The present invention relates to emergency lighting equipment or devices
of the type containing an emergency unit capable of providing an independent power
supply to a lamp or fluorescent tube if the mains supply fails.
The invention relates more particularly to an emergency unit for this
type of equipment or device.
The invention also relates to a method for controlling devices and
emergency units of the aforesaid type.
Emergency lighting devices of the type comprising a ballast supplying
at least one lamp, an emergency battery and an emergency unit are available at the
present time. The emergency unit comprises a battery charger, a control circuit
and an inverter for supplying the lamp in emergency conditions, in other words in
the absence of electrical mains power. In emergency, the power is supplied by the
battery, which is kept charged by the battery charger during normal operation.
An example of such an emergency lighting unit is disclosed in US-A-3833817.
EP-A-364371 discloses an emergency unit with a special device for controlling the
battery charge. Other examples of known emergency lighting units are disclosed in
US-A-4988889 and US-A-4158792. Unitrode Design note XP-001051082 discloses the use
of a certain family of resonant lamp drivers in floating lamp applications.
These devices must meet specific requirements established by the regulations
in force in various countries, primarily relating to the reserve time of the device
in emergency conditions, in other words the minimum guaranteed duration of operation
in emergency conditions. When the battery capacity and the minimum reserve time
of the device have been specified, the current which the battery can supply to the
lamp is determined. For example, in order to guarantee a reserve time of 3 hours
when a 4 Ah nickel-cadmium battery is used, the maximum discharge current, in other
words the current which the battery can supply to the lamp during operation in emergency
conditions, is 0.8 A. For a reserve time of 1 hour, the maximum discharge current
is 2.4 A. These values also allow for the exhaustion of the batteries over time
and the consequent reduction in their capacity.
The current absorbed by the lamp associated with the device depends
on the power of the lamp. Conventional devices are therefore designed in such a
way as to absorb the maximum permissible current (determined, as indicated above,
from the capacity of the battery and the minimum reserve time to be guaranteed)
when the lamp having the highest power is fitted to the device. Consequently, lamps
whose power exceeds the design limit for a given device cannot be supplied by the
device. Moreover, the current absorbed by the device and then supplied to the lamp
in emergency conditions is not always the maximum amount determined by the life
of the battery. This will be the case only for the highest-power lamp for which
the device has been designed. For other lamps, the current supplied by the battery
will be lower, and consequently the light flux in emergency conditions will be less
than that which could be achieved while still maintaining the guaranteed minimum
Fig. A shows schematically a self-oscillating inverter of an emergency
unit of a conventional type for supplying the lamp. The letter A indicates the inverter
in a general way, B indicates the two controlled switches, C indicates the control
circuit of the switches B, and Vbat indicates the battery voltage.
It is therefore necessary to provide a larger number of emergency
devices to obtain the minimum level of illumination required by the regulations.
If the BLF (ballast lumen factor) is defined as
BLF = (emergency light flux) / ((light flux in normal operation)) x 100
a given emergency device will provide the maximum possible BLF only with the highest-power
lamp for which the device was designed. Lamps with a lower power will operate, in
emergency conditions, with a BLF below the optimal value.
The object of the present invention is to provide an emergency lighting
device and an emergency unit of the aforementioned type which do not have these
disadvantages. More particularly, the object of the invention is to provide a device
which optimizes the BLF for each lamp used.
These and other objects and advantages which will be made clear to
a person skilled in the art by the following text, are essentially achieved with
an emergency lighting unit comprising
- a connection for a battery,
- an inverter for supplying, in emergency conditions, by means of said battery,
a lamp connectable to said unit,
- a circuit for controlling the current supplied by the battery to the inverter,
which maintains said current at a predetermined value independently of the characteristics
of the lamp connected to the inverter.
Essentially, when the current absorbed from the battery is controlled
in such a way that it is kept constant when the lamp used is changed and is kept
equal to the maximum permissible value according to the reserve time of the device,
the emergency light flux is maximized, independently of the lamp used.
