Abstract:
A pulse igniter for a discharge lamp has a transformer with a primary winding coupled to a first capacitor and a switching element and a secondary winding shunted by a series circuit of a diode and a second capacitor at its output. As a result, the generated ignition voltage is rectified and applied to the lamp for a relatively long time interval, resulting in improved ignition behavior.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to a circuit arrangement for igniting a lamp, comprising input terminals for connection to a power supply source, 
     a first capacitive element, 
     a first circuit component coupled to the first capacitive element and to the input terminals for generating a charging current from a power supply voltage supplied by the power supply source, said charging current being used for charging the first capacitive element 
     a transformer having a primary winding and a secondary winding, 
     a first branch shunting the first capacitive element and comprising a series arrangement of the primary winding and a switching element, 
     output terminals coupled to the secondary winding for connecting a load circuit comprising a lamp. 
     The invention also relates to an illumination unit. 
     A circuit arrangement as described in the opening paragraph is known from U.S. Pat. No. 4,342,948. The known circuit arrangement is very suitable for generating a voltage having a relatively high amplitude. In practice, it is often found for such a circuit arrangement that the voltage generated by the circuit arrangement has the shape of an attenuated AC voltage because the leakage inductance of the transformer resonates with the first capacitive element. The frequency of the attenuated AC voltage is often relatively high. Since the transport of charge carriers in the ignited lamp is relatively slow, the relatively high frequency of the attenuated AC voltage has the result that the maximum amplitude of the ignition voltage of the lamp is higher than the maximum amplitude of the attenuated AC voltage across the second capacitive element. The ignition behavior of the lamp is thereby influenced positively. A drawback of the relatively high frequency of the attenuated AC voltage is, however, that the ignition voltage is present across the lamp only for a relatively short time. The frequency of the attenuated AC voltage can be modified by modifying the dimensioning of the circuit arrangement. However, the latter would result in a relatively expensive and voluminous circuit arrangement. In practice, the relatively high frequency of the attenuated AC voltage has the result that some lamps do not ignite or only ignite after a relatively large number of attempts. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to provide a circuit arrangement for igniting a lamp, with which the lamp can be ignited rapidly and effectively. 
     According to the invention, a circuit arrangement as described in the opening paragraph is therefore characterized in that the secondary winding is shunted by a second branch comprising a series arrangement of a unidirectional element and a second capacitive element. 
     During operation of the circuit arrangement, the second capacitive element is charged to a voltage which is equal to the maximum amplitude of the attenuated AC voltage which would be present across the second capacitive element in the absence of the unidirectional element. However, due to the presence of the unidirectional element, the voltage across the second capacitive element in a circuit arrangement according to the invention is a DC voltage instead of an attenuated AC voltage. In theory, an infinitely long voltage pulse is present across the second capacitive element. The DC voltage across the second capacitive element also constitutes the ignition voltage across the lamp. In practice, the voltage across the second capacitive element decreases because it is discharged by means of the leakage current of the unidirectional element and the leakage current of the second capacitive element. Although the maximum amplitude of the ignition voltage across the lamp which is generated by a circuit arrangement according to the invention is relatively low, the period of time during which this ignition voltage is present across the lamp is relatively long. It has been found that lamps of various types and power can be ignited rapidly and effectively by means of the circuit arrangement according to the invention. 
     The switching element may be constituted by a breakdown element which becomes conducting at a given value of the voltage across the first capacitive element. However, it has proved to be advantageous to make use of a switching element provided with a control electrode and to provide the circuit arrangement with a control circuit coupled to the control electrode for rendering the switching element conducting and non-conducting. A very reliable operation of the circuit arrangement can be realized in this way. More particularly, this has been found for embodiments of a circuit arrangement according to the invention in which the switching element is a thyristor. It has also been found that a good control of the conductivity state of the switching element is possible when the control circuit comprises a diac and/or a zener diode. 
     The unidirectional element preferably comprises a diode. 
     In a preferred embodiment of a circuit arrangement according to the invention, the output terminals are connected by means of a third branch comprising a series arrangement of the second capacitive element and the input terminals. The ignition voltage across the lamp is thus formed by the sum of the power supply voltage and the voltage across the second capacitive element. 
     A circuit arrangement according to the invention is very suitable for igniting a discharge lamp having a filling which mainly consists of neon. A discharge lamp having a filling which mainly consists of neon is herein understood to mean a discharge lamp having a filling comprising neon in such a way that red light is generated in the plasma of the lamp during stationary lamp operation, with the color point in the C.I.E. chromaticity diagram being in the range bounded by the lines y=0.300, y=0.350, y=−x+1 and y=−x+0.99. Such a lamp is very suitable for use in an illumination unit used, for example, as a signal light in a motorcar. Such an illumination unit is preferably provided with a housing having a reflecting surface and with means for positioning the lamp in the housing. If the illumination unit is not used as a brake light but, for example, as a blinker, the wall of the lamp is preferably provided with a luminescent coating. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     These and other aspects of the invention will come apparent from and will be elucidated with reference to the embodiments described hereinafter. 
