I think I might be able to help explain this. In the most basic form, we generate electricity either by placing a conductor through a magnetic field, or by having a magnetic field pass by a conductor. In a car generator, there is a rotating shaft with windings on it we call an armature. Around this is a stationary magnetic field created by applying power to windings wrapped around soft iron on something we call a stator. As the armature turns inside the stators magnetic field, we induce current flow, and produce electricity. This electricity is carried out of the stator by a series of contact on the end of the armature shaft that rub against carbon brushes. To make sure there is always a steady magnetic field for the windings in the armature to pass through, a group of windings is switched on in sequence depending on the angular position of the armature. This is accomplished using a segmented series of contacts called a commutator. The commutator always makes sure the electricity produced is positive and direct. The regulator is designed to vary the amount of electricity into the stator, which in turn controls the strength of the magnetic field, and so the amount of voltage produced at the commutator….a closed loop control system.
The generator is not very efficient because the brushes don’t make perfect contact, and are always being connected and disconnected in rapid succession. Anyone who has looked at an electric drill motor in operation has seen the shower of sparks being produced at the commutator to know that some of that power is being wasted in the form of heat, not to mention broadcasting radio noise. Brushes also wear down because of the discontinuity in the commutator surface. In addition, there is a physical limit to how fine the angular switching of the armature windings – the brushes can be made only so thin before they break or are limited in current flow. So another approach was needed. When Bell labs developed the semiconductor in the late fifties, diodes and transistors, a means was now possible to make a more compact, yet more efficient, electricity generating device. The alternator, so named because it is actually producing an alternating or polyphase current just like the AC in a home, is really a 3-phase generator. The output is derived not with a segmented commutator, but smooth slip rings. This mitigates wear (and sparks hence heat hence losses), and produces more power ever 120 degrees than a generator. The electric field around this is controlled by the regulator. The output of the three phases is channeled through diodes, which only allow current to flow in one direction. As the output rises above or below ground, the connection of the diodes always results in positive current flowing to yield direct current, albeit with some ripple. Now for the idiot or charging light. For a light to work, obviously current from flow through the bulb or light emitting diode, back to a battery. An incandescent light or old style light bulb is polarity insensitive. A diode on the other hand must have a positive connection connected to the anode, and the output, or cathode, connected to the battery or power supply negative or ground. (I always envision the diode symbol anode as a big ‘A’, which makes it easy to know which side to connect to the positive). Now what if the diode or bulb has positive connected to both sides? No current flows, and so no light lights. We call this zero voltage drop. This is the basis of a charging system warning or idiot light. When you switch on the ignition, 12 volts is fed through the light and to the alternator field windings. When the engine starts and the rotation of the alternator gets up to speed and power is being produced, some of that power is fed back to the filed winding, in effect, creating the same potential on both sides of the bulb, so without a voltage drop, no light glows. In fact, for some cars, if that bulb is bad, the alternator won’t work. When I was young and foolish, or more foolish, I used to check an alternator by pulling off the battery positive terminal while the engine was running…if the car stayed running, the alternator was good because if it wasn’t putting out, there would be no power back to the filed to generate electricity to keep the car going. Now to the question of our LED replacement for the charging light in our SLs. An incandescent light has more resistance than an LED, and needs some resistance to heat up and glow. An LED is current sensitive and can burn out, requiring only about 20 milliamps forward biased for operation, and only about 5 milliamps for a high efficiency LED. So adding a resistor in series will be required to protect the LED and have a roughly equivalent load to the field circuit, depending on what else is connected to it. I would not connect an LED to the charging circuit without a series resistor. To be sure how best to do this, I’d like to see a schematic of the W113 charging system. I assume there is one somewhere on the forum? If 82 ohms works, I would suggest hooking an ammeter is series with the LED and this resistor and measure the current when the ignition is on but the engine not running…the LED should be on…if you are below approximately 30 mils, I think you’re good.
