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USB power

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edpav8r(at)yahoo.com
Guest

Posted: Mon Sep 17, 2012 4:33 pm Post subject: USB power

John, please be aware that simply installing a USB receptacle and placing 5V across pins 1 and 4 will not charge an Apple iPad (or iPhone, or Samsung Galaxy Tab, and probably others) as you might expect.



The USB standard only requires 500mA (one-half ampere) be available from a USB receptacle, and that's what you'll get from a computer's USB port and from many 110VAC and 12VDC USB adapters.  That's sufficient for many phones, but smart-phones and tablets with large batteries require larger currents to charge properly.



That's not the only problem.  Both Apple and Samsung have ignored the USB standards to make proprietary charging schemes.  Their devices look for voltages on the data pins, D- and D+, to indicate that the charger is able to supply their relatively high current demands.  If your solution doesn't provide this, your tablet will assume it's plugged into a standard 500mA source.  At best it will charge at a trickle, and may refuse to charge at all (Galaxy Tabs are famous for the "red X" indicating not charging).



Apple's scheme uses three different variations of data pin voltages to designate three levels of charger current delivery capability: 500mA, 1,000mA and 2,000mA.  iPhones require 1,000mA to quick-charge and iPads require 2,000mA.



Also note that Apple products are notoriously fussy about charge voltage falling between 5V and 5.5V.  Any less than 5V and they begin to ramp down their current draw, and therefore their charge rate.  Many cheap USB adapters are very poorly regulated and will suffer voltage dropout as current demand rises.  Furthermore, a switching regulator that's built to a price will likely omit properly selected filtering elements and may inject noise into other components sharing the same voltage source.  For example, I have an iPhone car charger (made overseas; you can guess where) that makes my car's AM radio receiver unlistenable!



So, a properly designed regulator with low voltage dropout, sufficient current overhead, and properly designed input and output filtering is important both for proper charging performance and to avoid noise in other aircraft systems.



Having said all of that, it's not rocket science!  Here's what your USB receptacle needs to provide to properly quick-charge your iPad (for iPhones, reverse pins 2 and 3):



Pin 1 (+V):  5V (no less; ideally ~5.2V)

Pin 2 (D-):  2.00V

Pin 3 (D+):  2.75V

Pin 4:  Ground



Assuming you start with a good supply, the data pin voltages are easy to accomplish with a resistor voltage divider.  Small 1/8-watt resistors are perfectly adequate as there's negligible current flow from these two pins; they're just signaling voltages.  See http://bit.ly/8JLUUe for a diagram of a resistor voltage divider and a calculator to figure resistor values.  Try to keep R1 at 10000 ohms or more, as this will ensure very little current flow to ground.



Cheers, Eric



P.S.  I hope I'm safe mentioning this here, since I have nothing yet to sell, but I hope to bring what I think will be an elegant solution to this problem to market in the near future.  Initial design has a panel-mounted single or dual-USB receptacle that occupies ~1 sq in of panel space and less than 2 in of depth, with a very small and light remote-mounted voltage source capable of simultaneously fast-charging two tablets or a tablet and a smart-phone.  If you're interested, send me an email at VersoElectronics(at)cox(dot)net and I'll get in touch only  if/when I finalize the design.

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nuckolls.bob(at)aeroelect
Guest

Posted: Mon Sep 17, 2012 7:40 pm Post subject: USB power

At 07:31 PM 9/17/2012, you wrote:

 	  Quote: 	
		  



John, please be aware that simply installing a USB receptacle and 

placing 5V across pins 1 and 4 will not charge an Apple iPad (or 

iPhone, or Samsung Galaxy Tab, and probably others) as you might expect.



The USB standard only requires 500mA (one-half ampere) be available 

from a USB receptacle, and that's what you'll get from a computer's 

USB port and from many 110VAC and 12VDC USB adapters.  That's 

sufficient for many phones, but smart-phones and tablets with large 

batteries require larger currents to charge properly.



