USB cable burnt

Hi everyone,

I have designed a PCB based on Arduino MEGA 2560 and RAMPS, with some differents:

- It works at 24V with drivers drv2580
- The communication is based on FTDI driver and an USB connection.
- There is a comparator which selects the 5V from USB or power supply, based on a PNP transistor. The 5V is obtained from 24 V using this DC-DC converter:
OKI-78SR-5/1.5-W36H-C.
- The 5V pin from USB has a fuse of 500mA, whereas the two pins of 24V have a fuse of 5A.
- The case of the USB connector is connected to ground through a ferrite core.
- There is an unique GND plane both USB and power source.

Recently I had a problem in my machine where the USB heats up and is burnt and the computer crashes, when it has been connected to the PCB and the 24V power supply turns on.

After this problem, I made the next analysis:

- The origin of the USB cable heat is due to the GND pin is absorbing too much current for some reason and this also causes that the computer crashes.

- The origin of this high current can be a short circuit, and the current comes from power source, not from USB connection, because it has a 500mA fuse and this component is still working perfectly. The 5A fuses are also working, therefore, this short drifts the current through the USB GND and not to power source GND.

- When I connect the PCB with the computer, the connection works correctly. Also when PCB with the power source. This suggests that the problem could be the 5V source selector, but as it is working correctly for both cases, I do not believe this is my problem. The source selector is similar to Arduino MEGA 2560 has integrated.

I have reviewed the machine design, but at first it not seems something is causing this problem.

I keep waiting some help of you, I really appreciate it.

Thank you very much.
Best regards,
Antonio.

Schematic and pictures of the physical board would be nice, no promises on making ability to help though

Also, tried poking around with a multimeter yet? See if the ground is actually being powered? And if so with the 5v or the 24v

Edited 1 time(s). Last edit at 03/01/2016 08:54AM by DaGameFace.

Hi everyone,

thank you very much for your very quick answers.

I have attached the schematics of the power source and FTDI to USB circuits. I am not sure what you mean with the ground powered: in my PCB design, the ground of USB and power source is connected in the same plane.

I continue searching the answer

Thanks again,
Antonio

Your 24V PSU probably has its negative output connected to mains ground. The PC also has its chassis connected to mains ground. So if the connection between the negative output of your PSU and your electronics has broken or become loose, then the current finds an alternative path via mains ground and the ground wire in the USB cable. This is what makes the USB cable get hot.

---

Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

Hi dc42,

really what you say make sense, and more if I take account that I had been working correctly with the pcb until, suddenly, one day I connect the USB and the power source and the USB cable was burned. And this matched with the machine was carried to another place. However, there is something missed, and it is that the pcb consumes aprox. 15mA when it is turned on, and it is strange that this current is able to burn the cable and crash the computer. Any suggestions?

Regarding to your description, what is your advice to protect the USB cable and the computer?

Thank you very much,
Antonio.

... some (cheap chinese) PSU's are internal not proper wired to mains ground, so the GND-potential of the PSU and the GND-potential of the PC can be some Volts to some ten Volts off.

When connecting this two "different" GND-potentials with the shield of your USB-cable, this can cause currents of up to some ten Amperes!

---

Viktor
--------
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Call for the project "garbage-free seas" - [reprap.org]

Quote
aj.moreno
Hi dc42,

really what you say make sense, and more if I take account that I had been working correctly with the pcb until, suddenly, one day I connect the USB and the power source and the USB cable was burned. And this matched with the machine was carried to another place. However, there is something missed, and it is that the pcb consumes aprox. 15mA when it is turned on, and it is strange that this current is able to burn the cable and crash the computer. Any suggestions?

As you say, 15mA load would not do that. Are you sure that you didn't have the bed heater operating, perhaps unintentionally, when the USB cable got hot?

Quote
aj.moreno
Regarding to your description, what is your advice to protect the USB cable and the computer?

The best advice is not to use USB. Except when uploading or debugging firmware, I use Ethernet to control my printer, which unlike USB provides galvanic isolation. But if you must use USB, here are some tips:

* Make sure that the 0V connection between the PSU and the electronics is sound

* Plug the PC and the printer, and nothing else, into the same mains distribution block (preferably a surge-protected one)

* Use a USB cable with the ferrite bead on it

The last two of these are aimed at preventing transients on the mains ground wire upsetting the USB comms, or the electronics generally.

---

Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

Hi everyone,

Quote
... some (cheap chinese) PSU's are internal not proper wired to mains ground, so the GND-potential of the PSU and the GND-potential of the PC can be some Volts to some ten Volts off.

I think you are in the right way. Actually I am using a cheap chinese PSU for generating the 24V. I have measured the voltage difference between the circuit with the USB cable (in the ground pin) and the power source (V- pin), but there is not voltage.

What you say, it makes totally sense with the next. I have integrated this pcb in a 3d system. I have done test in our office (in Spain) and the prototype works correctly. When the prototype has been carried to other countries (concretely Portugal and Scotland), it has been when this problem appears, the USB has been burnt and the computer crash.

If we take into account your answer, and that it is very probably the management of ground / earth in other countries will be different than in Spain, both can cause this horrible problem. I would like to get deep into this problem and I would thank explanations about these differences and suggestions.

Quote
As you say, 15mA load would not do that. Are you sure that you didn't have the bed heater operating, perhaps unintentionally, when the USB cable got hot?

Yes, it is sure. I have measure it a lot of times.

Quote
* Make sure that the 0V connection between the PSU and the electronics is sound

You mean that V- of PSU and ground of our PCB, are connected? In this case, it is correct.

Quote
* Plug the PC and the printer, and nothing else, into the same mains distribution block (preferably a surge-protected one)

* Use a USB cable with the ferrite bead on it

Both are ok.

Edited 1 time(s). Last edit at 03/02/2016 04:32AM by aj.moreno.

Hi everyone,

Probably my problem resides that grounds from computer and from power source are different. This is a very good result, because it explains mostly my problem. Thank you guys.

However, in order to solve this issue, I need to understand where it comes from, and then modify my design to avoid that the problem will be repeated again.

Analyzing my circuit from this perspective, I had:

- 24V Power source: has 3 inputs: Line, neutral and ground (earth), and six outputs (3 V+ and 3 V-, which are 24VDC and 0VDC, respectfully). I have opened the power source and I have seen that the ground is connected, through capacitors, to the 0VDC outputs and to the two outputs of a transformer which inputs are line and neutral (I think this is used to reduce voltage from 230V in the input). Also ground is connected to the chasis of the power source.

- PCB circuit: from power source, it has four inputs, two of 24V and two of 0V. From USB, it has 5V and 0V, and the computer ground is connected to 0V through a ferrite.

My questions are then:

- How I can introduce a ground line into the PCB? is it mandatory? what is the best way, using capacitors between the 0V line and the ground line?

- What considerations should I have for avoiding this voltage potential between the power source, the pcb and the computer, when I use the laptop connected to the electrical net and running with the battery?

- Maybe the ferrite of the USB case is causing the problem that the ground of the computer is different to the ground of the power source?

- Can the power source improve how it grounds the signals?

- Other best practices / advices?

Again thank you very much.

Antonio.