Emergency light control and battery charging system

An emergency light control and battery charging system utilizes a temperature compensated switched voltage regulator for charging the battery. Energization of the emergency light is effected through a field effect transistor which is turned on through a voltage doubler circuit enabled when there is a failure in the power supply. The system is arranged with a latch that insures that the light is not turned on when the battery is connected until after line power is initially applied. A low battery voltage detector insures that the battery is protected from a deep cycle discharge if line voltage is not restored.

BACKGROUND OF THE INVENTION 
This invention relates to emergency light control and battery charging 
systems and, more particularly, to such a system which provides extended 
battery life and high efficiency. 
In commercial establishments, such as stores and office buildings, 
emergency lighting systems are often required under municipal building 
codes and safety ordinances. Such systems typically include a battery 
which is maintained in its charged state whenever conventional electrical 
AC power is available. When such power fails, emergency lights are turned 
on and powered from the battery until the conventional power is restored. 
When power is restored, the battery is recharged. 
While such systems have been in fairly widespread use in the past, such 
systems suffer from a number of drawbacks. For example, such systems 
typically utilize a linear type of voltage regulation in the battery 
charging circuit. This results in a relatively large transformer and the 
requirement that the voltage regulator must have a heat sink. 
Another drawback in such prior systems is due to the fact that the battery 
needs a slightly higher charging voltage when the ambient temperature is 
low and a slightly lower charging voltage when the ambient temperature is 
high. The reason for critical battery voltage control is to extend the 
life of the battery. In emergency lights, the battery must last from three 
to five years in the standby mode and constantly trickle charging a 
battery will cause it to fail in a few months. The energy from the trickle 
charger, after a battery is fully charged, goes into converting its 
internal chemicals into unusable compounds or electroplating its terminals 
into useless stubs. Another problem is that a battery can be discharged 
and still show full voltage. For example, a 6 volt battery will measure 6 
volts even when discharged, so that to charge this battery more than 6 
volts must be applied to it for current to flow. At 75.degree. F, the 
battery can be fully charged at 6.8 volts, whereas the battery will be 
destroyed at 7 volts. 
Accordingly, it is an object of this invention to provide an improved 
emergency light control and battery charging system. 
It is a further object of this invention to provide such a system where the 
voltage regulator is switched, rather than linear, to allow a lower 
wattage and smaller transformer to be utilized and to avoid the 
requirement that the voltage regulator must have a heat sink. 
It is another object of this invention to adjust the charging voltage based 
on ambient temperature in order to achieve longer battery life. 
SUMMARY OF THE INVENTION 
The foregoing, and additional, objects are attained in accordance with the 
principles of this invention by providing a system adapted for connection 
to a source of conventional electrical AC power, a battery, and a light, 
the system being arranged to keep the battery charged from the power 
source and in the event of a disruption in the AC power source to energize 
the light from the battery. The system according to this invention 
comprises a power supply for converting the conventional electrical AC 
power to DC power between a supply line and a reference line, charging 
means including switched voltage regulating means coupled between the 
power supply and the battery for selectively charging the battery to 
maintain the battery voltage within a predetermined range, latch means 
coupled to the power supply for providing a latch signal upon initial 
connection of the system to the AC power source, and enable means coupled 
to the power supply and to receive the latch signal for energizing the 
light when the latch signal is present and the DC power is absent. 
In accordance with a feature of this invention, the system further includes 
a low battery voltage detector coupled to the battery and the latch means 
for sensing the battery voltage and disabling the latch means to eliminate 
the latch signal when the batter voltage is below a preset level. 
In accordance with another feature of this invention, the charging means is 
temperature regulated to increase the charging voltage to the battery as 
the ambient temperature decreases.

DETAILED DESCRIPTION 
Referring now to the drawing, wherein like elements in different figures 
thereof have the same reference numeral applied thereto, FIG. 1 shows a 
block diagram of a system constructed in accordance with the principles of 
this invention. As shown in FIG. 1, the system includes a power supply 10 
hard wired to a conventional AC power source 12. Typically, when such a 
system is installed, it is connected directly to the power wiring in the 
building. The power supply 10 functions to convert the AC power from the 
source 12 to DC power between a supply line 14 and a reference line 16. 
