I have a small control board with an STM32 controller on it that handles commands from VCP usb port and controls a large ledpanel of 400 leds.
My PWM circuit looks like this and should be able to handle currents up to 1.9A
My board is powered from a 12v DC adapter, but the touchpanel pc has it's own adapter.
As soon as the USB cable is plugged in, the leds started to turn on way brighter. So with unplugged usb cable I measure like 5 volt between the usb cable plug and my board ground. I was expecting a tiny bit of potential difference, because they don't share the same ground. My board ground is from DC 12V adapter and USB ground is from the Touchpanel PC adapter.
I really don't get it yet what is going on here. Can it be that the large ledpanel is dragging my board ground to -5V?
I'm PWMing at 32kHz, the load is around 1.5A maximum