PeachParts Mercedes-Benz Forum - View Single Post - Electrical trouble; replace Alternator, Brushes or Voltage Regulator?

JimSmith · #16 01-28-2002, 08:59 PM

Greg,

The way the system works in all alternators or generators, the need for something called a voltage regulator takes the inverse relationship you know about and kind of makes it a side effect. There is an energy storage device, the battery, in the system, and a source, the alternator. The combined effect of these components makes the phenomenon you described in the beginning actually occur, that is, as the resistance increases the alternator and the battery try to maintain the voltage in a fixed range. This causes the fuse to pop in the set of circumstances you and then I described. IF there was only a battery, and it was sized so it could only barely maintain the load alone, then Dave would be correct, and as resistance increased, the amperage would decrease. Since we have a regulated source, and it is regulated on the Voltage feature (which is kind of what a battery is, provided it is of sufficient rating), current will be a consequence of the load. If the Voltage is fixed and the load is 100 Watts, and then Voltage is kept fixed and the load goes to 200 Watts, then the only thing that can accomplish that is an increase in Amperes. In the lead acid battery of the size and rating to start a car, especially a Diesel, the Voltage is regulated by the chemical reaction and the number of cells wired in series. Once you exceed the current rating at 12 Volts the Voltage will droop and Dave's scenario takes over. For a circuit that has a light on it, the fuse will blow before that happens.

The voltage regulator is designed to limit voltage, not boost it, so the alternator has to be designed to make the minimum Voltage required to run the full load of the electrical system when the engine is idling. This means the Voltage will climb, as well as the available power, as the rpm increases. The alternator is designed to limit this Voltage rise so it is not linear, and the regulator can be sized to fit under the hood. As loads are added while driving the car, the alternator usually absorbs it, and at highway speeds, usually has lots of margin. At idle the situation is not so robust, and if you have a Voltmeter, you may see the Voltage flicker when the blinkers go on and off, when you step on the brake, turn the fans on, etc.

The dip in Voltage that accompanies the increase in demand for Amperes is an artifact of how the alternator, regulator and system are designed to work over the full range of possible operating conditions (high rpm - no load, high rpm - full load, idle - no load, idle - full load). If the Voltage continued to droop to say, 5 Volts, the Amperage would not continue to go up as at 5 Volts you just could not get the power out of the alternator. The lights would glow dimly, and the fans would slow down and so on.

I don't know if this has helped or not, but I hope it has. Jim