This DC power is then utilized by the system to maintain the charge on the 
battery 18 and to control energization of the emergency light 20 upon 
detection of a failure in the source 12. 
Accordingly, the system includes a charging circuit 22 which operates to 
selectively charge the battery 18 to maintain the battery voltage within a 
predetermined range. As will be described in full detail hereinafter, the 
charging circuit 22 includes a switched voltage regulator and also 
includes a temperature regulator which increases the charging voltage to 
the battery 18 as the ambient temperature decreases. 
The system also includes a line voltage latch 24 which provides a latch 
signal upon the initial connection of the system to the AC power source 
12. This is to insure that if the battery 18 is connected before power 
from the source 12 is applied, the light 20 will not be energized. Such 
energization is delayed until after there is an initial connection to the 
source 12. 
Energization of the light 20 is effected through a controlled switch 26 
which is controlled by the light enable circuit 28. The light enable 
circuit 28 is coupled to the power supply 10 and receives the latch signal 
from the latch circuit 24 to actuate the switch 26 when the latch signal 
is present and DC power is absent, DC power being absent upon failure of 
the AC source 12. 
The system also includes a low battery voltage detector 30 for protecting 
the battery 18 from a deep cycle discharge if power is not restored. The 
detector 30 operates by sensing the battery voltage and disabling the 
latch circuit 24 to eliminate the latch signal when the battery voltage is 
below a preset level. The latch circuit 24 will not be re-enabled until 
power is restored. 
Referring to FIG. 2, the power supply 10 comprises a transformer having a 
primary winding 32 and a center tapped secondary winding 34. The primary 
winding 32 is directly connected to the AC power source 12. The diodes 36, 
38, 40 and 42 are connected as a diode bridge across the secondary winding 
34 to convert the step down AC voltage across the secondary winding 34 to 
a DC voltage between the supply line 14 and the reference line 16. In 
particular, the diodes 36 and 38 supply the positive potential and the 
diodes 40 and 42 supply the negative potential. The light emitting diode 
44, connected through the dropping resistor 46 between the center tap 48 
of the transformer secondary 34 and the reference line 16, lights up 
whenever power is applied to the power supply 10 from the AC source 12. 
The charging circuit 22 includes a silicon controlled rectifier (SCR) 50 
having its cathode 52 connected to the positive terminal 54 of the battery 
18 and its anode 56 connected to the center tap 48 of the transformer 
secondary 34. The negative terminal 58 of the battery 18 is connected 
directly to the reference line 16. Since the cathode 52 of the SCR 50 is 
connected to the positive terminal 54 of the battery 18, when the gate 60 
of the SCR 50 goes 0.6 volts above the cathode 52, the SCR 50 will turn 
on, conducting current to the battery 18 and thereby charging it. The 
voltage at the gate 60 is regulated by a series string comprising a 
resistor 62, diodes 64 and 66, and Zener diode 68. The resistor 62 
regulates the current for the diodes 64 and 66, which are connected to 
have a 1.2 volt drop across them. The potentiometer 70 is connected across 
the diodes 64 and 66, with its variable tap connected to the gate 60, to 
allow the gate voltage to be adjusted over this 1.2 volt range. The Zener 
diode 68 provides regulation at 6.8 volts, which therefore is the minimum 
of the voltage range at the gate 60. Since the diodes 64 and 66 add 1.2 
volts, the maximum of the voltage range at the gate 60 is therefore 8 
volts. Because the voltage at the gate 60 is 0.6 volts above the voltage 
at the cathode 52 and the positive terminal 54 of the battery 18, the 
adjustment range of the battery voltage is 6.2 volts to 7.4 volts. 
In accordance with the principles of this invention, the diodes 64 and 66 
have a negative temperature characteristic. This is because when the 
ambient temperature is cold, the battery 18 needs a slightly higher 
charging voltage than when the ambient temperature is high. Critical 
control of the battery voltage helps to extend the life of the battery, as 
previously described. Illustratively, the diodes 64 and 66 provide -8 
MV/.degree. C voltage variation with temperature. This offsets the 
positive temperature coefficients of the other components, such as the 
Zener diode 68, the gate to cathode voltage of the SCR 50, and the battery 
18 itself. 
