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The Mystery of USB Chargers

By on July 9, 2014
USB Charger








Almost any new device such as mobile phone, tablet, iPod, GPS etc… will come with some sort of USB Charger and a cable. The cable will connect to your device through a dedicated connector, mini USB, micro USB or manufacturer’s specific. At the other end, most cables will be terminated with the familiar USB connector (USB type A), which you can connect either to the charger or to your computer.

USB Charger

If you are like the 50% of mobile phone users you will buy a second charger, just to have one at work and one at home. Over the years, you will end up with several USB chargers and you might wonder if you could simplify your life by having only one charger, or at least a couple of chargers, for all your devices. The problem is that they are not all compatible. Your Samsung charger will not work with your Apple device etc… In this article we will analyze some USB chargers and find out why they are so different. And may be find out that they are not so different at all…

Battery charging principles

Before going into the mystery world of USB chargers we need to do a refresh on (generic) battery charging. Charging a battery is much more than applying the right amount of voltage and current to its terminals and hoping for the best!  Despite their apparent simplicity, batteries are complex devices. They have their own charging and discharging characteristics. A battery will need a larger current at the beginning of the charge, decreasing as the charging process goes on. When the battery is fully charged it will need a very small amount of current to maintain the charge.  Too much current might damage the battery. Too less current and the charging process will take forever.

USB chargers

Battery chargers (talking about the generic ones) must have a certain amount of built in intelligence. Some can be quite sophisticated, like the 3 stage chargers for example. A 3 stage battery charger will maintain a constant current until 80% of the charge, then a constant voltage with decreasing current until full charge. Then it enters the float mode where a constant current and constant voltage is maintained to keep the battery charged without boiling it. These chargers are controlled by microprocessors and can be quite expensive.


What about USB Chargers?

Do our tiny USB chargers have all this intelligence built in? Not really… In the case of a mobile phone/tablet charger, the intelligence is at the other end of the cable, in the device itself. The devices will happily take the 5V input and regulate the current as it suits them. So why can’t we just apply 5V to the corresponding terminals, then have a charger that works with any one of the devices? The short answer is that the device, although not expecting a great deal of intelligence from the charger, is expecting some information to tell him what kind of charger it is… so it can decide how much current it will take!


The USB plug has 4 terminals only. Two are used for the power (GND and +5V) and two for the data transmission (D+ and D-). If you build a charger that supplies 5V to the corresponding power terminals it might work for some devices and not for others. Most devices will expect the presence of a voltage on D- and D+ which will tell them that it is a genuine charger and how much current it can supply. Some devices expect a short between D+ and D- to identify a “genuine” charger. Indeed most of the so called “universal charger” have the D- and D+ shorted together.

To illustrate this I made the following adapter:

mystery of usb chargers

It is very straight forward indeed. Any one of the 4 jumpers can be opened to insert an Ampere Meter as shown below. The test points provide enough space to connect a voltmeter or oscilloscope.


My first test was to connect a Galaxy Tab to its genuine charger through this adapter, with an Ampere Meter connected in series with the + 5V. The A Meter indicated a healthy 720mA when the battery was at 80% of its charge. At 100% the current went down to around 50 mA. This looks very good…

Now I checked the voltages at the data lines. Both were at a potential of +1.25 V. Disconnecting the power I measured zero Ohm between D+ and D- (obviously linked together).

What if I open the jumpers at D+ and D- so the charger only supplies the 5V, without any signal on D+ and D-? Surprise (really?) The device recognizes that a charger is connected but refuses to charge. In summary this one needs its particular signature (+1.25 V on D+ and D-) to recognize a “genuine” charger. Otherwise it will refuse to charge.

Doing the same measurements with my phone (Galaxy Tab using its genuine charger) the results were quite different: Straight through with the jumpers on, the current measured is 280 mA and doesn’t vary when the phone is switched on or off (well regulated). Remove the jumpers and the current goes up to 390 mA with noticeable variations when the phone is turned on or off. Measurements show that D+ and D- are connected together but no voltage is measured.  They are not connected to the ground, just floating!

 usb charger mystery

Surprisingly, this device will limit the current to 280 mA when it is connected to its genuine charger. Turning the phone on or off doesn’t seem to affect the current. Otherwise, when connecting the phone to a charger that doesn’t provide the right signature on D+ and D- the current goes up to 390 mA and doesn’t seem to be well regulated anymore (noticeable fluctuations when turning the phone on and off). Scary!


What about Apple Chargers:

Apple chargers are different. They provide a signature on D+ and D- lines using two different voltages, respectively 2.0 V and 2.7 V. Depending on the combination (D- = 2.7 V and D+ = 2.0 V, or D- = 2.0 V and D+ = 2.7 V or both D- and D+ at 2.7 V) Apple devices will identify what type of charger it is and behave accordingly!  This is done through voltage dividers. The figure below shows how one of those combinations could be done:




There is a huge market for USB chargers. As mentioned earlier in this article, about 50% of the mobile phone user will buy a second charger. Chargers are at the top of the list of forgotten items in hotel rooms. No surprise then that the device manufacturers have no interest in making universal chargers, because they want to sell their proprietary products, hence the confusion.

Help is on the way though; there are now official specifications for USB chargers. You can find them at the following link:

For more information you can also download the data sheet of the TPS2513, USB Dedicated Charging Port Controller (, used by Apple and other manufacturers. This provides interesting information about the D+ and D- signature.

