Re: positive ground

Posted by Gerald J. on February 01, 2009 at 11:46:29 from (4.254.70.142):

In Reply to: positive ground posted by matt knoeck on February 01, 2009 at 09:03:54:

There two ways to use the chassis as part of a DC electric system. Positive ground and negative ground. They both work and history has not shown one to work better than the other. In the more recent solid state electronics era, more loads are polarity sensitive and so the auto industry has settled on negative ground. Much of the world's electronics in all areas is planned negative ground, except for telephone offices where positive ground is the standard. Yet the same transistors and integrated circuit devices are used, just with care about which side of the supply is grounded.

I think the choice about corrosion went back to DC trolleys and corrosion of buried metal water and sewer pipes. The problem there was that a considerable portion of the ground current flows in the ground instead of the rail and the ground conductivity is improved by the pipe. And somewhere along the pipe, different for every pipe, the current may enter or leave in a small area and that current density either corrodes or protects the pipe. So for each pipe and rail line there was a polarity that minimized the local electrical corrosion of the pipe. The motors, lights, and controls in the trolleys didn't care what the polarity was and worked the same on low frequency AC.

I think that trolley/pipe experience argument was carried over the vehicular electric systems where before radios and before solid state it made little difference in performance or life which side was grounded. The ignition coil works best when its polarity matches the battery though, and its important to flash the generator when changing polarity so it charges with the right polarity. Some voltage regulators were polarity sensitive because of the choices of contact point materials. Today's solid state regulators and alternators are absolutely intolerant of reversed polarity, as are radios and engine control computers. Some gas gauges don't work well with reversed polarity. And the connections to an ammeter need to be reversed when the polarity is changed so it reads right. Otherwise it reads backwards, but its not harmed.

Gerald J.

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