A light bulb is designed to operate with a 12 volt drop across it. The amperage will never be more than the bulb is designed for. Adding the additional resistance of the field coil drops the amperage passing through it even more. In this case the bulb is the main factor limiting the amperage draw. With a diode the field coil is the limiting factor.
The voltage applied to the excite terminal will determine the speed at which the alternator starts to charge. The more resistance in the bulb the higher the rpms needed for the alternator to start to charge. If the resistance through the bulb is such that you have 4 - 6 volts at the excite terminal the alternator will start to charge in the 1000 - 1400 rpm range. If using a diode with low resistance you will have very nearly battery voltage on the excite terminal and the alternator will turn on at around 600 rpm. If you use only a diode that has very little resistance it should be large enough to handle the rotor amperage draw at battery voltage. A three amp diode will have sufficient capacity for this. Since most diodes will handle larger amperage loads than for which they are designed, they will usually carry the overload for at least a bit of time. A one amp diode may work as long as you turn on the switch and then start the engine. The same diode may fail if the switch is turned on and the alternator does not shortly begin to charge. A failure of the diode trio could also overload the diode even though the alternator could continue to charge.
An alternator excited by a diode could put out full amperage with a failed diode trio. If the same alternator was excited through a light bulb the maximum output would be considerably less due to lower voltage available to power the field coil in the rotor. In this case the light would remain on since there would be no voltage coming from the diode trio to turn it off.
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Today's Featured Article - When Push Comes to Shove - by Dave Patterson. When I was a “kid” (still am to a deree) about two I guess, my parents couldn’t find me one day. They were horrified (we lived by the railroad), my mother thought the worst: "He’s been run over by a train, he’s gone forever!" Where did they find me? Perched up on the seat of the tractor. I’d probably plowed about 3000 acres (in my head anyway) by the time they found me. This is where my love for tractors started and has only gotten worse in my tender 50 yrs on this “green planet”. I’m par
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