The battery, and the rest of the electrical system,
must be protected from excessive voltages. To prevent
early battery and electrical system failure, regulation
of the charging system is very important. Also the charging
system must supply enough current to run the vehicle's
electrical accessories when the engine is running.
not require current limiters, because they limit their
own current output. Current limit is the result of the
constantly changing magnetic field because of the induced
AC current. As the magnetic field changes, an opposing
current is induced in the stator windings. The inductive
reactance in the alternator limits the maximum current
that the alternator can produce. Even though current
(amperage) is limited by its operation, voltage is not.
The alternator is capable of producing as high as 250
volts, if it were not controlled.
voltage is done by varying the amount of field current
flowing through the rotor. The higher the field current,
the higher the output voltage. By controlling the amount
of resistance in series with the field coil, control
of the field current and the alternator output is obtained.
To insure a full battery charge, and operation of accessories,
most regulators are set for a system voltage between
13.5 and 14.5 volts.
If sensing voltage
is below the regulator setting, an increase in charging
output results by increasing field current. Higher sensing
voltage will result in a decrease in field current and
system output. A vehicle being driven with no accessories
on and a fully charged battery will have a high sensing
voltage. The regulator will reduce the charging voltage,
and current, until it is at a level to run the ignition
system while trickle charging the battery (2 to 4 amperes).
If a heavy load is turned on (such as the headlights)
the additional draw will cause a drop in the battery
voltage. The regulator will sense this low system voltage
and will reduce the field circuit resistance. This will
allow more current to the field windings. With the increase
of field current, the magnetic field is stronger and
alternator output is increased. When the load is turned
off, the regulator senses the rise in system voltage
and cuts back the amount of field current and ultimately
that affects regulation is temperature. Because ambient
temperatures influence the rate of charge that a battery
can accept, regulators are temperature compensated.
Temperature compensation is required because the battery
is more reluctant to accept a charge at lower ambient
temperatures. The regulator will increase the system
voltage until it is at a high enough level that the
battery will accept it.
To properly test
and service the charging system, it is important to
identify the field circuit being used. Automobile manufacturers
use three basic types of field circuits. The first type
is called the A circuit. It has the regulator on the
ground side of the field coil. The B+ for the field
coil is picked up from inside the alternator. By placing
the regulator on the ground side of the field coil,
the variable resistance will allow the control of field
current by varying the current flow to ground. A resistance
can be located anywhere in the series circuit and have
the same effect.
The second type
of field circuit is called the B circuit. In this case,
the voltage regulator controls the power side of the
field circuit. Also the field coil is grounded from
inside the alternator.
The third type
of field circuit is called the isolated field. The alternator
has two field wires attached to the outside of the case.
The voltage regulator can be located on either the ground
(A circuit) or on the B+ (B circuit) side.
There are two basic types of regulators: electromechanical
and electronic. Also, on many newer model vehicles regulation
is controlled by the computer. Even though the electromechanical
regulator is obsolete, a study of its operation will
help you to understand the more complex systems.
The external electromechanical regulator
is a vibrating contact point design. The regulator uses
electromagnetics to control the opening and closing
of the contact points. Inside the regulator are two
coils. One coil is the field relay and the second is
the voltage regulator. The field relay coil and contact
with no current flowing through the coil are shown.
The contact points are open, preventing current flow.
An electromagnetic field develops when current flows
through the field relay coil. It pulls the contact arm
down. Once the contact points close, current will flow.
The voltage regulator coil uses an electromagnetic
field to open the contact points. With the points closed,
current flows from the battery to the rotor. Current
also flows to the regulator coil. As the battery charges,
the battery's voltage increases. This increase in voltage
strengthens the coil's attraction for the contact points.
At a preset voltage level, the coil will overcome the
contact point spring tension and open the points. This
will prevent current from flowing to the rotor. Once
the points open, voltage output of the alternator drops,
and the battery will start to discharge. As the battery
voltage decreases, so does the regulator coil's magnetic
strength. Spring tension will overcome the magnetic
attraction and the points will close again. Once again
current flows through the rotor and the alternator is
producing voltage. This action occurs several times
When the ignition switch is in the RUN
position with the engine off, the regulator will be
in the position shown. With the ignition switch in RUN,
current flow for the field will go through the ignition
switch through the resistor and bulb, through the lower
contacts of the voltage regulator (closed), and out
of the F terminal to the alternator, through the field
coil, and to ground. The indicator lamp bulb will light
because the bulb is in series with the field coil.
Once the engine is started, the alternator
will begin to produce voltage. At this time the system
voltage may be below 13.5 volts, however, because the
alternator rotor is revolving, there is some production
of voltage. This allows voltage out of the R terminal
to energize the field relay coil. With the relay coil
energized, the contact points close and direct battery
voltage flows through terminal 3 and out terminal 4.
