Motor vehicle electrical system

A multivoltage electrical system for a motor vehicle that has a pair of storage batteries which, in a first charging mode are charged in series from the direct voltage output terminals of a bridge rectifier connected to an alternating current generator. In a second charging mode, only one of the batteries is charged and this takes place when a plurality of controlled rectifiers connecting the generator and a common junction of the batteries are gated conductive. The switching between modes is a function of the voltage of one of the batteries and the voltage of a triangular voltage waveform generator operating at a fixed frequency. The system is maintained in the second mode when generator speed is below a predetermined value.

This invention relates to a multivoltage motor vehicle electrical system 
and more particularly to a motor vehicle electrical system that is capable 
of providing first and second load supply voltages referenced to ground 
and a third voltage corresponding to the sum of the first and second 
voltages. 
Battery charging systems for motor vehicles that have two series connected 
batteries are known in the prior art, one example being the system 
disclosed in the U.S. Pat. No. 3,624,480 to Campbell et al. In that patent 
two batteries are connected in series and in one charging mode, the two 
batteries are charged in series from the direct voltage output terminals 
of a bridge rectifier that is energized by the polyphase output winding of 
an alternating current generator. In another charging mode only one of the 
two batteries is charged and this is accomplished by gating a plurality of 
controlled rectifiers conductive that are connected between the polyphase 
output winding of the generator and a common junction of the two 
batteries. 
One of the differences between the system of this invention and that 
disclosed in the above-referenced Campbell et al. patent is the manner in 
which the system switches from one charging mode to the other. In the 
Campbell et al. patent the system is switched to a mode where only one 
battery is charged whenever the voltage of the other battery exceeds a 
predetermined value. In accordance with one aspect of this invention the 
switching between modes takes place at a fixed frequency with the time 
periods of the modes being variable. This is accomplished by comparing the 
output voltage of a fixed frequency ramp voltage generator with a voltage 
that is a function of the voltage across one of the batteries. It 
accordingly is an object of this invention to provide a battery charging 
system in which two batteries are charged in series in one charging mode 
and only one battery is charged in another charging mode and wherein the 
switching between modes occurs at a constant frequency and with variable 
time periods for the respective charging modes. 
Another object of this invention is to provide a battery charging system 
wherein two batteries are charged in series in first mode of operation and 
only one of the two batteries is charged in a second mode of operation and 
wherein the system is maintained in the second mode until the speed of the 
generator that supplies charging current to the batteries exceeds a 
predetermined value. 
Another object of this invention is to provide a battery charging system of 
the type described wherein the two batteries are connected to a common 
junction that is at the electrical ground potential of the motor vehicle 
electrical system. With this arrangement the system is capable of 
providing a positive voltage which is referenced to ground, a negative 
voltage which is referenced to ground and a third voltage which 
corresponds substantially to the sum of the first and second voltages. By 
way of example, where two 12 volt batteries are utilized the system will 
provide a positive 12 volts referenced to ground, a negative 12 volts 
referenced to ground and 24 volts across both batteries. 
Still another object of this invention is to provide a battery charging 
system of the type described which includes a voltage regulator connected 
to the field winding of the generator to control field current and 
operating to sense the voltage across one of the two batteries of the 
system. The system is arranged such that a plurality of diodes are 
utilized to develop a field energizing voltage and is further arranged 
such that one of the batteries is capable of supplying field current to 
the field winding through a diode so that sufficient field current is 
available when the system switches between a mode wherein only one battery 
is charged to a mode wherein the two batteries are charged in series.

Referring now to the drawings and more particularly to FIG. 1, the 
reference numeral 10 generally designates an alternating current generator 
which has a field winding 14 and a three phase Delta connected output 
winding 16. The phase terminals 17, 19 and 21 of the output winding 16 are 
respectively connected to the AC input terminals 18, 20 and 22 of a three 
phase full-wave bridge rectifier circuit comprised of three positive 
diodes 24 and three negative diodes 26. The cathodes of diodes 24 are 
connected together and to a positive direct voltage output terminal 28. 
The anodes of the diodes 26 are connected together and to another negative 
direct output terminal 30 of the bridge rectifier. 
