Hello, I recently bought an arduino leonardo and a usb host shield (it's already soldered). I made a code so that when I connected my mouse to the USB host shield, the mouse would work on the PC through HID. Everything works perfectly, but as soon as I connect an external power source to the arduino through the jack (9V 1.5A) the mouse stops working
That sounds like a grounding problem, can you post your annotated schematic that shows all connections.
How can I do that
Thanks
@gilshultz
I've fixed your reply; the forum software thought that you were posting code 
Your external power supply goes through a voltage regulator on the Leonardo. Due to lack of cooling, this regulator can not supply a lot of current.
When powering your setup through USB, the regulator is skipped.
The above might be your problem.
If your setup does not have to communicate with a PC while it is externally powered you can use a cell phone charger.
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