Electrical System Regulation

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.

Alternators do 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.

Regulation of 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 alternator output.

Another input 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.

Field Circuits

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.

Electromechanical Regulators

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 per second.

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.

Electronic Regulators

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 (A circuit).

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 output voltage.

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.

Computer-Controlled Regulation

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 output voltage.

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.