The field winding 14 of the alternating current generator is carried by the 
rotor of the generator in a manner well known to those skilled in the art 
and the rotor is driven by the engine 31 of the vehicle, as illustrated by 
the dotted line in FIG. 1. The drive between the engine 31 and the 
generator 10 is by way of a belt and pulleys and generator speed varies 
with engine speed as is well known to those skilled in the art. One end of 
the field winding 14 is connected to a junction 32 which in turn is 
connected to the cathodes of three diodes 34. The anodes of the diodes 34 
are connected respectively with the AC input terminals of the bridge 
rectifier and therefore to the phase terminals of the three phase winding 
16. The opposite end of the field winding 14 is illustrated as being 
connected to the collector of an NPN transistor 36, the emitter of which 
is grounded, as illustrated. The transistor 36 forms part of a voltage 
regulator generally designated by reference numeral 38 and including 
voltage regulator circuitry shown as a block and designated by reference 
numeral 40. The voltage regulator is of a known construction and can be of 
the type, for example disclosed in the U.S. Pat. No. 3,597,654 to Harland 
et al. 
As disclosed in the above-referenced Harland et al. patent, the transistor 
36 can be comprised of a pair of Darlington connected transistors which 
switch on and off to control field current. The circuit for energizing the 
field 14 is from junction 32, through field winding 14 and through the 
collector-emitter circuit of the switching transistor 36 to ground. 
The circuitry within the block 40 is coupled to the base of transistor 36 
to cause this transistor to switch on and off in accordance with the 
voltage sensed by the voltage regulator. The voltage regulator has a 
voltage divider of the type described in the above-referenced Harland et 
al. patent that is connected between conductor 42 and grounded conductor 
44. The voltage regulator circuitry further is coupled to junction 32 by 
conductor 45 to provide an input voltage to the circuitry contained within 
block 40. 
The direct voltage output terminal 28 is connected to a conductor 46 which 
in turn is connected to junction 48. The junction 48 is connected to the 
positive terminal of a 12 volt storage battery designated by reference 
numeral 50. The negative side of the battery 50 is connected to junction 
52 and this junction is grounded as illustrated. The positive terminal of 
another 12 volt storage battery 54 is connected to junction 52 and the 
negative terminal of battery 54 is connected with junction 56. The 
junction 56 is connected to the negative direct voltage output terminal 30 
of the bridge rectifier via a conductor 58. 
The reference numeral 60 designates a 12 volt motor vehicle electrical 
load. When switch 62 is closed the electrical load 60 is connected with 
junctions 48 and 52 and therefore across battery 50. The reference numeral 
64 designates another 12 volt motor vehicle electrical load and when 
switch 66 is closed the load 64 is connected to junctions 52 and 56 and 
therefore across the battery 54. The reference numeral 68 designates a 24 
volt electrical load and when switch 70 is closed the 24 volt load 68 is 
connected to junctions 48 and 56 and therefore across batteries 50 and 54. 
It will be appreciated that the circuit arrangement that has been 
described provides a positive 12 volts referenced to ground via battery 50 
and a negative 12 volts referenced to ground via battery 54. The system 
further provides 24 volts to the 24 volt load 68. 
The system of FIG. 1 has three silicon controlled rectifiers, each 
designated by reference numeral 72. The anodes of these controlled 
rectifiers are all connected to a conductor 74 which is grounded. The 
cathodes of controlled rectifier 72 are connected respectively to the AC 
input terminals 18, 20 and 22 of the bridge rectifier and hence to the 
phase terminals 17, 19 and 21 of generator output winding 16. The cathode 
of one of the controlled rectifiers is connected to a junction 76 which in 
turn is connected to a conductor 78. The conductor 78 is connected to a 
circuit shown as block 80 which is a speed trip and generator control lamp 
circuit that will be described in detail hereinafter. 
The gates of the controlled rectifiers 72 are connected to a controlled 
rectifier gate driver circuit 82. The gate driver circuit 82 is connected 
to a mode switching control circuit 84 and to the speed trip and generator 
lamp control circuit 80. A generator charge indicator or warning lamp 85 
is connected between the circuit 80 and a conductor 86. The conductor 86 
is connected to the anode of a diode 88, the cathode of which is connected 
to junction 32. 
The system of FIG. 1 includes a pair of switches 90 and 92 which are closed 
whenever the ignition switch on the motor vehicle is in a position to 
energize the engine ignition system. One side of the switch 90 is 
connected to conductor 86 and the opposite side of this switch is 
connected to conductor 46 and junction 48. One side of the switch 92 is 
connected to conductor 94 and the opposite side of this switch is 
connected to junction 56 via conductor 96. 
