Working Principles of the Voltage Regulator

Updated: Feb 27

By Norman Nock

Originally published in the Austin-Healey Magazine, June 1994


The little black box called the voltage regulator (or "control box") is mounted to the passenger side firewall in the engine compartment. The factory service manual should be carefully followed if you contemplate any mechanical adjustment to the regulator.


The Cut-Out


Figure 1 shows the function of the cut-out circuit. When the output voltage from the generator rises, the magnetic field of the shunt coil overcomes the spring to close the cut-out points. This action joins the battery to the generator armature. The total output from the generator now will pass through the cut-out contacts and through the heavy series winding on the cut-out bobbin. This current will hold the contacts closed and stop them from chattering.


If these contacts are inadvertently closed when the engine is not running, they can be held closed by the battery current flowing back through the series winding. In this case, since the generator is not running and is not producing any electricity, the battery will try to motorize the generator (make the armature spin). However, since the fan belt will not allow the armature to turn, the armature windings will start to burn and let out smoke. If this happens the armature will have one of its windings burned black.


Voltage Regulation


There was a time when all you had on a car was a cut-out to join the battery to the generator. To avoid overcharging the battery you had to turn on your head lights. Inside the car was a switch for high or low charge (winter or summer) and a special third brush in the generator to be adjusted.


Then Mr. Lucas came along and made it easy. He developed the regulator. With the cut-out now closed, the electricity flows through the cut-out contacts, through two series windings, out the regulator at terminal A, and into the battery. Voltage control is achieved by opening and closing the regulator contacts and inserting a resistance in the field circuit. This will automatically lower the generator output to the point where the regulator contacts close and the output goes up again. This quick movement of the contacts gives voltage control over the range of 15.8 to 16.4 volts (open circuit).


The heavy series winding on the regulator also helps to lower the output of the generator to avoid damage. The second series winding on the regulator to terminal A1 is to lower the voltage output from the generator when the battery is too low (or the lights are turned on) and there is the possibility of a line voltage drop (referred to as "load turns") If too many amps (load) are taken from the generator, the armature will overheat and cause solder to be thrown from the commutator area.


Figure 2: To keep the battery from overcharging and the current levels in the field windings of the generator from rising too high, the output line (F Terminal) is controlled by the regulator circuit. When levels in the regulator-shunt coil rise to an unacceptable level, the regulator points open and the generator output is dumped into a resistance circuit. When generator output drops, the points close and power once again flow shack to the generator through terminal F.


One other thought: We have found that on many "look alike" Lucas regulators, cut-out contacts are not adjusted correctly causing an intermittent "no charge" after being in service for only a few thousand miles. Try to use original Lucas equipment. The “look-alikes" are poorly made and lack quality—a problem common to most look-alike Lucas parts.


As a final note, voltage regulators used on all Healeys except BJ8s use screw terminals. Many owners who replace their wiring harnesses solder the ends of the wires that go into these (and other) terminals. This is a fallacy. If the end of the wire is soldered, it will eventually come loose, because the soldered wires can't be compressed. If the wire ends are not soldered as it has always been done on new cars, the wires will compress into a small area and not come loose.


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