Injecting current into +5V line of USB port -- is it OK?

My apologies up front if this question is more appropriate for Orbitty carrier board supplier – unfortunately, I’m not an expert on USB, so not sure where to start.

I need to connect a USB camera and a G3/G4 LTE USB modem to the USB OTG port on Tx2 + Orbitty carrier board configuration. Total current draw from the two USB devices is expected to be up to 865 mA, which is more than the USB OTG port can supply. In this application, space and weight constraints make it difficult to use a powered USB 2.0 hub. Instead, I’d like to splice an external +5V power supply between the +5V and GND lines within a USB cable. In doing so, the +5V line going back into the Orbity carrier board would become an open circuit; however, the ground line would remain unbroken. My question is, will this work? More specifically, will my approach cause any problem for the USB circuitry when it tries to negotiate with connected devices to allocate power? Is there a way in software to disable current sensing on USB port so this injection scheme will work?

Thank you

Hi, please contact carrier board vendor for the detail design on USB port. Generally there is no way to disable output current limit, it is a hw request and also a safety request. USB enumeration can be done without involving vbus level.

Usually for that situation, connect the usb data lines and ground but on the device side (the ones plugging in) do not connect to the +5v line coming off of the usb connector, or if you need to splice wires just have the jetson side +5v line taped off and disconnected. Connect the device +5v to the other power source. Make sure to tie all three grounds together.

Thank you for fast reply and the key word “USB enumeration,” which lead me to basic online documentation explaining USB protocol.

Thank you for the fast and informative reply. If I’m understanding your description correctly, it sounds like the original scheme I proposed should work. Do you agree? 1) cut +5V line inside a USB cable and connect the slave device side of that line to an external +5V supply, 2) leave the Jetson side of the cut +5V line open (connected to nothing), and 3) tie the ground line from the +5V power supply into the ground wire inside the USB 2.0 cable but also leave the ground wire connected to the Jetson/Orbitty side (this is what you mean by tie all three ground wires together, correct?). This, in combination with Trumany’s post that USB enumeration can be done without involving vbus level seems to suggest that everything should work. I also need to supply additional current capacity to a device connected to the USB 3.0 port. As follow-up question, is the same scheme of splicing a +5V power supply into a USB 3.0 cable expected to work? Here, the actual plan for splicing in +5V power for both the USB 2.0 and 3.0 cases is to use small, commercially-available breakout boards that expose each line inside a USB cable. Any expected problem with using USB breakout boards in this way? Thank you again for the informative response.

You always want the grounds connected, and the data +/-. You only bother with +5V if the device itself requires 5V from the host computer. If your device is self-powered, then cutting the 5V is acceptable. Standards do not allow power going to a host from a device. There can never be two hosts, and there can never be two devices on a given port. Self-powered devices are quite common.

1] yes
2] yes
3] very much yes

So the main thing you need to worry about is trying to MINIMIZE disrupting the usb signal wires!!! For 2.0 those are data+ & data-. That is going to be the hard part about all of this. Keep the ground wire surrounding those data wires, and keep the signal cables twisted around each other. This will keep the signal out of those wires coherent and readable.

For 3.0 wires: This will be more difficult since those signals are going much faster and have a high chance of cross talk. Tbh, not sure if that is going to work, but you never know till you try. Let us know how it goes!

Thanks for excellent advice to all who have replied. Will post here what the outcome is after ordering some parts and giving this a try. It could be a while though as it seems Amazon orders are being delayed due to Covid 19.

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