I recently bought a 3d printer which runs on a Melzi v2 board [http://reprap.org/wiki/Melzi]. It came with a 12V 20A power supply unit (multimeter readings showed values ranging from 11.8V-12.1, im my books that seems ok). After completing all the assembly work and re-checking [multiple times] that all connections were correct and secure I powered it up. Everything seemed to be working perfectly up until I plugged the USB port into my computer, windows notified me that a power surge has occurred on the port and shut the port down. The USB cable was burning hot and one end of the connector started to smoke. I quickly turned everything off. I disconnected all the wires on the board and discovered that one of the IC's on the board had short-circuited. Further inspection showed that the GND pin on the usb was sinking a large amount of current (cable was permanently damaged so I threw it away). I contacted the manufacturer and they sent me a new board.
This time around I decided to place some safeguards between my pc and the board, so I placed two 50 ohm resistors on the D+ and D- lines and a 6V 0.1A fuse on the GND (see image below). When I powered everything up, once again everything seemed to be working fine up until I connected the USB cable in. R1 and R2 both burnt up so I disconnected everything (this time around board and pc were fine). I changed out R1 and R2 and decided to retry the experiment with my lab power supply set to 12V. The problem disappeared.
So my question is why would two different power supply units, both at equivalent settings, be giving me different results?
Any help/suggestions would be much appreciated!