Charging circuit: alternator

A vehicle 's charging circuit must be capable of supplying the electrical current needed to recharge the battery and supply all of the vehicle's consumers (lighting, signaling, ventilation, etc.). The value of the current supplied by the alternator depends on the rotational speed of the engine.

The charging circuit includes:

  • A battery.
  • A generator.
  • A bridge rectifier.
  • A voltage regulator.

As its name suggests, the alternator is a generator that delivers an alternating voltage that must be rectified by a diode bridge because the battery only accepts direct current.

A voltage regulator will be needed to limit the voltage to 14.4V for 12V circuits and 28.8V for 24V circuits.

2. The alternator

  • Rep1: Flask.
  • Rep 2: Diode support.
  • Rep 3: Rectifier diode.
  • Rep 4: Excitation diode.
  • Item 5: Brushes + incorporated voltage regulator.
  • Rep 6: Stator (Armature).
  • Rep 7: Rotor (Inductor).
  • Rep 8: Cooling fan.
  • Rep 9: Drive pulley.

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3. Charging principle

1 alternateur
  • Rep 1: Battery.
  • Rep 2: Contact Neiman.
  • Rep 8: Armature (Stator).
  • Rep 9: Inductor (Rotor).
  • Rep 10: Collector.
  • Rep 11: Brushes or carbons fed in positive.
  • Rep 12: Brushes or coals connected to ground.  


Inductor : Windings of the rotor (moving part of the alternator). When the rotor windings are energized, they create a magnetic field (magnetization).

Armature : Stator windings (fixed part of the alternator). When the windings are subjected to a magnetic field, they create a current.

Collector : Part of the rotor which enables the inductor windings to be supplied one after the other.

Brooms : Also called carbon brushes. They ensure the passage of current through the collector.

The operation of the alternator is based on the property of self induction phenomena.

Some notions :

When a current passes through a coil, the coil creates a magnetic field and behaves like a magnet.

When a magnet is moved in front of a coil, it creates positive and negative pulses. (Alternating current)

The alternator has 3 connections:

  • B+: (+) battery.
  • D+: (+) after contact.
  • A mass.

The bridge rectifier and the voltage regulator are an integral part of the alternator.

By supplying the field of the alternator, a magnetic field appears in the windings of the rotor and polarizes it. (Creation of a magnet). The magnet by rotating close to the stator windings creates an alternating current which must be rectified to recharge the battery.

4. Alternator operation

2 alternateur

In Fig. B we find:

  • Rep1: Battery.
  • Rep 2: Contact Neiman.
  • Charging indicator.
  • Voltage Regulator.
  • Broom.
  • Wound Rotor (Inductor)
  • Three-phase wound stator: star connection (Armature).
  • Diode bridge.
  • W: Output for revolution counter.
  • C: capacitor.


  1. Priming function (Fig. B)

The combustion engine is stopped and the alternator is not turning.

By actuating the contact of the Neiman, the + 12 V is put in relation with the indicator of load, the output of the indicator of load feeds in series the winding of the rotor by terminal D+, the voltage regulator and a brush. The other end of the rotor is grounded through the voltage regulator and the other brush.

It can be seen that the resistance of the bulb is much greater than that of the rotor.

The bulb lights up, the voltage difference across the bulb terminals is 10 Volts and the voltage difference across the rotor terminals is 2 Volts. The voltage of 2 V makes it possible to create the magnetization of the rotor.

3 alternateur
  1. Generator (Fig. C)

When the heat engine starts, the rotor powered by a low voltage behaves like a magnet and rotates close to the stator coils. The current induced in the stator is alternately directed to the bridge rectifier

4 alternateur
  1. Straightening (Fig. D)

The rectifier bridge has 9 diodes  :

  • 3 positive diodes.
  • 3 negative diodes.
  • 3 boot diodes.

The positive alternations of the stator are directed thanks to the 3 positive diodes of the rectifier bridge towards the B+ battery.

The negative alternations of the stator are directed from the ground to the windings thanks to the negative diodes of the rectifier bridge.

In Fig. B, the positive alternation is directed in L2 and goes to B+ of the battery by a positive diode.

The negative alternation directs the mass in L1 by a negative diode.

The positive alternation is also redirected to D+ of the voltage regulator.

Note that the charging voltage increases very rapidly. The charging voltage is reduced to D+, the charging indicator goes out since the voltage difference across its terminals is 0 V. On the other hand, the system tends to get carried away, the voltage on the rotor winding is higher , so the magnet is stronger and the current induced in the stator increases. The voltage regulator makes it possible to limit the voltage to 14.5 V by making cuts on the excitation.


6 alternateur
  1. Current smoothing (Fig. E)

The diode bridge makes it possible to rectify the voltage by suppressing the negative alternations. The current is not completely continuous. A capacitor installed between B+ and ground smooths the voltage.

5 alternateur
  1. Voltage Regulator (Fig. F & G)

In order to regulate the tension, it is necessary to carry out micro cuts on the excitation of the rotor to decrease “the power of the magnet”.

The voltage regulator comprises transistors which allow fast cuts and a zener diode which has the particularity of being conductive in the opposite direction when it reaches a breakdown voltage of 14.5 V.

We find :

  • DZ: zener diode.
  • PNP 1: PNP transistor (drives the closing of the 2nd transistor).
  • PNP 2: PNP transistor (powers the rotor).
  • R1 & R2: Limiting resistors (limits overcurrents in the transistors).
  • D1: Free-wheeling diode (protects the transistors from choke effects).

Reminder of PNP transistor operation:

When the base is grounded and the emitter is powered by a positive (+), the current flow is from the emitter to the collector.

In Fig. F, the rotor is powered by the PNP transistor 2. The zener diode remains closed because the voltage is lower than the breakdown voltage (14.5V).

In Fig. G, the voltage reaches the breakdown value of the zener diode. Diode DZ becomes conductive and connects PNP transistor 1 to ground, which activates. When activated, a positive (+) is brought to the base of PNP transistor 2 which closes the passage between E2 and C2. The rotor is no longer powered, which allows the magnetization to decrease and lower the output voltage of the alternator.

The current induced in the windings of the rotor creates a large reverse voltage in the electrical circuit when the excitation stops. This phenomenon is called self-effect.

Diode D1 is a free-wheeling diode installed to protect transistors from electric arcs.


5- Malfunction





No load

(Battery discharge)

The charging light comes on

The alternator does not turn

broken belt

replace it

Patina belt

Tender her

Seized bearing

Replace it

The alternator is spinning

Disconnected electrical wires

Check and reconnect

Bad contact, oxidation

Check voltage drops and eliminate them

Short circuit in the charging circuit or in the vehicle circuit

Check, isolate

The current does not arrive at the D+ terminal

Check the connections, fuses, contact, etc.

The current arrives at the D+ terminal

Check alternator output, brush wear, regulator, rectifier diodes, commutator, armature, inductor

Excess load

Excessive electrolyte bubbling

Excessive light intensity

The charging light comes on

Regulation circuit

Check charging voltage

Check regulator ground connection.

Voltage regulator change.



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