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In today’s modern car’s electrical current is its lifeblood. Without it there would be no power, no lights, no tunes and no go. So, what does the little red light that says ALT mean when it comes on? That little red light basically means “Alternator output voltage is lower than battery voltage or the battery voltage is lower that alternator output voltage” if the light is dimmer then mostly the problem is with alternator and if it gets brighter then battery is most likely bad.

To understand more we need to understand how alternator works. Below, is a block diagram, or a “functional” diagram, of an alternator, and its connections to the remainder of the automobile electrical system. Following the figure is a description of the various components that make up an alternator, and a description of how each operates to keep the battery charged in your car.

Alternator Functional Diagram


Let’s understand the alternator from where it all starts in the alternator itself – at the “Alternator Rotor”. The rotor consists of a coil of wire wrapped around the core. A magnetic field is produced around the core by the current through wire coil called “field” current. The field current is DC in nature because of which it flow in one direction only, and is supplied to the wire coil by a set of brushes and slip rings. . The rotor is driven by the alternator pulley, rotating as the engine runs, hence the name “rotor.”



Another set of coils surrounding the rotor are called stator. The stator does not turn and is fixed to the shell of the alternator. As the rotor turns within the stator windings, the magnetic field of the rotor sweeps through the stator windings, producing an electrical current in the windings. Because of the rotation of the rotor, an alternating current is produced.



In a DC system AC voltage is of little use, so it has be converted to DC before it can be used. This conversion to DC take place in Bridge Rectifier. Diodes allow current to flow in one direction, while blocking current flow in the other direction. The bridge rectifier consist six diodes, one pair for each winding. One of the pair is for negative half cycle, and the other half for the positive half cycle. As a result of this diode rectification, the output of the alternator looks as a pulsating DC. The output of the alternator is actually not a pure DC. Because there are three windings, each with a positive and a negative half, by the time the voltage is passed through the diodes, there are six pulsations for each rotation of the rotor. This is close enough to D/C for most automotive components. Critical components, such as radios, have their own internal filtering circuits to further smooth out the waveform to a purer DC.



The diode trio consists, as the name suggests, of three diodes, one per phase, which provides field current to the alternator regulator.



The regulator has two inputs and one output. The inputs are the field current supply and the control voltage input, and the output is the field current to the rotor. The regulator uses the control voltage input to control the amount of field current input that is allow to pass through to the rotor winding. If the battery voltage drops, the regulator senses this, by means of the connection to the battery, and allows more of the field current input to reach the rotor, which increases the magnetic field strength, which ultimately increases the voltage output of the alternator. Conversely, if the battery voltage goes up, less field current goes through the rotor windings, and the output voltage is reduced.



Field current supply is provided from two different sources – from the alternator itself, via the diode trio, and from the battery, via the alternator warning lamp. When you first get in the car and turn the key on, the engine is not running and the alternator is not spinning. At this time, the voltage/current source for the field current is from the battery, through the ignition switch, and through the warning lamp. After the engine is started, and the alternator is up to speed, the output of the diode trio is fed back to the regulator, and serves as a source of current for the field current. At this time, the alternator is self-sustaining, and the battery is no longer needed to power the automobiles electrical system

WARNING!!! : This is theoretical only – in actual practice, the voltage surges resulting from disconnecting the battery can seriously damage the regulator circuitry. All alternator manufacturers strongly advise NOT doing this! This test will not prove the functionality of the alternator anyway, as the engine may still run with a weak alternator output.



As can be seen from figure 5, a schematic for an actual alternator, there is a path to ground from the field current supply input to the regulator. As a result, when the key is turned on, current flows through the warning lamp, through the resistors, transistors, and field coil, and then to ground, causing the lamp to illuminate. Once the alternator is at full output, voltage from the diode trio, equals the battery voltage. At this time, with 12 volts on both sides, the lamp is out.

If the alternator should fail, voltage from the diode trio would drop, and once again the lamp would light from the battery voltage. If the alternator output is only a little low, the lamp will be dimly lit. If the alternator fails completely, and the output voltage goes to zero, the lamp will be lit at full brilliance. Conversely, if the battery should fail, and the battery voltage drops, with the output voltage of the alternator on one side and the low battery voltage on the other, the lamp will also light.

As stated earlier, if the light grows dimmer as the engine is revved up, it is because the alternator voltage is rising with the RPM, producing more voltage on the alternator side of the lamp. The closer the output voltage gets to the battery voltage, the dimmer the bulb becomes. By the same way, if the light gets brighter with increasing RPM, it is because as the alternator voltage increases, it is getting higher than the battery voltage. The higher the voltage with respect to the battery voltage, the greater the voltage difference across the lamp, and the brighter it gets.


Alternator Circut

The above write up explains the complete operation of an “Alternator” in a nut shell. So that next time you see the little red light in your dashboard, you will know exactly what it is trying to tell you.


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