In a practical embodiment, the control circuit of the emergency unit
- a system for measuring the current supplied from the battery to the inverter
- means of comparison which compare the value measured by the measurement system
with a value which can be selected, to generate a control signal;
- a device for regulating the current supplied from the battery to the inverter
according to the control signal.
In a particularly efficient embodiment, the regulating device comprises
a controlled switch, for example a MOSFET, and the control signal operates said
controlled switch, causing it to open and close in such a way as to maintain the
mean discharge current at the desired value. To ensure an essentially continuous
current at the power supply input, means are advantageously provided to maintain
a supply of current to the inverter during the intervals of opening of the controlled
switch. An inductance and a diode can be used for this purpose.
The inverter for use in the emergency unit can be a push-pull inverter
and more particularly an inverter of the current-sourcing push-pull parallel-resonance
The inverter is associated with a synchronizing circuit to generate
an inverter control signal at a frequency equal to the resonant frequency of the
inverter, independently of the lamp supplied by said inverter, in such a way that
the inverter is switched over at zero voltage and current. To reduce the current
consumption and the costs of the synchronizing circuit, this circuit can advantageously
be made with a logic gates configuration which gates generate a digital synchronizing
signal from a suitable input signal, for example the collector voltage of a transistor
whose switching is controlled by a signal synchronized with the inverter voltage.
The synchronizing circuit can also be equipped with means which prevent
the operation of the inverter when its resonant frequency departs from a preset
frequency range. For this purpose, it is possible to provide two oscillators having
frequencies equal to the maximum and minimum permissible for the resonant frequency
of the inverter.
The synchronizing circuit can also yield advantages if it is used
in other applications, for example in inverters which are not fitted in emergency
The invention also relates to an emergency lighting device, comprising,
in combination, a connection to a power supply line, a ballast for supplying at
least one discharge lamp from the voltage supplied by said line, and an emergency
unit as described above.
The invention also relates to a method for supplying a discharge lamp
in emergency conditions by means of an emergency battery, in which the lamp is supplied
from said battery through an inverter, characterized in that the value of the current
supplied by the battery is measured and the power supply conditions of said inverter
are controlled by maintaining the supplied current at a mean value which is essentially
equal to a maximum value which is determined by the capacity of the battery and
at which the battery provides a predetermined period of operation in emergency.
Further advantageous characteristics and embodiments of the invention
are indicated in the attached claims.
The advantages which are obtained with the emergency unit and with
the device according to the invention are numerous. In the first place, the emergency
lighting device supplies the lamp with a constant power which is independent of
the model and power of the lamp used. This power is equal (if the negligible losses
due to the power consumption of the device, and of the current control circuit in
particular, are disregarded) to the maximum value obtainable from the battery used
to guarantee the desired reserve time. This enables the BLF to be optimized for
a given battery and for each lamp.
The optimization of the BLF makes it possible to specify, at the design
stage, the use of a smaller number of ceiling lights fitted with the emergency unit.
The gain in percentage terms of BLF for 35 W T5 lamps has been found to be, for
example, more than 40% by comparison with conventional devices. Therefore, if, in
a given environment, ten 35 W T5 lamps supplied by conventional emergency units
are required to provide the level of illumination required by the regulations, only
six units will be required if use is made of the unit according to the invention
which increases the BLF by 40%.
Moreover, the emergency unit according to the invention makes it possible
to use different fluorescent lamps, for example T5, T8, TC-DD, TC-D/E, TC-T/E, TC-L,
TC-F, TC-S/E and T-R types, with a single type of device.
It is also possible to select the life of the battery at the time
of installation, by an external setting of the jumper, switch or equivalent type.
This is because, as will be made clear by the following description, the lamp supply
current in emergency conditions is controlled by comparison with a reference signal.
The value of this signal can be set in such a way that the device maintains the
current supplied by the battery at a value which provides the desired reserve time
of 1 hour or 3 hours (or another suitable value).
The invention will be more clearly understood from the description
and the attached drawing, which shows a practical and non-restrictive example of
the invention. In the drawing,
- Fig. A is a diagram of an inverter of a conventional emergency unit;
- Fig. 1 is a block diagram of the lighting device;
- Fig. 2 is a block diagram of the emergency unit;
- Figs. 3A and 3B are circuit diagrams of the inverter, the synchronizing circuit
for the inverter control signals, and the control circuit for the battery discharge
- Figs. 3C and 3D show details of modified embodiments of the circuit of Fig.
