do not provide any information on how much power devices are drawing.

    We solve these problems by with a DC input charger and a wattmeter LED display. Each port fast charges to 2.7A, and it is a robust build of aluminum plates, which makes it durable to be used at home, in a car, with lead acid batteries and battery packs.

    It takes DC input ranging from 7V to 17V, and can be used anywhere with wall outlets,

    We’re live on Kickstarter:

  11. Tomi Engdahl says:

    Power USB devices from a vehicle

    IC1 generates 5V from any 7.5 to 76V input—a wide enough range to include the complete range of car-battery voltage plus the 40V spike that can occur during a load dump. The IC is simple to use because it has an internal power switch and requires no compensation circuit.

    IC2 distributes to two outputs the 5V that IC1 generates. It not only distributes power but also protects against overload conditions. Most portable equipment receives its power or charge through a USB (Universal Serial Bus) interface, whose current limit is 500 mA. Because IC2 targets use in USB applications, it latches off any port that tries to deliver more than 500 mA but does not affect the other port. Automatic-restart capability ensures that the port recovers automatically after the removal of the overload condition.

  12. Tomi Engdahl says:

    Charge Your Phone with a Hand Crank Generator

    Wanna charge your phone in the apocalypse? Maybe my hand crank generator can help you.

    Anyway, let’s get started. Here are the major components: a servo motor, a diode, a polarised capacitor and a 5V boost module.

    Step one, convert the servo motor to a hand crank generator. Disassemble the servo motor. Desoldering all the wires on the board. Solder the wires and motor together. Also, remember to remove all the locking parts on the gear. After that, you can reassemble all the pieces together.

    Step two, solder the boost module, diode, capacity, and server motor together and turn them into a complete circuit. A diode is very important in this circuit. Since the hand crank mechanism can go either clockwise or anti-clockwise, the diode can prevent the current flow in a wrong direction.

  13. Tomi Engdahl says:

    Charge Your Phone with a Hand Crank Generator
    Wanna charge your phone in the apocalypse? Maybe my hand crank generator can help you.

    Solution : A Portable Hand Crank Generator

    In case of emergency, like Bear Grylls, we need a battery charging solution that is robust, accessible and portable. For god’s sake, coin battery and potato battery is apparently not an option.

    Therefore, I decided to make a hand crank battery charger. Using the human-powered generator to create electricity via motion and forces have already got a long history.

    Anyway, let’s get started. Here are the major components: a servo motor, a diode, a polarised capacitor and a 5V boost module.

    Step one, convert the servo motor to a hand crank generator.

    Step two, solder the boost module, diode, capacity, and server motor together and turn them into a complete circuit.

    A boost module is needed because the servo motor can only generate 1–3 volt which is far from the 5-volt standard to charge our mobile phone.


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