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USB OTG and Charging circuit question Answered

I've got an old tablet (some unknown chinese brand with zero support) that has one microUSB port for charging and connecting external USB devices (known as OTG). But I'd like to do both. Found a cable that has the required connections but it doesn't work and I'm trying to understand why. mobdro

I attached a device to the USB and attached charger, and the device was getting power, so VCC and ground are wired correctly. Connected USB device and tablet, and device was detected, so the OTG side is all good. But that means power is also flowing to the tablet from the charger. So connect charger and tablet, but not charging.
I mean, it makes my brain hurt more thinking about why this does work with my phone. Power flowing from charger to USB, fine, but the phone is also providing power to the USB, also fine (parallel power. nothing odd here), AT THE SAME TIME IT IS RECEIVING POWER TO CHARGE! https://sarkariresult.onl/

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Downunder35m

8 days ago

OTG comes in different flavours so to say.
The most basic and most implemented form is to allow a device to provide power output on the USB port.
Otherwise it would be quite impossible to use things like USB stick or card readers.
In this basic mode a device can do either but not both.
The electronics are not designed to have the battery connection open in both directions at the same time.
Never devices allow sometimes to do both but only if the attached device is using well below 500mA and then with very limited charging capabilities.

In terms of OTG adapters you can find two versions.
First is dirt basic and just transforms your USB port to a female one.
Second usually comes as a cable or USB hub and has a switch to select between charging and OTG mode.
If set to charging the device gets power but the OTG function is no longer wired through for the device to detect.
If set to OTG mode most of these cables (the better ones) will actually enable the OTG functions but cut the power connection to the device.
That means the connected USB device will get the power from the cable (hub) and NOT the device.

Way to figure out what exactly happens is to use two of these USB testers that show voltage and milliamps.
Needs the right adapter to go between tester and device though, I use the OTG adapter from a SD card reader for this.