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.


simulate this circuit – Schematic created using CircuitLab

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!


closed as off-topic by Leon Heller, Ricardo, Daniel Grillo, Dave Tweed Aug 11 '15 at 12:27

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  • \$\begingroup\$ What they need to do is send you a new power supply. \$\endgroup\$ – Dave Tweed Aug 11 '15 at 12:26

Sounds like the +12V supply output is somehow connected (or shorted) to earth ground.

Since the USB port is also grounded you will get severe problems under these conditions- good thing it didn't completely take out your computer motherboard.

Check that the +12V output is not connected to earth with a multimeter (power off and using continuity function).

  • \$\begingroup\$ To me it sounds like a earth referencing issue causing a short. If for e.g on the lab supply he is referencing V- to Earth and connecting to PC everything is fine. If the Melzi is floating and there is something clamping to earth via some element in the circuit then connecting common to earth via the USB connector can cause a large current to flow. I have experienced these kinds of reference issues debugging circuits with isolated grounds and different earth references. \$\endgroup\$ – crasic Aug 10 '15 at 17:50
  • \$\begingroup\$ @Spehro Thanks for the prompt reply, and yes before every test I was thorough when it came to probing the entire system for shorts including the power supply (with the continuity function), all connections are correct and secure, system has no shorts. +12 output is not connected to earth. \$\endgroup\$ – Zain Mir Aug 10 '15 at 23:48
  • \$\begingroup\$ @crasic any suggestions in further debugging techniques in regard to referencing issues? \$\endgroup\$ – Zain Mir Aug 10 '15 at 23:50
  • \$\begingroup\$ If you plug everything in but the USB cable and measure the voltage between the grounds (try both AC and DC volts scales) what do you see? \$\endgroup\$ – Spehro Pefhany Aug 11 '15 at 1:54
  • \$\begingroup\$ A fix would require isolation on usb side but for debug purposes you can also tie common (-) on 12 volt to earth to reference everything and solve isolation issues \$\endgroup\$ – crasic Aug 11 '15 at 3:31

There is a chance that the capacitor in the board's power supply has a short in it. This is the cap that connects the ground of the PSU to the mains (for noise suppression). Now, it should be a special cap, which when fails never shorts, but who knows? Check the voltage between the board's ground and the PC's chassis.


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