Delco 12SI draws 2-3 amps on the excite-field circuit. I've never blown a pair of 3 amp diodes in parallel yet and I've got 8 tractors wired that way (with Delcos or Hitachis).
Biggest problem I've had when using resistors instead of diodes is run-on with diesels. Many diesels with 12 volt electric shut-offs will keep going even with only 3 volts. Diode at less then $2 is kind of a no-brainer. Works with gas and diesels and always allows shut-off.
Another way to wire a Delco SI series with no resistor or diode is with a generic automotive type ignition switch. The kind that has separate ACC and IGN terminals. Wire the ignition to the IGN term, and the alternator excite wire to the ACC term, and you're all set. The two are isolated when you turn the key off.
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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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