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I know that when we charge devices via our computer, it is usually at 500ma (.5A). Does a cable exist that would allow the data transfer to happen to a computer via one USB A connector, but then it splits and has another USB A connector that I can connect to a power brick for 2A power?

              USB A                                                         
+-----------+ 2A charge                                                     
|Wall Outlet+-------+                                                       
+-----------+       |                                                       
                    |      Micro USB +----------+                           
                    +----------------+USB Device|                           
            USB A   |                +----------+                           
 +--------+ for data|                                                       
 |Computer+---------+                                                       
 +--------+    

Possibly related: Can the power source for a USB device be different from the data source?

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    \$\begingroup\$ USB cables are available exactly as you have asked for. They are commonly used with power-hungry portable hard drives and DVD / Blu-ray burners. \$\endgroup\$ – Dwayne Reid Sep 1 '15 at 2:03
  • \$\begingroup\$ @DwayneReid Ah! Yes! you are exactly correct. Can you link me to something or tell me the name of what I'm looking for? I am Googling all over and can't seem to get it. \$\endgroup\$ – boltup_im_coding Sep 1 '15 at 2:51
  • \$\begingroup\$ @DwayneReid will this do what I want? amazon.com/StarTech-com-USB2HAUBY1-Cable-External-Drive/dp/… From the reviews it seems so, but I don't understand how this isn't just .5A + .5A for 1A, which is still < 2A \$\endgroup\$ – boltup_im_coding Sep 1 '15 at 2:54
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Caution

As has already been pointed out, it is possible to purchase Y cables that have two Type A connectors on one end and the desired Micro B on the other end. These cables are intended to be used on computers or powered USB hubs where the +5V connections to both type A connectors come from the same +5V power supply. The +5V and GND leads of both Type A connectors are connected together. The design intention is to get around the 500mA current limit that exists on many computer/hub Type A USB jacks that contain circuitry to enforce the load current limit out any one connector.

You do not want to try to deploy one of these type cables for your application because it would mean that you would be connecting your external power source in parallel with the +5V output of the computing device. This is something you want to avoid at all costs because one voltage source will always be a higher voltage than the other one. The one with the higher voltage will try to assume driving all the load and back feeding the other source. Bad things can happen in this situation including overload of your external supply, oscillatory behavior of the regulators on both sources as they try to fight each other or other strange noise spike behaviors.

Cable Construction

Instead you want a cable that is built like this:

enter image description here

There are a couple of other factors that you need to consider as well:

Contact Ratings

The contact ratings of USB Micro B connectors may not be up to the current you are trying to put through them. So you need to check connector data sheets carefully. Take a look for example at the target device connector which solders to a circuit board of the device as in this data sheet. Notice that the contacts are only rated at 1A per contact if all contacts are used at rated capacity. In normal USB mode where the power pins are the ones carrying more current they are rated to 1.8A as long as the rest of the contacts carry less than 0.5A.

enter image description here

Cable Wiring

You also need to carefully consider the wire sizes in the cable to the target device. Many modern USB cables that terminate to Micro B type connectors use small guage wire to keep the cable diameter small and flexible. Small wire diameter at high currents can lead to a significant voltage drop along the cable. If the voltage drop gets too high the target device may fail to operate correctly due to the voltage drop. See the chart below for the type of voltage drop that will be seen for various wire sizes at 2A of current traveling down and back along a 2 foot cable length.

enter image description here

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  • \$\begingroup\$ Thanks for the great writeup, and you bring up a good point. So with the cable in the comments above, is each USB is getting .5A, for a total of 1A? \$\endgroup\$ – boltup_im_coding Sep 1 '15 at 17:32
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    \$\begingroup\$ @boltup_im_coding - Impossible to answer the question on the normal Y cable if it it connected to two different power sources. If it connected to two connectors on a computer and both those host connectors are attached into the same +5V supply then the current pulled by the target load will split close to half and half between the two Type A connectors. The amount of current will depend entirely on the load!! If both of the host ports had a current limit detector (not all do) that detector will kick in and turn off the output to the USB connector that exceeds the limit. (continued) \$\endgroup\$ – Michael Karas Sep 1 '15 at 18:22
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    \$\begingroup\$ (continued from above) Note that the current cutoff threshold will not be precisely at 500mA. Instead it will be at some threshold well above that that will actually have a fairly high tolerance. It could be from say 750mA up to maybe even 1A. Due to the wide tolerances involved one of the host connectors is likely to cut out first in an overload situation leading to the other one becoming overloaded at the instant the first goes off and then tripping itself. (continued) \$\endgroup\$ – Michael Karas Sep 1 '15 at 18:28
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    \$\begingroup\$ (continued from above) So if using the Y type cable it becomes very important to NOT operate with a load that tries to approach 2X the limit threshold of a single host USB port. \$\endgroup\$ – Michael Karas Sep 1 '15 at 18:29

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