-=> Roy J. Tellason said to Greg Mayman
 -=> about "VEHICLE LED'S" on 09-22-03  04:06.....

 RJT> Last one I remember seeing (taken out of a vehicle -- it was
 RJT> defective) was maybe 4 inches square,  3 inches high,  pretty big for
 RJT> just a couple of diodes being on there.

Yes, I remember they were big heat sinks.

 RJT> Yes.  The regulator I'm familiar with (Mopar stuff) is grounded,  and
 RJT> only has two other terminals,  one of which goes to the field winding. 

Some of the older alternators had their fields powered from the
switched ignition circuit. That way the regulator was looking
more or less directly at the primary battery, AFTER any isolator
diode that was charging it from the alternator. So the alternator
output would be automatically adjusted for the diode drop.

Unfortunately it doesn't seem to be done that way these days.

 RJT> I forget if the other end is grounded or tied to ignition,  and where
 RJT> the other wire from the regulator goes,  but it's pretty simple, 
 RJT> really.  Except for if you get a bad ground,  which I had happen to me
 RJT> one time and which boild out the battery in my truck,  before I caught
 RJT> it and cleaned things up. 

The regulator in a car my son owned a couple of years back was
stuck on full charge and destroyed one battery before he took my
advice and did something about it. He left it so long because
"an auto electrician might want to charge me $100 to fix it."

I checked the regulator and found a relay with normally closed
contacts that connected directly between the ignition circuit and
the alternator field. I assume the relay was supposed to operate
as soon as the alternator was putting out enough juice to power
the field itself and cut out the direct feed.

But for some reason it wasn't operating.

I couldn't do anything with the regulator as the PCB part of it
was potted and inaccessible, although all the parts were outside
the potting compound -- just enough to tease me into wanting to
trace the fault and fix it!

OTOH, a replacement from the junk yard cost him only $5. It had
already cost him about $25 to replace the battery from the same
yard!

 RJT> The other terminal on the alternator is the output,  and that's where
 RJT> you'd connect the isolator.  #10 wire,  as I recall.  So you'd need to

I'm not sure about that #10 wire as it is only 0.008 squ.inches.
At 40A out of the alternator, the current density would be about
5000 A/squ.inch.

#6 wire is 0.02 squ.inches. At 40A the current density would be
2000 A/squ.inch, a better figure considering the the elevated
temperatures it would get near the engine.

And if the second battery was any great distance from the
alternator, the heavier wire would reduce the voltage drop.

From Greg Mayman, in beautiful Adelaide, South Australia
   "Queen City of The South"    34:55 S  138:36 E

... Budget: A method for going broke methodically.
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