Aug 18

The Dirty Truth about USB device Charging

How many devices do you have that charge over USB?   How many devices require you to do something special, or have a special adapter to do it?

The last few cell phones I’ve had have USB micro or mini ports to charge from.  I figure I can take a USB A Male to USB micro/mini Male cable and just plug it into a USB port and get power… except that doesn’t work some times.  Then there are the Apple devices.  You’ve got your iPods, iPhones and iPads.  They do their own thing as well.  What’s a geek to do?

Well, the Chinese started mandating a specific configuration for cell phone charging via USB (shorting DATA- and DATA+ pins together) back in 2007.  The EU got the big cell phone manufacturers onboard in 2009, with the US wireless industry coming along also.  The bad news is that the EU standard doesn’t have to be implemented until 2012.

So that problem is resolved, right?  That’s just cell phones.  Others will probably jump on the band wagon, but there will be hold outs.  So, how do you design a USB charger in the mean time?  The Chinese standard is here, but that leaves our device non-apple compliant.  We don’t want to miss out on the apple market do we?  I hear you say, “but that means another product, and that’s expensive.”  We’ve got a handy generic circuit to allow you to use the same design for both charging standards by just replacing a couple resistors.  First, let’s look at the standards.

I don’t have the Chinese standard, but all articles agree that it is just shorting the D+ and D- lines together.  Easy enough.  Now, the Europeans did their own thing… sort of… not really.  They supported the USB Battery Charging Specification, which instead of shorting D+ to D-, puts a 200 Ohm resistor between them.  It’s practically the same thing.  So we can meet those two standards with a 200 Ohm resistor, or if you really want to short it out, a zero Ohm resistor.

European USB Charging Resistor

European USB Charging Resistor

Now onto Apple’s USB charging setup.  Luckily, there are intrepid explorers who did the hard work there.  There’s two Apple USB charging circuits, low and high current.  To change between the two we just need to switch out the resistors.  Here’s the circuit for the high current resistors.

Apple Hi-current charge resistors

Apple Hi-current charge resistors

Now, we just need to combine the two.  Merging the two circuits we get:

Hybrid Charging Resistors

Hybrid Charging Resistors

Now realize, that installing all five resistors will not meet either of the charging standards.  However, if we install R2, R3, R8 & R9, we will have an Apple format charging port.  If we install just R6, we will have an EU standard charging port.  If we replace R6 with a zero Ohm resistor and leave the other resistors off, we will have a Chinese standard charging port.

Now, instead of two different boards, we can use the same board with two or three assembly variants to support different devices.  First, here’s the layout for the resistors connected to a USB A Female board mount connector:

2D Charging Circuit layout

2D Charging Circuit layout

The circuit requires two layers if using surface mount components.Blue is the bottom layer and red is the top layer.  If using thru-hole, traces could be ran under the resistors.  The resistor layouts shown are for 0805 resistors.  They can be smaller, since they dissipate very little power (less than 1/8 W).  The same can be said for the traces.  I had the room, so I chose 0805s with larger traces.  Make sure to keep your GND and VCC traces large to support the current on them.  Here’s the 3D view of the layout:

3D Charging circuit layout

3D Charging circuit layout

And there you have it, one circuit and layout for multiple USB charging standards.  Enjoy!

Permanent link to this article: http://blog.curioussystem.com/2010/08/the-dirty-truth-about-usb-device-charging/


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  1. Joe Engineer

    You don’t need the 2nd set of voltage divider as your 200 ohms resistor would past the same +2.8V to the other data line.

    So your 5 resistors solution has became a 3 resistors one.

  2. Chad Kidder


    You never have all five resistors populated at the same time. Installing the 200 ohm resistor into the Apple charging circuit would screw up the voltages presented on the data pins and then it may not be recognized as a valid charger.

  3. tony

    Only 3 resistors needed, delete R3 and R8

    EU charger: R9=no fit, R6=200R, R2=no fit

    Apple charger: R9=100K, R6=20K, R2=100K
    (gives 2.72V and 2.27V and works perfectly)

  4. Tinker One

    Do you know of anywhere that sells one of these?

  5. Spacefish

    Thanks for you article, i have build a 1,5A charger, powered by 5-40VDC for charging on the go 🙂 it works pretty well an loads my mobile in just under one hour because of your info 🙂

  6. Chad Kidder

    I’m glad that it proved useful for you.

  7. Almost

    “which instead of shorting D+ to D-, puts a 200 Ohm resistor between them”

    Actually the USB charging spec says a resistance of *less than* 200 ohms, so a direct short is still compatible with the USB charging spec.

  8. Stan van Beek

    How much watts does the resistor has to handle?

  9. Chad Kidder

    The resistors have very little current running through them. You can use a 1/8 W rated resistor for the 200 ohm resistor. You can probably get away with less, but I don’t know what other resistors are in the end device.

  10. Rob Collingridge

    In your Apple high current image you have D- and D+ labelled the wrong way around. If you refer back to this article you will see this:

  11. petiepooo

    Using the 3-resistor layout suggested by tony (and others), what if you had 550, 200, and 500 Ohms? That’s a voltage divider that gives you 2.8v on D+ and 2.0V on D-, and also has 200 Ohms between the two. At a total of 1250 Ohms, at 5V, that draws 4mA, which isn’t too bad.
    With a little more effort, you could put a DPST switch to disconnect the Vcc and Gnd feeds to the voltage divider, and a LED off of the Vcc feed to indicate that it’s in “Apple” mode.
    Note: above idea needs testing. YMMV

  12. Richard

    My Samsung Galaxy Note 10.1 has slightly different layout
    10 K Ohm to +5 rail
    3 K Ohm to ground rail
    +d and -d connected to the junction.

    Only way I could get the stupid thing to go to full charge mode

  13. Christopher

    I am curious if this will work on ios 6 and ios 7.

    I have 2 uses for this: 1) a component video cable that is “not supported” that I would like to modify.
    2) I have previously modified a belkin car charger to support line out; all that was required was the addition of a 1M resistor i believe. I have hardwired this to a 12v source in my car and the cord works as it should for ipod/line out audio, however my iphone will not charge with this cord and I would really like to fix this.

  14. Jacob Christ

    We have Samsung Tab3 10.1. We have a custom USB controller that we are trying to communicate with, while charging at greater that +500mA (Quick240.com). We have yet to get this to work. I have a call in to Samsung to see if there is a work around.

    We also verified that the following will cause the table to charge as well:

    10 K Ohm to USB +5V
    3 K Ohm to USB ground
    +d and -d connected to the resistor network junction.

    This also worked:
    250 K Ohm to USB +5V
    75 K Ohm to USB ground
    +d and -d connected to the resistor network junction.

    But connecting
    500 K Ohm to USB +5V
    150 K Ohm to USB ground
    to each d+ and d- (four resistors, two networks) but leaving d+ and d- not connected didn’t work.

    Also, using the first configuration, if you short and unshort d+ and d- the tablet will go into and out of drawing full current.

  15. Ovigo

    Just to clear something up: the USB battery charging specification actually specifies the resistance between D+ and D- to be up to 200 Ohm. So the ‘European’ and Chinese standard are actually pretty much in agreement. Ay sight on Apple coming to agree to this standard by now?

  16. Joe Dubner

    Chad, is it possible the polarity of the connections to the voltage divider is reversed? In your diagrams the 2.8V connection goes to +Data while in https://learn.adafruit.com/minty-boost/icharging it goes to the the D- connection. Or maybe my brain is confused from peering at the screen all day.


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