Quoting Removed, click Modern View to seeDecreasing current & voltage: Many ballast resistors (including the original 12250 on N tractors) increase resistance as they heat up.
How to measure amperes when running? A current probe on an oscilloscope. It looks like this (lower trace):[URL=http://s192.photobucket.com/user/JMOR_photo/media/Ign/full_cycle_withcurrent_sml.jpg.html][/URL]
Is my tractor operating at 2.82A when running? No, it will be somewhat higher at 'peak', because the average current is lower due to both the exponential rise (see scope current trace) and the fact that points are closed only for the dwell percentage of the cycle. Since the average current is lower, the ballast resistor will cool down relative to steady ON situation, thereby dropping less voltage and giving you a somewhat higher than 4v coil input & thus higher than 2.82A peak.
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Today's Featured Article - Third Brush Generators - by Chris Pratt. While I love straightening sheet metal, cleaning, and painting old tractors, I use every excuse to avoid working on the on the electrics. I find the whole process sheer mystery. I have picked up and attempted to read every auto and farm electrics book with no improvement in the situation. They all seem to start with a chapter entitled "Theory of Electricity". After a few paragraphs I usually close the book and go back to banging out dents. A good friend and I were recently discussing our tractor electrical systems when he stated "I figure it all comes back to applying Ohms Law". At this point
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