The battery charging system illustrated in FIG. 1 is capable of charging 
the two 12 volt batteries 50 and 54 in series which will hereinafter be 
called the 24 volt charging mode. The electrical system is further capable 
of charging only the 12 volt battery 50 which will hereinafter be called 
the 12 volt charging mode. In the 12 volt charging mode the controlled 
rectifiers 72 are conductive. When controlled rectifiers 72 are 
conductive, anode to cathode, the 12 volt battery 50 can be charged from a 
circuit that can be traced from direct voltage output terminal 28, through 
conductor 46 to junction 48, through battery 50 to grounded junction 52 
and then via ground to conductor 74 and the anode-cathode circuits of 
controlled rectifiers 72. A simplified equivalent circuit for the 12 volt 
charging mode is illustrated in FIG. 2. Thus, as shown in FIG. 2, when 
controlled rectifiers 72 are gated conductive the conductor 58 is 
effectively opened. During this 12 volt charging mode, and assuming 
continuous duty electrical loads, the battery 50 is being charged and 
power is supplied to the loads as shown by currents depicted by arrows in 
FIG. 2. The current identified as I.sub.12 is the current being supplied 
to the 12 volt load 60 and the current identified as I.sub.24 is the 
current being supplied to the 24 volt load 68. Current I.sub.24 is in a 
direction to discharge the battery 54. 
The system operates in the 24 volt charging mode when the controlled 
rectifiers 72 are not conducting. The simplified equivalent circuit, for 
this mode of operation, is depicted in FIG. 3 where the arrows again 
indicate current flow. In the 24 volt charging mode the circuit between 
conductor 74 and grounded junction 52 is effectively opened since 
controlled rectifiers 72 are not conductive. During this mode of operation 
the batteries 50 and 54 are charged in series from the direct voltage 
output terminals 28 and 30 of the bridge rectifier. The generator supplies 
charging current to battery 50 and supplies load currents identified as 
I.sub.12 and I.sub.24. In this mode battery 54 is charged at a rate equal 
to the sum of current I.sub.12 plus the current supplied to battery 50. 
As will be more fully described hereinafter, the system is switched between 
the 12 and 24 volt charging modes by applying or removing gate drive 
signals to the gates of the controlled rectifiers 72. 
Referring now more particularly to FIG. 4, a circuit diagram is illustrated 
which illustrates in detail specific circuitry for the blocks illustrated 
in FIG. 1. In FIG. 4 the same reference numerals have been utilized as 
were utilized in FIG. 1 to identify corresponding circuit elements. In 
FIG. 4 the reference numeral 82 again designates the controlled rectifier 
gate driver circuit. This circuit includes a PNP transistor 100 having an 
emitter connected to conductor 86 and a collector connected to conductor 
102. The conductor 102 is connected to the anodes of three diodes, each 
designated by reference numeral 104. The cathodes of the diodes 104 are 
connected respectively to the gate electrodes of controlled rectifiers 72 
via resistors 106. The base of transistor 100 is connected to a conductor 
107 and this conductor is connected to conductors 108 and 110. A resistor 
is connected across the emitter and base electrodes of transistor 100 as 
illustrated. Whenever the transistor 100 is biased conductive in its 
emitter-collector circuit the direct voltage on conductor 86 is applied to 
each gate electrode of the controlled rectifiers 72 to cause these 
controlled rectifiers to be gated conductive. The transistor 100 will be 
biased conductive when the voltage on conductor 108 or conductor 110 drops 
to a value permitting sufficient base current to flow to bias the 
transistor 100 conductive. The voltage on conductor 108 is controlled by 
the speed trip circuit 80 which in its detailed FIG. 4 form is designated 
by reference numeral 80B. The voltage on conductor 110 is controlled by 
the mode switching control circuit 84 and the specific form of this 
circuit is designated by reference numeral 84A in FIG. 4. The mode 
switching control circuit 84 can take two forms. In FIG. 4 the mode 
switching control circuit, as designated by reference numeral 84A, is a 
fixed frequency variable pulse width circuit. The mode switching control 
84 can alternatively be a load determined load switching control which is 
illustrated in FIG. 5 and generally designated as 84B. The circuit of FIG. 
4 will be described first as including the fixed frequency mode switching 
control 84A. 