That's not the only problem.  Both Apple and Samsung have ignored 

the USB standards to make proprietary charging schemes.  Their 

devices look for voltages on the data pins, D- and D+, to indicate 

that the charger is able to supply their relatively high current 

demands.  If your solution doesn't provide this, your tablet will 

assume it's plugged into a standard 500mA source.  At best it will 

charge at a trickle, and may refuse to charge at all (Galaxy Tabs 

are famous for the "red X" indicating not charging).



Apple's scheme uses three different variations of data pin voltages 

to designate three levels of charger current delivery capability: 

500mA, 1,000mA and 2,000mA.  iPhones require 1,000mA to quick-charge 

and iPads require 2,000mA.



Also note that Apple products are notoriously fussy about charge 

voltage falling between 5V and 5.5V.  Any less than 5V and they 

begin to ramp down their current draw, and therefore their charge 

rate.  Many cheap USB adapters are very poorly regulated and will 

suffer voltage dropout as current demand rises.  Furthermore, a 

switching regulator that's built to a price will likely omit 

properly selected filtering elements and may inject noise into other 

components sharing the same voltage source.  For example, I have an 

iPhone car charger (made overseas; you can guess where) that makes 

my car's AM radio receiver unlistenable!



So, a properly designed regulator with low voltage dropout, 

sufficient current overhead, and properly designed input and output 

filtering is important both for proper charging performance and to 

avoid noise in other aircraft systems.



Having said all of that, it's not rocket science!  Here's what your 

USB receptacle needs to provide to properly quick-charge your iPad 

(for iPhones, reverse pins 2 and 3):



Pin 1 (+V):  5V (no less; ideally ~5.2V)

Pin 2 (D-):  2.00V

Pin 3 (D+):  2.75V

Pin 4:  Ground



Assuming you start with a good supply, the data pin voltages are 

easy to accomplish with a resistor voltage divider.  Small 1/8-watt 

resistors are perfectly adequate as there's negligible current flow 

from these two pins; they're just signaling voltages.  See 

http://bit.ly/8JLUUe for a diagram of a resistor voltage divider and 

a calculator to figure resistor values.  Try to keep R1 at 10000 

ohms or more, as this will ensure very little current flow to ground.



Cheers, Eric



P.S.  I hope I'm safe mentioning this here, since I have nothing yet 

to sell, but I hope to bring what I think will be an elegant 

solution to this problem to market in the near future.  Initial 

design has a panel-mounted single or dual-USB receptacle that 

occupies ~1 sq in of panel space and less than 2 in of depth, with a 

very small and light remote-mounted voltage source capable of 

simultaneously fast-charging two tablets or a tablet and a 

smart-phone.  If you're interested, send me an email at 

VersoElectronics(at)cox(dot)net and I'll get in touch only  if/when 

I finalize the design.


	


    Good and useful data Eric. Thanks!

    Your product sounds like it would have

    some appeal to OBAM aircraft owners so

    advertising availability here on

    the List is welcome also.

   Bob . . .

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edpav8r(at)yahoo.com
Guest

Posted: Tue Sep 18, 2012 9:12 am Post subject: USB Power

Bob Nuckolls wrote:

 	  Quote: 	
		   Your product sounds like it would have some appeal to OBAM aircraft owners so advertising availability here on the List is welcome also.

 

 Bob . . .


	


Thanks, Bob.  I almost didn't mention it at all; too many of these forums get polluted with either excessive commercial messages of off-topic garbage.  I promise not to make my shilling a nuisance!



Eric

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Use the List Feature Navigator to browse the many List utilities available such as the Email Subscriptions page, Archive Search & Download, 7-Day Browse, Chat, FAQ, Photoshare, and much more:

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nuckolls.bob(at)aeroelect
Guest

Posted: Tue Sep 18, 2012 9:46 am Post subject: USB Power

 	  Quote: 	
		  Thanks, Bob.  I almost didn't mention it at all; too many of these 

forums get polluted with either excessive commercial messages of 

off-topic garbage.  I promise not to make my shilling a nuisance!


	


    No worries.

   Bob . . .

- The Matronics AeroElectric-List Email Forum -

Use the List Feature Navigator to browse the many List utilities available such as the Email Subscriptions page, Archive Search & Download, 7-Day Browse, Chat, FAQ, Photoshare, and much more:

http://www.matronics.com/Navigator?AeroElectric-List

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