     In the drawings: 
     FIG. 1 shows diagrammatically an embodiment of a circuit arrangement according to the invention, with a load circuit comprising a lamp connected thereto, and 
     FIG. 2 shows diagrammatically an embodiment of an illumination unit according to the invention. 
     FIG. 3 shows a cross-section of the illumination unit. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In FIG. 1, the references K 1  and K 2  denote input terminals for connection to a power supply source. Input terminals K 1  and K 2  are interconnected by means of capacitor C 0 . Capacitor C 0  is shunted by a series arrangement of an ohmic resistor R 3 , a zener diode D 7  and a transistor Q. A common point of zener diode D 7  and transistor Q is connected to a base electrode of transistor Q via a series arrangement of ohmic resistors R 9  and R 12 . The series arrangement of zener diode D 7  and ohmic resistor R 9  is shunted by a capacitor C 1  which constitutes a first capacitive element in this embodiment. Ohmic resistor R 9  is shunted by a capacitor C 3 . A common point of capacitor C 3  and ohmic resistor R 9  is connected to a first end of diac D 9 , and a further end of diac D 9  is connected to a control electrode of thyristor S, which constitutes a switching element in this embodiment. Capacitor C 1  is shunted by a series arrangement of primary winding L 1  of transformer T and thyristor S which together constitute a first branch. A common point of capacitor C 1  and thyristor S is connected to output terminal K 3 . Input terminal K 1  is connected to output terminal K 4  via diode D 1 . Output terminals K 3  and K 4  are connected by means of a series arrangement of an ohmic resistor R 1  and a discharge lamp LA, which series arrangement constitutes a load circuit in this embodiment. Ohmic resistor R 1  constitutes a stabilizing element for limiting the current through the discharge lamp LA. In this embodiment, diode D 1  constitutes a unidirectional element. Diode D 1  is shunted by a series arrangement of secondary winding L 2  of transformer T and capacitor C 2  which constitutes a second capacitive element in this embodiment. Diode D 1  and capacitor C 2  jointly constitute a second branch. R 3  constitutes a first circuit component coupled to the first capacitive element and to the input terminals for generating a charging current from a power supply voltage supplied by the power supply source, which charging current is used for charging the first capacitive element. Ohmic resistors R 3  and R 9 , zener diode D 7 , diac D 9  and capacitors C 1  and C 3  jointly constitute a control circuit coupled to the control electrode of the switching element for rendering the switching element conducting and non-conducting. The series arrangement of capacitor C 2 , secondary winding L 2 , input terminal K 1 , capacitor C 0  and input terminal K 2  constitutes a third branch in this embodiment. 
     The embodiment shown in FIG. 1 operates as follows. 
     If input terminals K 1  and K 2  are connected to a power supply source supplying a DC voltage, capacitor C 1  is charged via resistor R 3  to the zener voltage of zener diode D 7 . After zener diode D 7  has become conducting, capacitor C 3  is charged via ohmic resistor R 3  and zener diode D 7  until the breakdown voltage of diac D 9  is reached. When diac D 9  becomes conducting, a current flows through the control electrode of thyristor S and thyristor S also becomes conducting. Capacitor C 1  is subsequently discharged via primary winding L 1  and thyristor S. Consequently, a voltage is generated between the ends of the secondary winding L 2 , so that capacitor C 2  is charged to a DC voltage. Before the discharge lamp LA ignites, a DC voltage is present across the discharge lamp LA, which is equal to the sum of the power supply voltage and the voltage across capacitor C 2 . If the discharge lamp LA ignites, a current flows from input terminal K 1  to input terminal K 2  via diode D 1 , output terminal K 4 , ohmic resistor R 1 , discharge lamp LA, output terminal K 3 , resistor R 12  and the base-emitter junction of transistor Q. Due to this current, transistor Q becomes conducting so that capacitor C 3  is prevented from being charged to the breakdown voltage of diac D 9 , so that no ignition voltage is generated anymore. 
     FIG. 2 a  in FIG. 2 is a front-elevational view of an illumination unit according to the invention. FIG. 2 b  is a side-elevational view of the same illumination unit. LA is a bent discharge lamp provided with a plasma which consists of neon. The wall of the discharge lamp has a luminescent coating. H constitutes a housing having a rectangular opening. The housing accommodates a mirror reflector R which constitutes the reflecting surface in this embodiment. The rectangular opening of the housing is closed by means of a light-transmissive lid D. In this embodiment, pins P 1 -P 5  constitute means for positioning the discharge lamp in the housing. In FIG. 2 b,  the reference BC denotes an embodiment of a circuit arrangement according to the invention. The coupling between circuit arrangement BC and the lamp LA is shown diagrammatically by means of broken lines. FIG. 2 c  is a cross-section of the illumination unit in accordance with FIG. 2 a  and FIG. 2 b  through the broken line shown in FIGS. 2 a  and  2   b  and perpendicular to the plane in which the discharge lamp LA is bent.