The mode control 84A comprises a quad operational amplifier comprised of 
four sections designated respectively B1, B2, B3 and B4. This device may 
be a National Semiconductor LM-124 quad operational amplifier or 
equivalent. In FIG. 4 reference numerals adjacent the sections of the 
operational amplifiers designate the terminals of the respective 
amplifiers when utilizing the LM-124 device. In regard to the power supply 
for these operational amplifier sections, the terminals 4 and 11 of 
amplifier B4 are shown connected respectively to grounded conductor 112 
and conductor 94. The remainder of the power supply connections for 
amplifiers B1, B2 and B3 have not been illustrated but would be connected 
the same as the connection for amplifier section B4. 
The negative terminal of amplifier B1 is connected to a junction 114 
located between potentiometer resistor 116 and a resistor 118. A diode 120 
is connected between resistor 118 and conductor 112. The conductor 112 is 
connected to a grounded conductor 121. The positive terminal of amplifier 
B1 is connected to conductor 112 via resistor 126. A Zener diode 128 is 
connected in series with resistor 126 and between the positive terminal of 
amplifier B1 and conductor 94. The output of amplifier B1 is connected to 
the negative terminal of amplifier B4 via a conductor 130. The positive 
terminal of amplifier B4 is connected to the output of amplifier B3 and 
the output of amplifier B4 is connected to a junction 132. The junction 
132 is connected via a resistor to the base of an NPN transistor 134. The 
emitter of this transistor is connected to conductor 94 and the collector 
of this transistor is connected to conductor 110 via resistor 136. The 
amplifiers B2 and B3 and associated circuitry connected thereto, including 
the NPN transistor 138, form a ramp voltage generator which generates a 
ramp voltage of constant frequency designated as V.sub.REF in FIG. 6A. 
This ramp voltage is applied to the positive terminal of amplifier B4 and 
is compared to the voltage applied to the negative terminal of amplifier 
B4. The voltage applied to the negative terminal of amplifier B4 is a 
conditioned and filtered representation of the voltage across battery 54 
and this is accomplished by the circuitry connected to battery 54 
including amplifier B1 and capacitor 137. The conditioned voltage applied 
to the positive terminal of amplifier B4 will vary as the voltage across 
battery 54 varies and, as an aid in explaining the operation of this 
invention, two conditioned voltage levels V.sub.1 and V.sub.2 that 
represent the voltage of battery 54 are illustrated in FIG. 6A. These are 
only two of many conditioned voltage levels that may exist, dependent upon 
the voltage of battery 54. If it is assumed that the voltage across 
battery 54 is of such a magnitude as to produce conditioned voltage 
V.sub.1 the triangular voltage waveform produced by the ramp voltage 
generator (B2 and B3) and applied to terminal 10 of B4 will exceed V.sub.1 
at time periods identified as T.sub.1 and T.sub.2 in FIG. 6A. Thus, during 
the time period from T.sub.1 to T.sub.2 the triangular voltage applied to 
the positive terminal of amplifier B4 exceeds the reference voltage 
V.sub.1 applied to the negative terminal of amplifier B4 with the result 
that the output on amplifier B4 biases NPN transistor 134 conductive. With 
transistor 134 conductive base current for transistor 100 can flow through 
the collector-emitter circuit of transistor 134 with the result that 
transistor 100 is biased conductive to thereby gate controlled rectifiers 
72 conductive. The gate drive signal, under this condition of operation 
that is applied to the gates of controlled rectifiers 72, is illustrated 
in FIG. 6B. Thus, the cross hatched square waves also identified as on are 
indicative of the gate drive signal applied to controlled rectifiers 72 
and are also indicative of the time period that the system is operating in 
the 12 volt charging mode, that is where only battery 50 is being charged. 
The off periods, shown in FIG. 6B, correspond to the periods of time in 
which the system is operating in the 24 volt charging mode and in FIG. 6B 
it has been assumed that current I.sub.12 is greater than current I.sub.24 
as they are depicted in FIGS. 2 and 3. 
If it is assumed that the voltage across battery 54 produces conditioned 
voltage V.sub.2, which is applied to the negative terminal of amplifier 
B4, and that current I.sub.12 is less than current I.sub.24 the gate drive 
signal to the controlled rectifiers 72 will be as depicted in FIG. 6C 
where again the cross hatched square waves, designated as on, are the 
periods of time that the controlled rectifiers are gated conductive and 
also the periods of time that the system is operating in the 12 volt 
charging mode. It can be seen, from a comparison of FIGS. 6B and 6C, that 
the time periods that the system is operating in the 12 volt charging mode 
is decreased in FIG. 6C from the corresponding time periods of FIG. 6B. 