An example is useful for explaining the way the charging circuit 22 
operates. Assuming that the potentiometer 70 is set such that the gate 60 
of the SCR 50 is at 7.6 volts, the cathode 52 would be at 7 volts. 
Accordingly, whenever the voltage at the positive terminal 54 of the 
battery 18 exceeded 7 volts, the SCR 50 is switched off. If the battery 
voltage drops below 7 volts, the SCR 50 is switched on, and remains on 
until the battery 18 is fully charged to 7 volts. 
The function of the line voltage latch circuit 24 is to prevent the light 
20 from being energized when the battery 18 is initially connected before 
the system is wired to the power source 12. After the line voltage from 
the source 12 is present, the latch 24 allows normal functioning of the 
system. This feature is valuable because it prevents the battery from 
being drained if the lights are installed before line voltage is present. 
Battery life is severely affected if the battery is left in a discharged 
state for long periods of time. To provide the described function, the 
latch circuit 24 includes resistors 72 and 74 connected as a voltage 
divider to lower the voltage from the supply line 14. This lowered voltage 
is filtered by the capacitor 76 and is conducted by the diode 78 to 
terminal 80 of the exclusive OR gate 82. The other input 84 of the gate 82 
is connected to the reference line 16. Whenever power is applied to the 
supply line 14, the input 80 will go high, and since the input 84 is 
normally low, the output 86 will go high. The resistor 88 connected 
between the output 86 and the input 80 acts as a positive feedback loop to 
maintain the output 86 at a high level after it initially goes high, even 
if the level on the supply line 14 subsequently drops. The capacitor 90 
acts as a filter for the terminal 80 so that no noise may enter, and holds 
it low during connection of the battery 18. 
The output 86 of the exclusive OR gate 82 is connected to the input 92 of 
the exclusive OR gate 94 in the enable circuit 28. The other input 96 of 
the gate 94 is connected to the power supply 10 through the voltage 
divider made up of the resistors 72 and 74. Accordingly, under normal 
operating conditions with full power applied, the inputs 92 and 96 to the 
gate 94 will both be high and the output 98 will be low. However, if there 
is a failure in the source 12, the voltage on the line 14 will go low, 
allowing the input 96 of the gate 94 to go low. Since the input 92 is 
connected to the latched output 86, it will remain high. Accordingly, 
failure of the source 12 will result in the output 98 of the gate 94 going 
high. As will be described, this will cause the light 20 to be energized. 
The light 20 is energized when the controlled switch 26 completes a 
conductive path between the positive terminal 54 of the battery -8, the 
light 20, and the negative terminal 58 of the battery 18. Preferably, the 
controlled switch 26 includes a field effect transistor 100 having its 
source 102 connected to the negative terminal 58 of the battery 18 and its 
drain 104 connected to the light 20. To turn on the field effect 
transistor 100 so that it conducts requires a voltage of 10 volts to be 
applied to the gate 106. This is a greater voltage than that which can be 
supplied by the battery 18, even if it is fully charged. Accordingly, the 
enable circuit 28 includes a voltage doubler section to double the battery 
voltage to provide enough gate drive to turn on the transistor 100. 
This voltage doubler section includes the exclusive OR gate 108. When the 
output 98 of the gate 94 goes high, this causes the input 110 of the gate 
108 to go high. The input 112 of the gate 108 is connected to the 
reference line 16 through the capacitor 114, so it remains low. 
Accordingly, the output 116 of the gate 108 goes high. This initiates 
charging of the capacitor 114 through the resistor 118. This continues 
until the capacitor 114 is charged sufficiently that the voltage at the 
input 112 is high enough to cause the output 116 of the gate 108 to go 
low. This causes the capacitor 114 to discharge through the resistor 118. 
Thus, the circuitry goes into oscillation. The values of the components 
are illustratively chosen for oscillation to occur at approximately 3 
kilohertz. The capacitor 120 and the diode 122 perform the voltage 
doubling. When the output 116 goes low, the capacitor 120 charges through 
the diode 122. When the output 116 goes high, the diode 122 is open 
circuited, and the voltage at the junction 124 rises to twice the power 
supply voltage. This voltage is supplied through the diode 126 to the gate 
106 of the transistor 100. The gate 106 of the transistor 100 has internal 
capacitance between it and the source 102, and charge transferred into 
that internal capacitance causes the transistor 100 to turn on. This 
source-to-gate capacitance filters the oscillation of the voltage doubler. 