The bulb will go out with battery voltage on both sides
of the bulb. Simultaneously, battery voltage flows through
the voltage regulator contact points to the field coil
in the alternator. Because the voltage regulator coil
is also connected to the battery (above terminal 4),
the lower than 13.5 volts is unable to produce a sufficient
electromagnetic field to pull the contact points open.
This condition will allow maximum alternator output.
As the battery receives a charge from
the alternator, the battery voltage will increase to
over 13.5 volts. The increased voltage will strengthen
the electromagnetic field of the voltage regulator coil.
The coil will attract the points and cause the lower
contacts to open. Current will now flow through the
resistor and out terminal F. Because an additional series
resistance is added to the field coil circuit of the
alternator, field current is reduced, and alternator
output is also reduced to one half its rating.
If the alternator is producing more voltage
than the system requires, both battery and system voltage
will increase. As this voltage increases to a level
above 14.5 volts, the magnetic strength of the coil
increases. This increase in magnetic strength will close
the top set of contact points and apply a ground to
the F terminal and the alternator's field coil. Both
sides of the field coil are grounded, thus no current
will flow through it and there is no output.
Once the engine is shut off, there is
no current from the R terminal; the field relay coil
is deenergized, allowing the points to open. This prevents
battery draw into the charging system.
The second type of regulator is the electronic regulator.
Electronic regulators can be mounted either externally
or internally of the alternator. There are no moving
parts, so it can cycle between 10 and 7,000 times per
second. This quick cycling provides more accurate control
of the field current through the rotor. Electronic regulation
control is through the ground side of the field current
Pulse width modulation controls alternator
output by varying the amount of time the field coil
is energized. For example, assume that a vehicle is
equipped with a 100-ampere alternator. If the electrical
demand placed on the charging system requires 50 amperes
of current, the regulator would energize the field coil
for 50% of the time. If the electrical system's demand
was increased to 75 amperes, the regulator would energize
the field coil 75% of the cycle time.
The electronic regulator uses a zener
diode that blocks current flow until a specific voltage
is obtained, at which point it allows the current to
flow. A simplified electronic regulator is shown.
Alternator voltage from the stator and
diodes first goes through a thermistor. Current then
flows to the zener diode. When the upper voltage limit
(14.5 volts) is reached, the zener diode will conduct
current to flow to the base of transistor 1. This turns
transistor 1 on and switches off transistor 2. Transistor
2 controls field current to the alternator. If transistor
2 is off, no current can flow through the field coil
and the alternator will not have any output. When no
voltage is applied to the zener diode, current flow
stops, transistor 1 is turned off, transistor 2 is turned
on, and the field circuit is closed. The magnetic field
is restored in the rotor, and the alternator produces
Many manufacturers are installing the
voltage regulator internally in the alternator. This
eliminates some of the wiring needed for external regulators.
The diode trio rectifies AC current from the stator
to DC current that is applied to the field windings.
Current flow with the engine off and the
ignition switch in the RUN position is illustrated.
Battery voltage is applied to the field through the
common point above R1. TR1 conducts the field current
coming from the field coil, producing a weak magnetic
field. The indicator lamp lights because TR1 directs
current to ground and completes the lamp circuit.
Current flow with the engine running is
illustrated. When the alternator starts to produce voltage,
the diode trio will conduct and battery voltage is available
for the field and terminal 1 at the common connection.
Placing voltage on both sides of the lamp removes any
voltage potential, and the lamp goes out.
Current flow as alternator output is being
regulated is illustrated. The sensing circuit from terminal
2 passes through a thermistor to the zener diode (D2).
When the system voltage reaches the upper voltage limit
of the zener diode, the zener diode conducts current
to TR2. When TR2 is biased it opens the field coil circuit
and current stops flowing through the field coil. Regulation
of this switching on and off is based on the sensing
voltage received through terminal 2. With the circuit
to the field coil opened, the sensing voltage decreases
and the zener diode stops conducting. TR2 is turned
off and the circuit for the field coil is closed.
On many vehicles after the mid-1980s,
the regulator function has been incorporated into the
vehicle's engine computer. The operation is the same
as the internal electronic regulator. Regulation of
the field circuit is through the ground (A circuit).
The logic board's decisions, concerning
voltage regulation, are based on output voltages and
battery temperature. When the desired alternator output
voltage is obtained (based on battery temperature) the
logic board duty-cycles a switching transistor. This
transistor grounds the alternator's field to control
General Motors introduced an alternator
called the CS (charging system) series. This alternator
is smaller than previous designs. Additional features
include two cooling fans (one external and one internal),
and terminals designed to permit connections to an onboard
computer through terminals L and F. The voltage regulator
switches the field current on and off at a fixed frequency
of about 400 times per second. Varying the on and off
time of the field current controls voltage output.