The system thus provides a constant frequency switching system for 
switching between the 12 and 24 volt charging modes but with variable time 
periods for the respective modes as depicted in FIGS. 6B and 6C. 
The generator lamp circuit 80A and the speed trip circuit 80B, illustrated 
in FIG. 4, use sections of a National Semiconductor LM-139 quad comparator 
or equivalent. These sections are designated respectively as A1, A2, A3 
and A4. The reference numerals adjacent the comparator sections designate 
terminals for an LM-139 quad comparator. The power supply for the section 
A3 is achieved by connecting terminal 3 to conductor 86 via conductor 140. 
Terminal 12 of section A3 is connected to conductor 124 by conductor 141. 
The conductor 124 is grounded since it is connected to conductor 121. It 
is to be understood that the other sections of the quad comparator are 
similarly connected in regard to power supply. 
The purpose of the speed trip circuit 80B is to cause the system to operate 
in the 12 volt charging mode to charge only battery 50 whenever generator 
rotor speed is below some predetermined value. Thus, at low engine speeds, 
particularly at idle, the generator 10 cannot supply as much current in 
the 24 volt mode as it can in the 12 volt mode. It is preferable to leave 
the generator in the 12 volt mode at low engine speeds in order to 
effectively utilize the output that the generator is capable of developing 
at low speeds. 
The positive terminal of comparator section A2, of the speed trip circuit 
80B, is connected to a junction 142 located between resistors 144 and 146. 
These resistors are connected in series with a diode 148 which in turn is 
connected in series with conductor 78. A junction 150 connected to 
conductor 78 feeds the generator lamp circuit 80A which will be described 
in detail hereinafter. The voltage applied to conductor 78 is the voltage 
at the AC input terminal 18 of the bridge rectifier and is an alternating 
voltage, the frequency of which is a function of generator speed and hence 
vehicle engine speed. This voltage is applied to the positive terminal of 
the comparator A2 which develops a square wave voltage at its output. The 
output of A2 is differentiated by capacitor 151 and resistor 153 to obtain 
a constant pulse width voltage which is then amplified and squared by 
section A3 of the comparator. This output voltage of A3 is applied to 
capacitor 152 by the circuit that includes diode 154. The capacitor 152 is 
charged by the output voltage pulses of comparator section A3 and the 
voltage on the capacitor is therefore proportional to engine and generator 
speed. This speed voltage is applied to the positive terminal of 
comparator section A4 which operates to compare the voltage on junction 
160 with the generator speed voltage. The voltage at junction 160 is a 
reference voltage developed by a voltage divider comprised of a resistor 
162 and a potentiometer resistor 164. As long as the speed voltage is less 
than the reference voltage on junction 160 the output of comparator 
section A4 will cause the transistor 100 to conduct which causes the 
controlled rectifiers 72 to be gated conductive which in turn places the 
system in the 12 volt charging mode. Thus, as long as engine and generator 
speed are below some value the output of comparator section A4 is low 
providing a path for base current via diode 170 and resistor 172 to bias 
transistor 100 conductive. 
When engine and generator speed exceed some value the output of comparator 
section A4 goes high biasing transistor 100 nonconductive. The transistor 
100, however, after this occurs can be biased conductive when the voltage 
on conductor 110 goes low. The voltage on conductor 110, as previously 
described, is controlled by the mode switching control 84A. 
In summary, the speed trip circuit 80B will always force the system to 
operate in the 12 volt charging mode as long as engine and generator speed 
are below some predetermined value and this will occur regardless of the 
mode that is being selected by the mode switching control 84A. 
By way of example, the speed trip circuit 80B may be tripped to cause the 
system to be controlled by the mode control 84A when generator rotor speed 
reaches 2400 rpm. Once the trip point has been exceeded, the forced 12 
volt mode will not occur again until generator speed decreases to a lower 
speed, for example, 2100 rpm. This hysteresis feature is obtained by the 
use of a feedback resistor 174 that connects the output of comparator 
section A4 to the positive input terminal of this comparator section. The 
purpose of this hysteresis feature is to prevent the output of comparator 
section A4 from turning on and off rapidly, if the generator speed would 
happen to stay at the trip speed of 2400 rpm for a period of time where 
ripple on the speed voltage signal or power source could make the 
comparator section A4 switch at the ripple frequency. 