- Fig. 4 shows the variation of the voltage at the central tap of the transformer
of the power supply inverter;
- Figs. 5 and 6 show the variation of the waveform in the synchronizing circuit;
- Fig. 7 shows the signal at the outputs of the control flip-flop of the switches
of the inverter as a function of the clock signal;
- Fig. 8 shows the variation of the waveform in the current control circuit;
- Fig. 9 is a circuit diagram of a battery charger;
- Fig. 10 shows the variation of the charging current for a type of battery that
can be used with the emergency device; and
- Fig. 11 is a diagram of a circuit for recognizing the battery voltage and setting
Fig. 1 shows a block diagram of the device. This comprises an emergency
unit 1, a ballast 3 for supplying the fluorescent lamp L in normal supply conditions,
in other words when the power is supplied from the mains, and a battery 5 for the
supply in emergency conditions, connectable to the unit 1.
The switching from the mains supply to the emergency supply takes
place in a known way and the corresponding parts of the circuit are not illustrated
The emergency unit 1 comprises a battery charger 7 capable of charging
the battery 5 when the mains power is present, in other words in normal operating
conditions. It also comprises an inverter 9 capable of supplying the lamp L with
the power taken from the battery 5 when the mains voltage is absent, and a control
circuit 11 which serves to maintain the absorbed current equal to the maximum value
(Imax) compatible with the reserve time of the device in emergency operating
conditions, independently of the type of lamp L connected to the inverter 9.
The power supplied by the battery in emergency operating conditions
is equal to
P = V * I
where V is the voltage of the battery and I is the current supplied. Essentially,
the control circuit acts in such a way that the current I supplied by the battery
is always equal to Imax, in other words to the maximum current that can
be supplied to achieve the guaranteed reserve time of the emergency device.
Fig. 2 shows a block diagram of the current and inverter control circuit
for supplying the lamp L in emergency conditions. In this case also, the number
5 indicates the battery, 9 indicates the inverter, 9A indicates the synchronizing
circuit for synchronizing the control signal of the controlled switches of the inverter
and 11 indicates the current control circuit. As described in greater detail below
with reference to an example of an embodiment of the circuit, the control circuit
11 measures the current supplied by the battery 5 to the inverter 9, in other words
the discharge current of the battery, and generates a feedback signal which causes
the opening and closing of a controlled switch 47 to maintain the mean value of
the inverter power supply current equal to the maximum value Imax at
which the minimum reserve time of the battery 5 is guaranteed.
One solution in circuit form for the construction of the blocks 9,
9A and 11 is illustrated in Figs. 3A and 3B. Here, the frame of broken lines indicated
by 21 encloses the components forming the inverter and the synchronization control
circuit which causes the opening and closing of the controlled switches of the inverter.
23A and 23B indicate two blocks which together make up the control circuit of the
supply current in emergency conditions, and L indicates the fluorescent lamp, which
in this diagram is represented as a set of three resistors Rf,
Rt and RI, representing the resistances of the two
filaments and the internal resistance of the lamp. Here again, 5 indicates the supply
battery. The letters A, B, C indicates the points of connection between the portions
of circuit shown in Figs. 3A and 3B.
More particularly, the inverter comprises a pair of MOSFETs or other
controlled electronic switches 31, 33 in half-bridge configuration, in parallel
with which are connected a capacitor 35 and the primary winding 37 of a transformer
with a central tap connected to the load circuit, the secondary of this transformer
being indicated by 39. The numbers 41 and 42 indicate three capacitors of the charging
circuit which also comprises the inductance 40 and the resistors Rf,
Rf, Ri representing the lamp L.
The inverter is connected to the battery 5 through an inductance 45.
The configuration described up to this point is known and is called a current-sourcing
push-pull parallel-resonance inverter (CS-PPRI).
