Device for controlling the light intensity of a fluorescent tube fed from a D.C. voltage

A device for controlling the luminous intensity of a fluorescent tube supplied with a direct voltage comprises a stabilized supply, a ramp generator, a modulator receiving the ramp signal responsive to a control means sending thereto a voltage for generating a chopped signal whose chopping rate is determined by the control means. The signal is sent to the converter of the tube through an interface circuit. The device is applicable to the regulation of the intensity of tubes on an airship, automobile or portable lamp.

The present invention relates to a device for controlling the luminous 
intensity of a fluorescent tube supplied with a direct voltage as, for 
example, a fluorescent illuminating tube of an airship, an automobile or a 
portable lamp working off a battery. 
It is known that, for the supply of fluorescent tubes from a DC network, 
converters are used which are supplied by the network and usually 
comprises a power oscillator whose output is connected to the primary 
winding of a transformer whose secondary winding is connected to the tube. 
The restrictions imposed by the operation of the fluorescent tube in 
practice do not allow the control of the luminous intensity of the tube by 
a modification of the supply conditions. Now, it is highly desirable to be 
able to control and modify the luminous intensity of a fluorescent 
illuminating tube supplied from a DC network in many applications. 
There has already been described in patent application PCT/81/00184 a 
process for controlling a fluorescent lamp supplied with direct current, 
in which, for the purpose of reducing the luminous intensity, the mean 
value of the duration of pulse trains is decreased by piloting a 
transistorized switch which interrupts and allows in an alternating manner 
the supply of the converter of the lamp. Such an arrangement is 
complicated and is not easily applicable to the case of the control of a 
plurality of lamp ballasts by the same control voltage. 
There has also been described, in U.S. Pat. No. 4,251,752, a variable 
cyclic ratio control but which is applied directly to the operating 
frequency of the ballast, with the associated drawbacks. 
An object of the present invention is consequently to provide a device for 
controlling the luminous intensity of a fluorescent tube supplied from a 
direct voltage, which is capable of being adapted to all the needs of the 
user and to many different configurations of converters. 
Another object of the invention is to provide such a device which is 
capable of employing control means which are simple, easy to arrange, but 
little sensitive to parasites and non-disturbing. 
Yet another object of the invention is to provide such a device which while 
it is capable of being adapted to many types of converters, conserves an 
acceptable efficiency even at a low rate of operation, i.e. for low values 
of the luminous intensity. 
A still further object of the invention is to provide such a device which 
is simple and cheap in construction. 
The invention has for subject a device for controlling the luminous 
intensity of a fluorescent tube supplied from a direct voltage through a 
converter, in which the duration of voltage pulses allowing the ignition 
of the tube is varied, characterized in that it comprises a control means 
and means for generating a chopped signal with a chopping rate determined 
by said control means, said chopped signal being sent to a responsive 
element of the converter so as to block temporarily and in a repeated 
manner said converter at the rhythm of said chopped signal. 
Advantageously, the blocking of the converter at a variable cyclic ratio 
thus achieved is effected at low frequency, which facilitates the 
adaptation to many types of converters and permits retaining an acceptable 
efficiency even at a reduced rate of operation. 
The repetitive blocking of the converter results in a corresponding 
blocking of the fluorescent tube in a brief and repetitive manner, the 
variation in the luminous intensity thus occurring without visual 
inconvenience, without flashing and without stroboscopic effect. 
In a preferred embodiment, the control means is a means permitting the 
establishment of a direct control voltage, for example a variable 
potentiometer. This potentiometer may be supplied for example from the 
direct voltage supply of the network and has at least two positions, 
namely a position permitting the operation at full power of the 
fluorescent tube and a position allowing only operation at low power. 
The device then comprises advantageously a ramp generator and a modulator 
mainly comprising a comparator one of the inputs of which receives the 
signal from the ramp generator whereas there is introduced on its other 
input said direct control voltage so as to generate a chopped signal of 
constant potential whose chopping rate varies as a function of the value 
of the introduced control potential, the chopping rate being zero when 
this potential is equal to the peak potential of the ramp generator and 
becoming greater as this potential is decreased. The chopped signal 
issuing from the modulator is advantageously sent to a converter blocking 
circuit. The latter may advantageously comprise a transistor whose base 
receives the chopped signal and which permits, when it is thus rendered 
conductive, earthing a power transistor of the converter, for example a 
transistor of a power oscillator of the converter. 
Preferably, the device comprises a supply circuit stabilized by the D.C. 
network so as to permit a stable operation of the whole over a wide range 
of variation of the supply voltage of the network. 
According to an improvement of the invention, the control means may be so 
designed that it does not send a voltage lower than a certain threshold so 
as to avoid blocking the converter beyond the limit permitting a stable 
operation of the fluorescent tube.