The purpose of the generator lamp control circuit 80A is to cause the 
generator tell-tale or warning lamp 85 to light in the event that the 
generator and rectifying apparatus are not developing a voltage of 
sufficient magnitude. This circuit includes a conductor 180 connected to 
junction 150 and therefore to terminal 18 of the bridge rectifier via 
conductor 78. Connected between conductor 180 and conductor 124 is a 
capacitor 182 and a resistor 184. The negative terminal of comparator A1 
is connected to a junction 186 located between resistors 188 and 190. A 
capacitor is connected in parallel with these resistors. The resistors 188 
and 190 are connected in series with a diode 192 that is connected to 
junction 194. 
The alternating voltage at junction 18 is coupled to junction 194 by the 
capacitor 182 and is half-wave rectified by diode 192. The magnitude of 
the voltage at junction 186 therefore represents the magnitude of the 
output voltage of the alternating current generator 10. The voltage at 
junction 186 is compared with the voltage at junction 200 which is 
developed by a voltage divider comprised of resistors 202 and 204 
connected across conductors 86 and 124. When the output voltage of the 
generator reaches a normal value, the output of amplifier section A1 
biases the Darlington connected NPN transistors 210 nonconductive. It can 
be seen that the collector-emitter circuit of the Darlington connected 
transistors are connected in series with the lamp 85 via conductors 212 
and 214. The lamp is connected with conductor 86 via conductor 216. Thus, 
when generator voltage is normal Darlington connected resistors 210 are 
biased nonconductive and the generator tell-tale lamp is not lit. 
When generator voltage is abnormally low the voltage at junction 186 is low 
and as a consequence the comparator section A1 biases Darlington connected 
transistors 210 conductive. Accordingly, the lamp 85 is lit, indicative of 
a low or no voltage output of the alternating current generator 10. 
It will be appreciated from the foregoing that the voltage on conductor 78 
performs two functions in the system. First of all, the frequency of this 
voltage is utilized as an indication of generator speed in order to 
control the tripping of the speed trip circuit 80B. The other function of 
the voltage on conductor 78 is to control the energization of signal lamp 
85 and in this regard the amplitude or magnitude of the voltage on 
conductor 78 is utilized for this purpose. 
As previously mentioned, the mode switching control 84 can be of the fixed 
frequency type, shown in FIG. 4, or can be of the so-called load 
determined type. The fixed frequency type of control has already been 
described in connection with FIG. 4 and in FIG. 4 is identified by 
reference numeral 84A. The load determined type of control is illustrated 
in FIG. 5 and identified as 84B. When it is desired to utilize the load 
determined type of control the circuit of FIG. 4 is modified by completely 
eliminating the circuitry identified as 84A and connecting the circuit 84B 
of FIG. 5 to the FIG. 4 circuit in a manner illustrated in FIG. 5. Thus, 
in FIG. 5 the same reference numerals have been utilized as were utilized 
in FIG. 4 in order to illustrate how the FIG. 5 circuit is connected to 
the FIG. 4 circuit, assuming of course that the circuitry 84A has been 
eliminated. 
The load determined circuitry 84B comprises an NPN transistor 220 having an 
emitter connected to a conductor 222. The conductor 222 is connected to 
junction 56 whenever switch 92 is closed. The collector of transistor 220 
is connected with a resistor 224 and this resistor is connected to 
conductor 108. The base of transistor 220 is connected to a junction 226 
via resistor 228. A capacitor 230 and a resistor 232 are connected across 
the base and emitter electrodes of transistor 220. A voltage divider 
comprised of a resistor 234 and a potentiometer resistor 236 is connected 
between conductors 121 and 222 and therefore across battery 54. A Zener 
diode 238 is connected between the base of transistor 220 and the junction 
240 of the voltage divider. 
The operation of the circuit of FIG. 4, assuming that the circuit 84B is 
the mode switching control, will now be described. The circuit 84B 
responds to the magnitude of the voltage at junction 52. Assuming that the 
system is in the 24 volt charging mode the batteries 50 and 54 are charged 
in series and the voltage at junction 52 will increase at the instant the 
24 volt charging mode is initiated. As this voltage increases it will 
eventually attain a value sufficient to cause the Zener diode 238 to 
conduct in a reverse direction to thereby bias transistor 220 conductive. 
