Hi everyone. We are manufacturing electrical device and must be test function device in testing machine. Kindly have a look with schematic below. The device is connected with adapter 24VDC, then device is connected to USB comport by 4 pins: 5V, GND, D+, D-. The problem is PC was broken sometime. We checked hardware and suspect that the 24VDC has caused computer failure through 5V and 0V wires. Kindly please let me know if you have any suggestions to solve this issue.

 

CERTAINLY, connecting 24 volts DC into the 5 volts connection on a PC 's USB port will cause damage, with the extent of that damage depending on the current available from that 24 volt source.
IS the TS able to measure the actual voltages at the USB connection??
It might also be that there is some mains power leakage to the "ground" connection of that USB connector cable, which could also cause damage to a mains powered PC.
Of course, these are only possibilities, based on not much information.

 

There is no way to know what is happening without more information. Something I recommend in these cases is to use isolators for your computer's USB ports.

 

I have tried use isolator, but the isolator quickly broke down

 

What voltage is on the USB data lines?

What voltage is on the USB Vbus line?

What voltage do you measure between USB power ground to outside power ground?

What voltage do you measure between the USB connector shell and outside power ground?

If the device is sending serial data with the USB device appearing as a com port, achieving complete isolation is pretty simple. Connect the micro's Tx and Rx pins to a USB converter chip like a CH341 via opto isolators. Keep all other connections completely separated from the rest of circuit, including Vbus and ground. Power for the CH341 is provided by the PC when connected.