When the mains voltage fails, the lamp L is supplied by the inverter
to which energy is supplied by the battery 5 through the controlled switch 47 in
series with the inductance 45. The controlled switch 47 is opened and closed by
the current control circuit 23A, 23B described below, in such a way that the lamp
L is supplied with a mean current equal to the maximum current Imax which
can be supplied in accordance with the conditions of minimum reserve time guaranteed
for the battery with which the emergency device is fitted.
The inductance 45 is associated with a diode 49 which provides an
essentially constant current to the inverter when the controlled switch 47 is open,
by using the energy temporarily stored in the inductance 45. When the controlled
switch 47 is open, the current flows through the diode 49, the inductance 45 and
The inverter which has been described operates by switching the switches
31 and 33 at zero voltage and zero current if the control frequency of the switches
is equal to the resonant frequency. This depends on the load applied to the inverter,
in other words, ultimately, on the lamp fitted in the device. It is therefore necessary,
in order to obtain switching at zero current and voltage, for the control signal
of the switches 31 and 33 to be locked to the resonant frequency. The synchronizing
circuit 9A is provided for this purpose.
The synchronizing circuit comprises a transistor 51 whose base is
connected through a voltage divider 52 to the central tap of the transformer of
the inverter 9, in other words to the point indicated by X in Fig. 2A. The collector
of the transistor 51 is connected to a first input of a first NAND gate indicated
by 53, whose output is connected to a first input of a second NAND gate indicated
by 55. The output of the latter is connected to the input of a NOT gate indicated
by 57 and to the clock input of a flip-flop of the "T" or toggle type, indicated
in a general way by 59, whose two signals at the outputs Q pass through an amplification
stage 61 to control the opening and closing of the two controlled switches 31, 33.
The circuit also comprises two oscillators consisting of two RC networks
indicated by 63 and 66, and formed, respectively, by a resistor 64 and a capacitor
65, and by a resistor 67 and a capacitor 68. The network 63 is connected to the
second input of the NAND gate 53, while the RC network 66 is connected to the input
of a NOT gate indicated by 71, whose output, in turn, is connected to the second
input of the NAND gate 55.
The output of the NOT gate 57 is connected to the two networks 63,
66 through two corresponding diodes 73, 75 and a resistor 77.
The operation of the synchronizing circuit described here is as follows.
At the point X there is a rectified sinusoidal voltage represented by the curve
C1 in Fig. 4. A voltage with a similar variation is applied to the base
of the transistor 51 (curve C2 in Fig. 5). Whenever the voltage at the
point X falls below a predetermined value, the transistor 51 is turned off, so that
its collector changes from a voltage of approximately zero to a voltage equal to
the battery voltage Vcc. In Fig. 5, the variation of the voltage on the
collector of the transistor 51 is represented by the curve C3.
Consequently, a high signal, which is synchronized with the rectified
sinusoidal voltage at the point X of the transformer of the inverter, and therefore
with the resonant frequency of the inverter, appears at the input of the NAND gate
53 to which the collector of the transistor 51 is connected. This signal is used
as the clock signal for the flip-flop 59. The outputs Q of the flip-flop 59 are
inverted at each rising front of the clock signal, as shown in Fig. 7, to provide
a duty cycle of 50%. The clock signal, determined by the collector voltage of the
transistor 51, is indicated by the curve C4 in Figs. 5 and 6.
The synchronizing circuit is made in such a way as to have two limit
frequencies above and below which the inverter cannot be made to operate. These
limit frequencies are determined by the resonant frequencies of the RC networks
63 and 66, where the former determines the maximum frequency and the second determines
the minimum frequency. Because of the NAND gate 53, the synchronizing signal on
the collector of the transistor 51 will be disregarded if a low signal is present
at the input of said gate connected to the RC network 63. Similarly, the synchronizing
signal will be disregarded if the signal at the input of the NAND gate 55, connected
through the NOT gate 71 to the RC network 66, is low.
The connection of the output of the NOT gate 57 through the diodes
73 and 75 to the oscillators 63, 66 causes resetting, in other words the discharge
of the capacitors, at each clock signal. Fig. 6 also shows the curves C3
and C4 and the variation of the voltage of the capacitors 65 and 68 (curves
C65 and C68).