When transistor 220 is biased conductive it provides a path for base 
current for transistor 100 thereby causing transistor 100 to conduct. The 
conduction of transistor 100 applies gate signals to the gate electrodes 
of controlled rectifiers 72 causing the controlled rectifiers to conduct 
and placing the system in the 12 volt charging mode. 
With the system in the 12 volt charging mode the voltage at junction 52 
decreases and will decrease to a point in which the Zener diode 238 
resumes its blocking state thereby biasing transistor 220 nonconductive. 
This causes the transistor 100 to be in a nonconductive state thereby 
removing the gate drive to the controlled rectifiers 72 and placing the 
system back into the 24 volt charging mode. The system therefore operates 
to continuously switch between the 12 and 24 volt charging modes in 
response to variation in the voltage of junction 52. 
FIGS. 7 and 8 illustrate waveforms of the gate drive voltage applied to the 
gate electrodes of controlled rectifiers 72 for various load conditions 
when the system of FIG. 5 is utilized. FIG. 7A illustrates a condition in 
which I.sub.12 is greater than I.sub.24 and FIG. 7B illustrates a 
condition in which I.sub.12 is less than I.sub.24 these currents 
corresponding to the currents identified by the arrows shown in FIGS. 2 
and 3. FIG. 8 illustrates relatively heavy total load conditions which 
causes the switching rate to increase. FIG. 8A illustrates a condition in 
which I.sub.12 is greater than I.sub.24 and FIG. 8B illustrates a 
condition in which I.sub.12 is less than I.sub.24. 
The rate at which the voltage builds up and decays across battery 54 and 
accordingly the voltage at junction 52 will determine the basic switching 
frequency of the system. This rate is in turn a function of the 12 and 24 
volt loads. Thus, the switching action is load determined. When the loads 
are very light (FIG. 7) the voltage of battery 54 changes slowly and the 
switching frequency is low. When the loads are heavy (FIG. 8) voltage 
changes rapidly and the switching frequency increases. The switching rate 
may be, for example, typically 5 hertz with the conditions of operation 
illustrated in FIG. 7. During a heavy load condition (FIG. 8) the 
switching frequency may be typically 50 to 100 hertz. 
The operation of the voltage regulator 38, in controlling field current and 
hence system voltages, will now be described. The voltage regulator, as 
previously described, includes a battery voltage sensing circuit connected 
between conductor 42 and ground which is comprised of a voltage divider 
network, as disclosed in the above-referenced Harland et al. patent. This 
voltage sensing circuit senses the voltage across battery 50 and therefore 
develops a voltage which is a function of the voltage across battery 50. 
The voltage regulator is arranged such that when the voltage across 
battery 50 is above some predetermined value the field controlling 
transistor 36 is switched nonconductive, and when the voltage across 
battery 50 drops to some predetermined value the field controlling 
transistor 36 is switched conductive. The magnitude of the battery voltage 
that will cause the transistor 36 to switch nonconductive may be, for 
example, 13.5 volts where the battery 50 has a rated terminal voltage of 
12 volts. Assuming this to be the case, the voltage regulating transistor 
36 would switch off when the voltage across battery 50 exceeds 13.5 volts 
and switches back on when this voltage drops below 13.5 volts. 
It should be noted that even though the voltage regulator 38 responds to, 
for example, a 13.5 volt trip point, the actual output voltage appearing 
between output terminals 28 and 30 of the bridge rectifier may be, for 
example, 27 volts when the system is operating in the so-called 24 volt 
mode, that is, where the bridge rectifier is charging the batteries 50 and 
54 in series. When controlled rectifiers 72 are biased conductive to place 
the system in the 12 volt charging mode, the output voltage appearing 
between junction 28 and the grounded anodes of the controlled rectifiers 
may be, for example, 13.5 volts since the voltage regulator 38 cuts back 
the average field current to provide the 13.5 volts. From the foregoing it 
can be seen that the voltage generated by the output winding 16 is about 
twice as much when the system is operating in the so-called 24 volt mode, 
as compared to the voltage generated in the output winding 16 when the 
system is operating in the so-called 12 volt mode. This is accomplished by 
the voltage regulator 38 which responds only to the voltage across the 
battery 50. 
