When enable Heatbed laptop switches off?

iya guys

been using MK2 heat bed no problems.

Then all of sudden when i enable heatbed on using pronterface the laptop cuts out as if theres been a short on the USB port.

so i check all cable for shorts etc and the thermistor etc nothing.

then starts working agian.

worked for 2 days

now its back again.

any ideas iv check for shorts on the board and cabling many times.

i was thinking maybe a PSU problem but im using a new 750W ATX PSU and the PSU does not cut out only the laptop

Which electronics board?

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[www.hydraraptor.blogspot.com]

Sanguinololu 1.3a board

just taken off heatbed again

no shorts anywere

just spent the last hour doing more tests.

looks like its a problem with the heated bed itself as when moving cables round to switch with the heated bed no problem is present with the bed connection.

took out heated bed now and taken all components off

when i test ohmns across the bed i get 1.6ohm which i think is very low resistance and probs why its killing the usb connection and causing the computer to short?

what do you guys think? iv check all over the board and cannot see any trance damage anywere

1.5 to 1.7 ohms is the correct resistance for the heated bed.

Possibly a poor ground connection causing current to flow through the USB ground lead. You must have a short thick ground between the sanguinololu and the power supply if the power supply ground is connected to mains earth and the laptop is also.

Does it happen if the laptop is battery powered?

Another possibility is it is something to do with the bug I found on the sanguinololu in that the USB VBUS is connected to the output of the 5V regulator. I removed that connection before I ever connected it to a PSU.

Edited 1 time(s). Last edit at 05/01/2012 04:51PM by nophead.

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[www.hydraraptor.blogspot.com]

yeah was a ground problem !

wire had worked loose so wasnt fully connected

thanks for the help