The current control circuit 11, enclosed within the frames 23A and
23B, comprises a resistor 81 through which a voltage signal, proportional to the
current supplied by the battery 5, is obtained. The value of the resistor 81 is
sufficiently low to make the losses across it practically negligible. The voltage
present across the terminals of the resistor 81 is suitably filtered by an RC filter
82 and amplified by an amplifier 83 (Fig. 3B). The output of the amplifier 83 is
connected to the positive input of an error amplifier 85, at whose negative input
there is a reference voltage, equal to 2.5 V in this example, and which forms part
of an integrator 87. The integrated error signal at the output of the integrator
87 is sent to the inverting input of a comparator 89, whose non-inverting input
is connected to a branch comprising a resistor 91 and a capacitor 93 and is connected
(at C) to one of the outputs of the flip-flop 59. Consequently, at the positive
input of the comparator 89 there is a triangular-wave signal, represented by the
curve C6 in Fig. 8, where C1 again represents the variation
of the voltage at point X of the inverter. The curve C7 represents the
signal at the output of the integrator 87. Fig. 8 also shows the squarewave signal
C8 at the output of the comparator 89. This signal represents the control
signal applied to the base of the controlled switch 47 to control the opening and
closing of this switch.
The control system is such that the switch 47 is opened when the triangular
wave C6 takes a value greater than the value of the error signal (curve
C7), and to be reclosed when the triangular wave C6 returns
to a value of less than the value of the error signal.
As shown in Figs. 3A and 3B, the output of the comparator 89 is not
connected directly to the base of the controlled switch 47, but to a first input
of a NAND gate 95, whose output is connected (connection at point B) to the inputs
of two NOT gates 97, whose outputs are connected to the base of the controlled switch
47. At the second input of the NAND gate 95 there may be an ON/OFF control signal,
which causes the control of the controlled switch 47 to be temporarily disabled
when the emergency inverter is first switched on, for a time interval which can
be specified and during which the switch 47 always remains closed, regardless of
the value of the discharge current of the battery.
Essentially, the operation of the controlled switch 47 controls the
mean value of the current supplied by the battery 5 to the inverter 9, preventing
this current from exceeding a maximum value Imax determined by a suitable
setting of the reference voltage applied to the negative input of the error amplifier
85. To ensure that the mean absorbed current does not fall below a minimum value,
it is sufficient to provide a correct specification of the inverter, and particularly
of the capacitor 41. The latter is specified in such a way that the current absorbed
by the inverter with the minimum load (the lowest-powered lamp of those used) is
equal to or slightly greater than the value Imax.
By maintaining the discharge current, in other words the current supplied
by the battery in emergency operating conditions, constantly equal to the maximum
value compatible with the reserve time required in the emergency device, considerable
percentage increases in the BLF are obtained with respect to similar devices available
on the market at present. In the following table, Column 1 shows the type of lamp
used, Column 2 shows the values of BLF obtainable with a device according to the
invention, and Column 3 shows the corresponding values of the BLF obtained with
a type XW.3NC device made by Existalfte® (United Kingdom). Both devices are
supplied from 14.4 V batteries.
type of lamp
BLF of the device according to the invention
BLF of the XW.3NC
The described circuit can also permit adaptation to different operating
conditions; for example it can permit a modification of the guaranteed reserve time
of the emergency unit. By suitable adjustment, it is possible to set a discharge
current corresponding to the maximum permissible current for different lengths of
battery life in emergency conditions. A first possibility of adaptation of the circuit
is shown in Fig. 3A, where a second resistor 81' is connected in parallel with the
resistor 81 and is indicated in broken lines to signify that this arrangement can
be optional. The resistor 81' can be connected in parallel to the resistor 81 or
isolated by a suitable contact, indicated schematically by 80. A change in the configuration
of the resistance (81, or 81 in parallel with 81') modifies the value of the signal
proportional to the discharge current which is supplied to the current control circuit
Fig. 3C shows a detail of the control circuit 11, namely the amplifier
83 and the corresponding network. In this modified embodiment, one or two resistors
84, 84', connected to earth, can be connected the inverting input of the amplifier
83. The resistor 84' can be isolated by operating a jumper or switch 86. Thus the
gain of the amplifier, and consequently the value of the output signal, can be modified.
