first time using StackExchange so please be gentle (and pardon my ignorance with wiring and USB standards). I'm trying to create a USB power booster that allows Android Auto to function over my car's USB port, while receiving power from an external 2.4A cigarette lighter charger. I have created a wiring diagram of a design that I think will accomplish my goal, but I want to make sure that I won't fry anything (or that it will even work). enter image description here

The USB breakout boards are to both increase the longevity of the device, and avoid cutting and soldering the tiny wires inside the USB cables. Also, I don't believe I have to worry about the 56k resistor in the type C cable? In order to improve this design, would it make sense to add diodes anywhere to prevent sensitive electronics from being fried?

I would be happy to provide any clarification necessary, thank you very much for your help!

  • \$\begingroup\$ Why not use USB Y-cable with a standard car USB charger? And usually the USB ports supporting Android Auto have enough power output. Why do you even need it? \$\endgroup\$ – Eugene Sh. Aug 14 '18 at 21:34
  • \$\begingroup\$ I agree with Eugene. The first major problem with your sketch is that there is no connection between the grounds of the two circuits The data lines will have no reference. \$\endgroup\$ – Transistor Aug 14 '18 at 21:42
  • \$\begingroup\$ @EugeneSh. I could only find one USB type C Y-cable on amazon, some people said it worked for my purpose and others had trouble with charging speeds. Also, I was looking for a project involving some soldering. \$\endgroup\$ – Slater Aug 14 '18 at 21:43
  • \$\begingroup\$ @Transistor would it be possible to wire the grounds together in some way, and use a diode to prevent damage? \$\endgroup\$ – Slater Aug 14 '18 at 21:51
  • \$\begingroup\$ I think it's just a bad idea. A diode in the ground path would really screw things up. \$\endgroup\$ – Transistor Aug 14 '18 at 21:54

As I understand the OP wants to use some standard USB port from car equipment to communicate with a smartphone, but provide an alternative power source (5V 2.4A) to boost smartphone charging functionality. This function (boost) was frequently achieved with "Y-cables", which were used to connect external USB-HDD enclosures in the past. One of Y-end would contain D+/D- data lines, and the other Y-end holds just GND and VBUS.

enter image description here

So this kind of cable should work for a smartphone, except that the phone has Type-C connector. Unfortunately, on a quick search on Google I wasn't able to find the exact configuration, but it should be possible to make such cable with some effort and luck.

First, if you take a standard USB A to USB-C cable (preferably a thick one, with imprinted text "AWG24/AWG28" on cable) and carefully open it up near type-A end, you can cut red wire, and re-connect it to another piece of cable with Type-A plug. The ground wire should be split between two Type0A plug cables. The original Type-A plug (with no VBUS) should go to the USB car port (for data transfer), and the attached extra (Y) end would go to the 2.4 A charger, just as your diagram indicates (except that grounds have to be connected). This should be done without any "breakout boards", they are bad. Also, before doing this you need to check if the grounds between your boost charger and ground on USB port are either not connected, or connected to each other with no significant potential difference, otherwise the entire project will fail.

Unfortunately, a standard good Type-A-C cable would have 56k pull-up on CC pin. So yes, you need to worry very much about the 56k pull-up. The 56k pull-up would make the smartphone to draw only 500 mA for charging, so you will have no "boost". To have the boost, the Type-C end should have 10 k pull-up, so the phone would be informed to draw full charge. Therefore, for this kind of project you will need to find an "illegal" USB-A to USB-C cable, with 10k (or 22k) pull-up on CC line, opposite to the ones recommended by "Google engineer Benson Leung", the "bad" ones. Unfortunately, this pull-up resistor is mounted inside Type-C overmold. One other way is to open the overmold carefully, and change the resistor, the task which will require some dexterity.

Alternatively you can use the pictured above Y-cable and replace the common Type-A end with Type-C DIY connector like this one, with proper value of pull-up resistor. This would be the best and neat variant.

ADDITION: This Y-cable with Type-C should work if it has 10k (or 22k) pull-up.


It is a bad idea to split off USB power from data lines with pure wiring as you have shown.

  • At a minimum, you must join the two ground lines, because USB signaling is not fully differential so there must be a stable ground reference.
  • If I remember correctly, it is also required for a USB host to not bring up power until it is ready to start talking (or some other similar timing requirement), which your connection of the +5V line separately will violate if the car computer starts up slower than the charger.

Instead, get a standard USB hub with a power jack, and hook that up to your separate 5V source. The hub will manage the combination properly, since that is its own function (in addition to providing multiple ports).

Since you are using a USB C phone, you could use a USB C charge-through hub and hook up the power input using a (third) standard USB cable.

  • Advantages: you're likely to get higher power (through USB Power Delivery) than with a traditional hub, and you don't need to adapt USB to a barrel jack, and everything is theoretically USB spec compliant.
  • Disadvantages: more expensive, and the newness and complexity of USB C means that you are adding more possibility of a poor implementation interacting badly such that your phone and car won't talk to each other through the hub.
  • \$\begingroup\$ Thanks for your answer. Out of curiosity, how would I have to change my circuit to prevent this from being an issue? \$\endgroup\$ – Slater Aug 14 '18 at 21:52
  • \$\begingroup\$ @Slater I've updated my answer with more details and an additional suggestion. \$\endgroup\$ – Kevin Reid Aug 14 '18 at 22:25

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