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USB Cable Hot!

Posted by ReprapDan 
USB Cable Hot!
April 22, 2013 09:14AM
Hi All,

New to Repraps - I have just built my Pursa running RAMPS 1.4 - with MARLIN Firmware.

I have an interesting issue when I connect my Heat bed. When I turn on the machine, my USB cable gets really hot. My heat bed turn on immediately and can not be controlled by Pronteface. I have to turn off the power supply to prevent my USB cable from melting. I think it also killed the battery on my laptop (maybe just coincidence).

If the bed is disconnected, I can safely run all the motors on my machine including the extruder heater with out any issues, and without the USB Cable getting hot. I am running a 12V - 6Amp Switching Power supply to the 5A side, and an ATX 350WD - 12V for the 11A heater side. This does not work. I tried to use the ATX to run both 5A and the 11A - but he USB cable gets hot. So at present I have the heatbed dis-connected so I can Calibrate my machine. I do not have the Diode 1 installed on the motherboard as I am a little un-sure when and why I should install it. It seems there is a problem somewhere with the heatbed drawing current from the computer via the USB (or the other way round).

Has anyone else had a similar problem. Appreciate any help on this as its the last thing mechanical I need to fix before I start printing.

Thanks in advance - Dan
Re: USB Cable Hot!
April 22, 2013 09:24AM
... measure the voltage/potential between your heat-bed ground and USB-ground or shield/ground of your netbook-PS - there's some wrong ground configuration (maybe missing GND-connections?), so there's a lot of current going through your USB-shielding the wrong way!

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Re: USB Cable Hot!
April 22, 2013 09:30AM
If you're running with separate supplies you need to tie their grounds together. Your USB cable should not be getting hot. Are you sure that you have the polarity correct on your power feeds?

- akhlut

Just remember - Iterate, Iterate, Iterate!

Re: USB Cable Hot!
April 22, 2013 09:51AM
Thanks for the fast responce.

The polarity appears to be correct (2x Yellow to the '+' and 2x Black to the '-', but my ATX is only producing 11.2V. This is only on the 11amp side as my other power supply powers the 5amp side (measures 12.1V)

I have a 10 Ohm 10 watt resistor on the ATX 5V to draw a load.

Will the ATX being under voltage effect the operation of the 11Amp side? If so, I will purchase a better power supply, and remove the complication of the ATX.
Re: USB Cable Hot!
April 22, 2013 10:15AM
I have helped debug a similar issue in the past. It was as VDX suggests.

To test for the differential ground issue: Disconnect USB from RAMPS. Connect and turn on your power supply. Put one voltage (multi-)meter probe on GND on RAMPS, the other probe to the metal USB shield from your computer. Check for DC volts and AC volts. Both should read 0V.
Re: USB Cable Hot!
April 23, 2013 09:11AM
Thanks for the responce.

I think I have found the problem. There is a resistor or a diode that has burnt. It is located on the top of the Arduinio board above the USB Case Shield. It is a surface mounted component, and it crumbled when touched. It is connected to the USB Case and track to the MegaGA16U2 Chip.

Do you know what I have to replace this with - and would that have been the cause of the problem?

I did the test you suggested with the following results:

12V 5A measured +/- returned 12.01V - then negative to the USB Case = 0.00V
12V 11A measured +/- returned 11.71V (separate power supply) - then negative to the USB Case = 0.00V

D10 measured +/- returned 0.00V but the negative and the positive returned 11.75V. Check D09 and D08 all returned 11.7V between each terminal and the USB Case. Not sure what this means.

Hope this sheds more information.

Re: USB Cable Hot!
April 23, 2013 11:43AM
Is the board new? Can you exchange?

How are you at soldering?

Which component is it?

- akhlut

Just remember - Iterate, Iterate, Iterate!

Re: USB Cable Hot!
April 25, 2013 01:46AM
Thanks Akhlut,

I have found the problem with the USB Cable.

The USB Metal Shield on the Arduino Board was coming into contact with one of the base of one of the transistors on the RAMPS Board. It only shorted when the USB cable was attached as this moved the card down a fraction. When you un-plug the cable the board would slightly lift and everything would test OK.

To remedy the problem I trimmed the excess wire close to the solder on the transistor to avoid them touching and insulated the top of the USB Casing with some Kapton Tape. Everything now works as expected.

The part that has burnt is the L1 BLM21 SMD EMI Suppression Ferrite Bead. I have attached the picture with the part circled.

If the resistance is not critical I can order a spare part from RS Components and solder myself. This part does not effect any of the operation of the Ramps boards as I believe it is a EMF noise filter only.

I have also linked the common on between power supplies as you have suggested.

Thanks for your help.
open | download - Arduino1.jpg (6.6 KB)
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