The configuration of Fig. 3D has an arrangement of two Zener diodes 88, 88' and
a jumper or switch 90. The value of the reference voltage on the inverting input
of the comparator 85 can be modified by closing or opening the switch 90.
Any one of the aforesaid configurations, or combinations thereof,
can be used as a means of modifying the value at which the current control circuit
11 maintains the discharge current during emergency operation, in accordance with
the minimum reserve time which the unit has to provide.
As mentioned above, the power supplied by the emergency battery 5
depends on the current and on the voltage. Therefore, in addition to the control
of the current which ensures the supply of the maximum current compatible with the
reserve time required from the battery, it is also possible to increase the light
flux and therefore the BLF by increasing the battery voltage.
Conventional emergency devices are not capable of using batteries
whose voltages differ from those for which the devices were designed, because of
In the first place, an increase in battery voltage causes an increase
in the recharging time of the battery, with the result that a device having a battery
charger designed to provide a specific recharging time for batteries of 12 V, for
example, is not capable of charging a battery of higher voltage, for example 14.4
V, in the same time.
Secondly, the battery voltage decreases during discharge and the emergency
operation of the device has to be interrupted when the voltage across the terminals
of each cell forming the battery reaches the minimum voltage of 0.8 V. This minimum
voltage corresponds to a voltage across the terminals of the battery which depends
on the number of cells making up the battery. In emergency lighting devices of the
conventional type, which use, for example, 12 V batteries consisting of ten cells
(1.2 V per cell), the supply of current in emergency conditions is made to cease
when the voltage across the terminals of the battery reaches the value of 8 V (called
the "undervoltage") which corresponds to a voltage of 0.8 V per cell.
If a battery with a larger number of cells is used in a device of
this type, it will continue to supply current even when the voltage of the individual
cell falls below 0.8 V. When a twelve-cell (14.4 V) battery is used, for example,
the undervoltage of 8 V is equivalent to a voltage of 0.67 V across the terminals
of each individual cell. This is not.acceptable, since it causes irreversible damage
to the battery.
In an improved embodiment of the invention, therefore, use is made
of a battery charger which, on the one hand, enables the battery to be recharged
in optimal conditions, regardless of the battery used, and, on the other hand, makes
it possible to use batteries of different voltages (for example 12 and 14.4 V) without
a risk of damage to the battery due to operation below the minimum permissible voltage
for the individual cell.
With these arrangements it is possible to use the same device with
different batteries, whereas at the present time it is necessary to design a different
emergency lighting device for each type of battery.
Fig. 9 shows a circuit for a battery charger capable of supplying
a constant recharging current to the battery 5 independently of the voltage of the
battery, in other words of the number of cells making up the battery. The battery
charger comprises a connection 101 to the electrical mains, a switching power supply
103, a connection 105A, 105B to the battery 5, and a resistor 107 connected across
the terminals of an operational amplifier 109. The output of the amplifier 109,
on which there is a signal proportional to the current Icharge which
the power supply 103 supplies to the battery 5 during recharging, is compared by
means of a comparator 111 with a reference signal Iref.
The error signal generated by the comparator 111 is used as a feedback
signal for controlling the switching power supply 103. The control is such that
the current Icharge is kept at a constant value over time.
For NiCd batteries, this value is typically 200 mA for batteries with
a capacity of 4 Ah, and 100 mA for batteries with a capacity of 2 Ah.
The circuit of Fig. 9 can be improved to make it possible to use batteries
of another type, for example metal-iodide nickel batteries. These batteries require
a charging current which is not constant over time, but varies as illustrated in
Fig. 10, in other words being equal to a value I1 for a first charging
period, typically of 16 hours, and then equal to a lower value I2. To
obtain this variation of the charging current it is simply necessary to modify,
by means of a suitable timer, the value of the reference signal Iref
at the input of the comparator 111.