To conclude, here is a word of caution. There are many cheap copy chargers out there. Many can be dangerous, for you and for your devices. The following link will provides excellent information about various chargers and their counterfeits:

It is generally much safer to spend a bit more money for a genuine charger than putting your device and yourself at risk.





This article was contributed by Gerald Musy from Penang, Malaysia.


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  1. Mark

    July 9, 2014 at 8:57 am

    Thanks Gerald,

    Excellent article and very informative.

    • temba ngwenya

      July 10, 2014 at 6:01 pm

      i am in zimbabwe and i am very pleased with your site

  2. Robert

    July 9, 2014 at 10:43 am

    Thanks for the article Gerald, it's very helpful.

  3. Tam K. H.

    July 9, 2014 at 11:08 am

    Totally overwhelm by your infors which open up wide informations on just the matter of usb chargers.

    Normally, I'm very reluctant to write in BUT your contributions really move me to reply my many thanks.

    T.Q. very much

  4. Mervin

    July 9, 2014 at 11:17 am

    interesting, thanks

  5. Shiharan

    July 9, 2014 at 11:24 am

    We can make a small and portable usb charger with pp3 battery an electrolytic, pair of ceramic caps and a 78 series voltage regulator.

    Also constant current and voltage configurations can also be made. A fancy device can be made by controlling the functions with a 8051 microcontroller

  6. moshe

    July 9, 2014 at 11:29 am

    very very interesting...thanks

  7. marco tapia

    July 9, 2014 at 12:54 pm

    I love your articles cause you always make understanding electronics so easy, thank you.

  8. Charles Lim

    July 9, 2014 at 12:56 pm

    Excellent study of a deceptively simple subject. Invaluable info. Bravo and thanks!

  9. Keith

    July 9, 2014 at 4:47 pm

    I thought they were all the same and nearly bought one. I will be careful now to get the right type.
    I add my grateful thanks for this info.

  10. GIRISh Wagh

    July 9, 2014 at 6:06 pm


  11. dicksy

    July 9, 2014 at 6:20 pm

    thanx Gerald.what a nice article.

  12. Amir Mukhtar

    July 9, 2014 at 6:25 pm

    Thanks you for nice article Mr. Gerald Musy.

  13. Allal Sahel

    July 9, 2014 at 7:37 pm

    Very useful information, best article. Thank you for everything.

  14. Luciano

    July 9, 2014 at 8:09 pm

    Hi Gerald
    Thanks! This article is very informative. Keep it up!

  15. Jon

    July 9, 2014 at 8:11 pm

    This is very helpful... thanks. Can you please confirm what size resistor you used on your testing PCB board, and what size LED as well, as i feel like doing a bit of tinkering to check my current chargers to see if any are fakes, as there is a lot of info on the web about bad chargers catching fire or failing.


    • Gerald Musy

      July 10, 2014 at 9:55 am

      Hi Jon,

      Thanks for your comment. I used a 470 Ohm resistor for the LED. It is a standard LED (from my old stock). The current in the LED is around 6 mA as the voltage drop in the LED is around 2 V. This is enough to indicate the presence of the 5V. I used a 1/4 watt resistor, any one smaller would do as the power dissipated is less than 20 mW. Smaller resistors would be too small for my old eyes to see :).

      Cheers, GM

  16. Isnachun

    July 9, 2014 at 9:26 pm's cool..., I never though before. Thank you

  17. Gary Gemmell

    July 9, 2014 at 10:15 pm

    Brilliant article i had no idea it was so involved.
    Great lunchtime read!

    Keep up the good research work Gerald its very interesting stuff.

  18. Igor Levin

    July 9, 2014 at 10:58 pm

    Very useful article.

  19. Joshua oloo

    July 9, 2014 at 11:31 pm

    Thanks Mr.Gerald for the Article.Its great to inform us on matter Usb/chargers.

  20. Mehdi

    July 10, 2014 at 7:54 am

    Thanks a lot
    But I think some thing is wrong.!
    Guess what? You should change D+ & D- place because D+ is near the negative voltage and D- is near the
    Positive voltage.
    Good luck

    • Gerald Musy

      July 10, 2014 at 10:18 am

      Hi Mehdi,

      Well done, you are right!

      Sorry for the confusion. I was mistaken by the first picture taken from the Internet.

      Thanks for your comment.

      Cheers, GM

  21. Gerald Musy

    July 10, 2014 at 10:19 am

    Thank you to all of you for your positive and constructive comments.


  22. Gopal Sharma

    July 10, 2014 at 10:21 am

    Thanks, it is very informative, i really don't know such details about USB chargers.

  23. Humberto

    July 10, 2014 at 7:19 pm

    Good and interesting article, Mr. Gerald Musy. God bless you

  24. Andre Gopee

    July 11, 2014 at 11:41 pm

    Excellent article and well informative. Thanks for sharing.

  25. Evuazeze Emmanuel

    July 20, 2014 at 3:53 am

    Thank you Sir, Ur article is really educating.

  26. Sam Montero

    July 31, 2014 at 11:44 pm

    I always have to repair chargers for my relatives but I never got to study the detail of how the charge is controled or limited. Thinking that any cellphone is smart enought for it. Anyway Thanks for your technical presentation.

  27. Raymundo Saura

    August 27, 2014 at 3:34 pm

    wow great article for sharing with us thanks sir

  28. Gurpreet

    September 17, 2015 at 8:30 pm


    Very good and knowledgeable article. Thanks for sharing.

  29. Chris

    September 23, 2015 at 5:13 am

    I found this article now, It's an old article but a very informative.
    It should be sorted somewhere in a list like "LOI" List Of Interest.
    Great article, good job.


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