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Say I have a normal USB "Y" cable - female and male with data and power connections, and an extra male port with just power connections - normally used to draw extra power from an additional port on the host.

Would it be safe to, instead, use the cable as a normal USB extension cable, and use the power-only male plug to power a small external device, like an Arduino?

edit: It's a solution I suggested for someone who wants a simple, small device with a multicolor LED, attached to their laptop (very low power - it can certainly run off a USB port power, that's not the question). Using just a USB cord would mean losing one of the two USB ports on the laptop. Using a commercial USB hub would be bulky and impractical. My suggestion solves both of these issues, If it actually works safely. I don't necessarily think this is a good idea - I just want to know if it would work.

Maybe I should rephrase the question: It looks to me like the power connectors on the "power plug" of a USB-Y cable are simply connected to the power connectors on the "host plug" in parallel. This is designed to supply additional power, but as long as there are no diodes or other control circuitry, I believe it could draw additional power instead. My question: is this accurate? Is my understanding of the USB "Y" cable circuit correct?

  • Depends on whether such a device would actually accept power from a port that usually provides it. Of course, there’s also the problem that a single USB port supplies very little power. – Daniel B Nov 6 '15 at 20:41
  • you have to check the volts and amps are enough. USB3 might be high ampage and USB is 5V apparently USB so 5V is fine for arduino forum.arduino.cc/index.php?topic=3435.0 Amps need to be >= the device's requirements – barlop Nov 7 '15 at 3:24
  • @monguin, looking back on my answer, I'm not sure that's the whole picture. The device lets the port know whether it's a low current device. If so, the port may limit the current. In a situation where you plug in something like a mouse as the USB device and then want to power something that needs some current, the port might not provide it, even though the Y cable could do the job. It might require something like a dongle between the port and all of this to trick the port into making its maximum current available. This is non-standard territory, so it might come down to just trying it. – fixer1234 Oct 2 '16 at 7:36
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I double-checked a Y cable and there was no internal circuitry, just wires. The power-only plug parallels only the power conductors. As long as the total load of both devices doesn't exceed the port capacity, what you describe will work. For USB 2.0, that's 500 mA; for USB 3.0, it's 900 mA.

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