The purpose of the diode 88 is to ensure adequate field current when the 
system is being switched from the 12 volt charging mode to the 24 volt 
charging mode. The field current for field winding 14 can be supplied via 
a circuit that includes the three diodes 34, the field winding 14, the 
collector-emitter circuit of transistor 36 to ground, and then back to the 
terminals 18, 20 and 22 via the anode-cathode circuits of the controlled 
rectifiers 72 when they are gated conductive. When the controlled 
rectifiers 72 are nonconductive, the path for field current is from the 
grounded emitter of transistor 36 to grounded terminal 52, through battery 
54, through conductor 58, and then through diodes 26 to terminals 18, 20 
and 22. When the system is going from the 12 volt to the 24 volt mode, the 
current developed by the generator and applied to the field winding 14 via 
diodes 34 may not be sufficient to obtain a fast build-up of generator 
voltage. Full field power is available, however, since the battery 50 can 
supply field current to the field winding 14 via a circuit that can be 
traced from the positive side of the battery 50, through diode 88 to 
terminal 32, through the field winding 14 and collector-emitter circuit of 
transistor 36 to ground, and then through grounded terminal 52 to the 
negative side of battery 50. The diode 88 prevents current flow from 
junction 32 to conductor 86 as a result of the voltage developed at 
junction 32 by the diodes 34. 
In regard to the operation of the fixed frequency control 84A, shown in 
FIG. 4, it is pointed out that amplifier B1 has a constant voltage applied 
to its positive terminal due to the provision of Zener diode 128. The 
voltage applied to the negative terminal of amplifier B1 varies, dependent 
upon the voltage across battery 54, since battery voltage is applied 
across the circuit comprised of diode 120, resistor 118 and potentiometer 
resistor 116. Battery voltage is filtered by capacitor 137 to obtain an 
average direct voltage level that is applied to the negative input 
terminal of amplifier B1 via junction 114 and this voltage is compared to 
the Zener reference voltage applied to the positive input terminal of 
amplifier B1. The circuit establishes the time ratio between the 12 and 24 
volt modes for the existing load conditions that will hold the average 
direct voltage at junction 114 and the negative terminal of amplifier B1 
substantially constant. The variation in time ratios, for certain load 
current conditions and conditioned battery voltages V.sub.1 and V.sub.2, 
is illustrated in FIGS. 6B and 6C. FIGS. 6B and 6C are generalized 
waveforms and are intended only to illustrate the general concept of 
varying the time ratios with different conditions of operation. Further, 
and by way of example, the frequency of the triangular waveform voltage 
generator, comprised of amplifier sections B2 and B3, may be about 25 to 
30 hertz. 
As has been described, the switching from 12 and 24 volt modes and vice 
versa can be controlled by the fixed frequency mode control 84A, shown in 
FIG. 4, or alternatively by the load determined control 84B shown in FIG. 
5. It is preferred to utilize the fixed frequency control 84A since the 
switching frequency can be selected to be high enough to prevent any 
possibility of light flicker of the vehicle headlamps. Thus, when the load 
determined control 84B is utilized, the switching between modes may occur 
at such a low frequency as to cause a slight light flicker. The fixed 
frequency mode switching control 84A is also considered to be superior to 
the load determined control 84B in that the ripple voltage applied to the 
batteries is less when using the fixed frequency mode switching control 
84A. 
The following describes the types of motor vehicle loads that can be 
supplied by the electrical system of this invention. Examples of the 24 
volt loads 68 are the electric motor driven engine radiator cooling fan 
and the rear window heater. Examples of the +12 volt loads 60 are the 
vehicle headlamps, radio, and other vehicle lamps. The electric engine 
cranking motor can be the -12 volt load 64. On the other hand, if a 24 
volt electric cranking motor is utilized, it can be energized across 
terminals 48 and 56. 
One of the advantages of providing a system which can supply both a 
positive and negative 12 volts referenced to ground is that it simplifies 
the circuitry required to energize reversible direct voltage electric 
motors that are used on motor vehicles. Thus, one end of a motor can be 
connected to ground and a single-pole double-throw switch connected 
between the opposite end of the motor and respective positive and negative 
terminals 48 and 56. The direction of the current flow through the motor 
will then be dependent upon whether terminal 48 or terminal 56 is 
connected to the motor by the switch. When switching from terminal 48 to 
56 or vice versa, the current in the motor will reverse. 
The batteries 50 and 54 are illustrated in FIG. 1 as two separate 
batteries. Batteries 50 and 54 could be provided in a single case or 
package that would have a positive terminal corresponding to junction 48, 
a negative terminal corresponding to junction 56, and an intermediate 
terminal corresponding to junction 52.