why d3 ?? anti reverse voltage going into the rectifier bridge ?
voltage drop ?
I suggest this picture
The diode rectifier bridge will never isolate any supply, or protect your pc from any 12vdc problems, you have to use an dc - dc converter
Ie 12 vdc in / 5vdc out, there is some models with 5vdc in / 5vdc out
Personally i would not play with usb ports, sure some have protection, but old pcs can supply little current amounts, and newest can go up to 1 amp in some cases, maybe more.