To adapt the device to the use of batteries of different voltages,
it is possible to provide for manual setting of the undervoltage, by means of jumpers,
switches or other devices. In a particularly advantageous alternative embodiment,
a circuit for automatic recognition of the voltage of the battery connected to the
charger can be provided, to permit automatic setting of the undervoltage. An example
of an embodiment of this circuit is shown in Fig. 11. It comprises a time delay
switch 121 which is closed after a predetermined time interval, of the order of
30-60 s after the start of the discharge of the battery, in other words after the
start of operation of the device in emergency lighting conditions.
This time interval is necessary because the battery voltage can be
read only during the discharge of the battery, and at this stage it is also sufficiently
independent of the temperature. In particular, for NiCd batteries the manufacturers
state that there is a voltage of 1.3-1.35 V per cell after a discharge time of 30-60
The switch 121 connects a terminal 123, at the battery voltage Vbat,
to the non-inverting inputs of a set of K comparators 125.1-125.K, where
K = Nmax - Nmin
- Nmin = minimum number of cells making up the battery
- Nmax = maximum number of cells making up the battery
A reference voltage VR1 - VRK, where
VR1 > VR2 > ... > VRK
is applied to the negative input of each comparator 125.1-125.K. The output voltage
V1-VK of each individual comparator is connected to the non-inverting
output of a corresponding amplifier 127.1 - 127.K, whose inverting terminal is connected
to earth. The outputs of the amplifiers 127.1 - 127.K are connected to corresponding
resistors R1 - RK, where
R1 < R2 < ... < RK
which, in turn, are connected to a node 129. This is connected through a resistor
RC to the voltage Vbat and through a resistor RN
In the circuit of Fig. 11, when the switch 121 is closed the generic
output Vi of the generic comparator 125.i goes to the high value and
the corresponding resistor Ri is connected in parallel with the resistor
Rc. An increase in the value of the battery voltage (in other words an
increase in the number of cells making up the battery) is accompanied by an increase
in the number of outputs V1 ... VK which go to the high value,
and consequently an increase in the number of resistors R1 ... RK
connected in parallel with the resistor RC.
The voltage in the node 129 is the undervoltage (Vundervoltage)
which will be equal to
Vundervoltage = RN / (RN + Req)Vcc
where Vcc is a reference voltage and
Req =1 / ((1/R1)+(1R2) ...+ (1/Ri)+(1/Rc))
where i is the number of outputs V1 - VK brought to the high
value, which depends on the battery voltage.
For example, if
VR1 = Nmax*Vcell-ε
VRK = Nmin*Vcell-ε
- Vcell is the voltage across the terminals of the individual cell
(typically 1.2-1.3 V),
- ε is the tolerance on the battery voltage:
- if the battery consists of Nmin cells, only the output VK
will be at the high value and the remaining outputs will be at the low value;
- if the battery consists of Nmin + 1 cells, the outputs VK,
VK-1 are at the high value, while the remaining outputs are at the low
value and the resistance Rc will be connected in parallel with two resistors
The final voltage at the node 129 will therefore depend on the number
of cells making up the battery, and will be accepted as the minimum voltage (undervoltage)
at which the emergency unit will interrupt the discharge current. This value is
stored by means of the amplifiers 127.1-127.K which form corresponding latch or
storage circuits, so that the value of the undervoltage is not modified during operation
in emergency conditions, despite the fall in voltage across the terminals of the
battery as a result of the gradual running down of the battery.
Other configurations for the battery voltage recognition circuit are
possible. In general, this circuit will carry out a battery voltage reading operation
consisting of the following stages:
- switching on the emergency unit;
- reading the battery voltage, after a wait time (approximately 30-60 s);
- setting the undervoltage;
- storing the undervoltage.
It is to be understood that the drawing shows only a possible embodiment
of the invention, which can be varied in its forms and arrangements without departure
from the scope as defined by the appended claims. The presence of any reference
numbers in the attached claims does not limit the scope of protection of the claims,
and has the sole purpose of facilitating the reading of the claims with reference
to the preceding description and to the attached drawings.