The first transistor, with its base circuit connected to a zener diode and voltage divider to terminal #2, never turns on until the battery voltage rises to a voltage pre-determined by the resistor values in the voltage divider and the zener diode breakover 'on' voltage. The battery charging current is initially set by the two biasing resistors (and a forward-biased diode) in the second transistor's base circuit until the battery voltage at terminal #2 rises enough to turn on the first transistor. Once the battery voltage at terminal #2 rises enough to switch on the zener diode at the base of the first transistor, its colector-emitter circuit robs (shunts) 'on' biasing current away from the second transistor base-emitter circuit, thus lowering the emitter-collector current through the 2nd transistor in series with the alternator field winding. Lowering the field current lowers the Alt. output voltage/current.
Leaving the #2 terminal disconnected does mean that the first transistor will never turn on and allows the alternator to full-charge continually, but the Alt. output current is still limited by the base-biasing circuits of the 2nd transistor, battery condition (internal resistance and state of charge) and the resistance of the Alt. stator windings (snd the alternator current rating).
The GM-type alternator works in much the same way as older generator charging systems - always on full-charge until rising battery voltage intervenes through some type of a voltage regulator. The GM-type alternator will not start charging unless: 1) there is enough residual magnetism and rotational speed to generate enough voltage to turn on the second transistor in the VR and apply a small current through the field winding (as in one-wire Alt. setups), OR 2) Terminal #1 receives a start-up voltage/current, typically from the battery through a 'charge lamp', resistor, or diode, to initially furnish a small amount of power to the 2nd transistor in the VR and the Alt. field winding to get charging started. Once started, a 3-diode rectifier connected to the stator windings furnishes the power to the VR, and the start-up circuit becomes idle (charge lamp goes out).
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Today's Featured Article - A Cautionary Tale - by Ian Minshull. In the early 1950s my father bought an Allis Chalmers B and I used it for all the row crop work with the mangolds and potatoes, rolling and the haymaking on our farm. The farm and the Allis were sold and I have spent a lifetime working on farms throughout the country. I promised myself that one day I would own an Allis. That time event
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