The supply line 1 of the dc network at voltage +Vcc, for example 25 volts, 
supplies power to a converter of a fluorescent tube comprising a usual 
power oscillator 2, for example constructed around a single transistor 
whose collector is charged by the tuned primary winding of a leak 
transformer 3, the secondary winding of the transformer comprising high 
tension and heating windings of the fluorescent tube 4, the choke of the 
high-tension winding performing the function of current regulating ballast 
in the tube. The oscillator is for example designed to have an operating 
frequency of the order of 80 kHz so as to reduce the size of the large 
components such as capacitors and transformers. 
The control device according to the invention comprises first of all a 
supply circuit 5 stabilized for example at 15 volts with a resistor 6, a 
15 volt Zener diode 7 and a transistor 8 whose emitter delivers the 
stabilized voltage of 15 volts. The latter supplies a ramp generator 
generally designated by the numeral 9 and a modulator generally designated 
by the numeral 10. 
The ramp generator, which is of usual type, comprising in particular an 
integrated circuit, for example of the type 555, sold by National 
Semi-Conductor, and set to 200 Hz, is connected to the input (2) of a 
comparator 12 which is part of the modulator 10. The other input (3) of 
the comparator receives a conductor 13 in which may be introduced a 
variable direct voltage Uc by a suitable control means (not shown), for 
example a system having three positions, namely stop, operation at 100% 
and operation at 25%. The input (3) of the comparator 12 is filtered by an 
RC circuit and protected by two diodes 14, 15. The output 16 of the 
comparator is connected to a blocking interface 17 of the oscillator 
comprising in particular a transistor 18 whose collector is connected to 
the base of the power transistor (not shown) of the oscillator 2 so that, 
when the required voltage is applied to the base of the transistor 18, the 
latter is conductive and earths the transistor of the oscillator 2. 
Advantageously, the blocking circuit 17 may be connected to the supply 
network 1 through a resistor 19 and also comprise RC filtering means. 
The operation is then as follows: 
When the voltage Uc applied by the control means is lower than the initial 
potential of the ramp (3.5 V), i.e. if Uc&lt;3.5 V, the comparator 12 
generates a constant voltage at its output 16 irrespective of the ramp 
potential which is always &gt;3.5 V. Under these conditions, the base of the 
transistor 18 is constantly excited and the transistor 18 is rendered and 
maintained conductive. The power oscillator consequently cannot operate 
and the fluorescent tube remains extinguished. If thereafter, there is 
applied by the control means a voltage Uc higher than 9.5 V, the final 
ramp potential, for example 10 V or more, the output 16 of the comparator 
generates no signal. The chopping rate is zero, the power oscillator 
operates permanently and the fluorescent tube is supplied at 100%. 
If thereafter there is introduced an intermediate voltage Uc, for example 
25% (5 V), the comparator 12 will emit a chopped signal whose chopping 
rate, i.e. the ratio of the duration of zero potential of the signal to 
the total duration of the signal, i.e. the period of the ramp, will be 
1/4. This signal sent by the conductor 16 to the transistor 18 will thus 
block the converter at a frequency of 200 Hz during each time the 3/4 of 
the period corresponding to this frequency. Consequently, the luminous 
intensity in the fluorescent tube will be about 1/4 of the maximum 
intensity. 
Of course, the invention may have many variants. First of all, the control 
means adapted to introduce a control potential of Uc may be piloted 
manually, or, on the contrary, under the control of some system. It may be 
constituted very simply by a switch having three positions permitting the 
setting for example of zero potential, a potential exceeding 100% of the 
peak of the ramp and an intermediate potential, for example at 25%. It may 
also be constituted by a mechanically controlled potentiometer which may 
be set at discrete values or, on the contrary, varied in a continuous 
manner. It may also be constituted by an electronic circuit permitting the 
introduction of a given potential Uc as a function of a manual or 
automatic instruction. 
The other illustrated electronic means may also be replaced by equivalent 
means available to one skilled in the art. The interface 17 will be 
constructed as a function of the nature and characteristics of the 
converter it must pilot, the interface 17 possibly being advantageously 
constituted by an interchangeable or adjustable module. 
Further, it is of interest, according to the invention, to prevent the 
illumination of the fluorescent tube for high chopping rates, for example 
higher than 75%, i.e. for values at which the stable operation of the 
fluorescent tube can no longer be ensured. This may be achieved for 
example by rendering the input (3) of the comparator 12 only responsive to 
a potential which is equal to or higher than 5 V so as to obtain a 
forbidden zone shown by the cross-hatching in FIG. 2.