The resistor 128 is a pull down resistor so that when the voltage doubler 
shuts off, the gate 106 will discharge to zero. However, the discharge 
through the resistor 128 is slower than the 3 kilohertz oscillation of the 
voltage doubler, so that once the field effect transistor 100 is turned on 
by the voltage doubler, it will remain conductive until the voltage 
doubler is turned off. 
The low battery detector circuit 30 includes the exclusive OR gate 130 
having its input 132 connected to the reference line 16. This means that 
when the input 134 is high, the output 136 is high and when the input 134 
is low, the output 136 is low. The input 134 is connected to the junction 
between the Zener diode 138 and the resistor 140. The resistor 140 has its 
other end connected to the reference line 16 and the Zener diode 138 has 
its anode connected to the positive terminal 54 of the battery 18. The 
Zener diode 138 is preferably rated at 2.7 volts, so that when the battery 
voltage drops below about 4.75 volts, the voltage on the input 134 is low 
enough to cause the output 136 to go low. When the output 136 goes low, 
this causes the diode 142 to become conductive and cause the input 80 of 
the gate 82 to go low. This disables the latch circuit 24 and causes the 
elimination of the latch signal on the output 86. With the latch signal on 
the output 86 gone, the enable circuit 28 is turned off, which turns off 
the voltage doubler section. Once the latch circuit 24 is disabled, it 
will not become re-enabled until power is reapplied at the source 12. This 
is an important feature because as soon as the light 20 is turned off, the 
battery voltage tends to go back to normal even though it is not being 
charged. For example, a battery discharged to 4.75 volts can rise up again 
to 6.5 volts as soon as the load is removed, even though no charging has 
taken place. This could fool the system into thinking that there is enough 
battery power to run the light 20, and the lights could come back on, 
resulting in damage to the battery, and a flashing of the lights. 
Accordingly, with this feature, once the battery 18 is discharged, the 
light 20 will remain off, until power is reapplied and there is a 
subsequent power failure. 
The disclosed system also includes the provision for testing whether the 
light 20 will come on upon a power failure. This is accomplished through 
the push-to-test switch 144. When the actuator of the switch 144 is 
pressed, this causes the input 96 of the gate 94, to go low, simulating 
loss of power on the supply line 14. The diode 78 prevents the input 80 of 
the gate 82 from going low. The previously described action will cause the 
light 20 to be energized as long as the switch 144 remains closed, the 
latch circuit 24 remains latched, and the battery voltage is not too low. 
When the switch 144 is released, the system will go back to its normal 
standby mode of operation. 
In order to appreciate the advantages of the disclosed design, certain 
features should be highlighted. One feature is the switched nature of the 
voltage regulation in the charging circuit 22. The silicon controlled 
rectifier 50 is preferably a type TO-92 device which can supply up to 0.5 
amp of charging current to the battery 18. Because of its ON/OFF switching 
nature, it dissipates very little heat and very little power is lost. 
Accordingly, a heat sink is not required. Additionally, the transformer in 
the power supply 10 can be smaller in physical size and of less wattage 
than would be required with a linear type voltage regulator. A further 
advantageous feature is the use of the Zener diode 68 in conjunction with 
the two diodes 64 and 66 for temperature regulation so that ambient 
temperature variation will cause the charging voltage to vary. 
Another advantageous feature is that the logic elements 82, 94, 108 and 130 
are all exclusive OR gates. This use of a single type of logic element to 
perform multiple functions allows the use of a commonly available single 
integrated circuit having the four gates thereon. 
Also, providing a voltage doubler allows the use of a field effect 
transistor 100. Such a device only has a voltage drop of 0.4 volts at 3 
amps of current, while a standard transistor would drop at least 1 volt. 
When dealing with a 6 volt circuit, this difference in voltage drop is 
critical. Furthermore, the field effect transistor 100 is of significantly 
lower cost than a relay and requires no driving current. 
Accordingly, there has been disclosed an improved emergency light control 
and battery charging system. While a preferred embodiment of the present 
invention has been disclosed herein, it will be apparent to those of 
ordinary skill in the art that various modifications and adaptations to 
that embodiment are possible and it is only intended that the present 
invention be limited by the scope of the appended claims.