Monday, March 8, 2010

Ground Loops

Last week at work, we proved that ground loops with an oscilloscope can be a bad thing. Here is a description of what happened, but the pictures mostly speak for themselves. As I understand it (I was not present) the issue occurred when a setup involving 277 volts was present. Our oscilloscope was not plugged into earth ground through the third prong, which was supposed to help look at AC signals. Unfortunately, the 'scope was plugged into the PC via the serial port (to wave capture) and then the serial ground provided at path for potential disaster. Basically, all the grounds in the circuit should be at the same point (voltage), ours were not.This picture is from the serial port on the back of the 'scope. Notice anything odd?
Same board, different angle. In this picture you can almost see the lifted traces on the board as well. That trace goes from one of the pins on the DB9 to the chip. The yellow cap. next to the chip isn't looking great either.
As it turns out, since the loop was through the PC, it was not spared either. Shockingly similar failure to the 'scope. Lucky for all of us, the hard drive was spared any damage and a duplicate PC was located to replace this lost one. The 'scope, however, could not be replaced as easily.

1 comment:

  1. I blew up my Elecraft K2 with a nasty ground loop on Field Day a couple of years ago. I had connected the radio to a 12VDC battery. I had also connected a DC-DC converter to the battery to power my laptop at 18VDC. Then I connected the radio to the computer for voice keying, and POP! The magic smoke came out. Apparently the DC-DC converter output had a floating ground. The computer was unscathed but some of the ground traces on the K2 were fried, along with it's CPU. Fortunately it's a K2 so I was able to fix it with a new CPU from Elecraft and some bus wire.

    The moral of the story is that opto-isolation is a good thing.