Abstract:
A control gear for a fluorescent lamp, comprising a voltage source and an ignition and supply circuit comprising an inductive component and a capacitance. The control gear also comprises a transformer whose primary coil is connected parallel to the capacitance, and a switching element which is connected to the secondary coil of a transformer for short-circuiting it.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a control gear for a fluorescent lamp, comprising a voltage source and an ignition and supply circuit comprising an inductive component and a capacitance. 
     Fluorescent lamps are generally used owing to their good lighting power. In addition, the long operating life of fluorescent lamps and the various tones of color available enable their use in various applications. 
     A control gear is required for burning fluorescent lamps, the control gear supplying the ignition voltage necessary for the fluorescent lamp and the supply voltage necessary during its use. When using electronic control gears, a problem arises from the fact that a lamp circuit has very high voltages which set extremely high demands on the switches used in the lamp circuit. Due to the high voltage levels, it is possible that the lamp ignites prematurely before the electrodes of the lamp are heated enough. This may lead to quick wear of the electrodes and uncertain ignition. In addition, in present prior art solutions, there are significant problems in the control of the switches in the lamp circuit and in determining the working condition of the lamp, which are due to said high voltage levels. 
     When a long ignition durability is required in fluorescent lamps, they should be ignited so that heating voltage is first switched on in the heater circuits for approximately one second so that they will reach the temperature required for a thermal electron emission. Only after this, an ignition voltage is switched on over the lamp, generating an arc discharge in the filler gas of the fluorescent lamp. 
     The above-mentioned function can also be performed by preventing the fluorescent lamp from igniting during preheating by short-circuiting it with a switch. The conventional use of a series choke and a glow discharge igniter in 50 Hz mains voltage use, for instance, is based on this method. 
     The problem in this case is that the used switch is greatly loaded due to the high voltages in the lamp circuit. Another drawback in the solution in question is that the switch over the lamp and its control electronics cannot be galvanically separated from the high-voltage lamp circuit. 
     BRIEF DESCRIPTION OF THE INVENTION 
     It is an object of the present invention to provide a control gear for a fluorescent lamp, which avoids the above drawbacks and makes it possible to keep the fluorescent lamp unignited during preheating of the electrodes and to measure the voltage over the lamp with a simple apparatus while the lamp is operational. This object is achieved by a fluorescent lamp control gear of the invention, characterized in that the control gear also comprises a transformer whose primary coil is connected parallel to a capacitance, and a switching element which is connected to the secondary coil of the transformer for short-circuiting it. 
     The control gear of the invention is based on the idea that by adding to the lamp circuit a transformer connection whose secondary coil can be opened and shut with a switch, it is possible to ensure that the preheating of the fluorescent lamp is long enough. The transformer connection also makes it possible to monitor the condition of the fluorescent lamp in a simple manner by monitoring the magnitude of the voltage in the secondary coil of said transformer. 
     The fluorescent lamp control gear of the invention provides significant advantages with respect to the reliability of the lamp ignition, because the preheating of the fluorescent lamp is arranged by means of the control gear of the invention so that the lamp cannot ignite before the electrodes are heated. Another significant advantage provided by the control gear is the possibility to monitor the condition of the lamp by using simple voltage measurement from a voltage that is considerably lower than the actual voltage of the lamp circuit. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES 
     In the following, the invention will be described by means of preferred embodiments and with reference to the attached drawings, in which 
     FIG. 1 shows a circuit diagram illustrating the principle of a control gear of the invention having a transformer feed, and 
     FIG. 2 shows a circuit diagram illustrating the principle of a control gear of the invention having a half-bridge feed. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the embodiment of the invention shown in FIG. 1, the voltage feed of the lamp circuit is a DC/AC chopper to whose output the primary coil of a transformer T 1  is connected. The secondary coil of the transformer T 1  forms a part of the lamp circuit so that the secondary coil is connected parallel to a fluorescent lamp. A capacitance C 1  is connected to the second poles of the fluorescent lamp electrodes as shown in the figure. The inductive component, i.e. the distributed inductance of the transformer T 1 , in FIG. 1 forms together with the capacitance C 1  a resonant circuit enabling the generation of the voltage required to ignite the fluorescent lamp. 
     According to the invention, a transformer T 2  is connected parallel to the capacitance C 1  of the lamp circuit. The primary coil N 1  of the transformer T 2  is connected to the second poles of the capacitance and the Lamp According to the invention, a switching element S 3  for opening and closing the secondary circuit of the transformer is connected to the secondary coil of the transformer. A control block Ctrl is arranged to control the switching element S 3 . 
     In the embodiment in FIG. 2, the voltage supply in the control gear of the invention is formed by a half-bridge connection made up of a control circuit A, switches S 1 , S 2  and diodes D 1 , D 2 . Such a high-frequency half-bridge-connected chopper voltage source enables the supply of alternating voltage to the lamp circuit in a simple manner. In a half-bridge connection, the state of the switches S 1 , S 2  is changed at a high frequency to achieve the desired voltage to burn the fluorescent lamp. By changing the pulse ratios of the switches, the magnitude of the voltage fed to the fluorescent lamp can be altered. According to the invention, an inductive component L 1  is connected to the half-bridge output, i.e. to the point between the switches S 1 , S 2  and the diodes D 1 , D 2 . In the embodiment of the invention in FIG. 2, the inductive component L 1  is an inductance which forms a series choke for the fluorescent lamp Lamp. The second pole of the inductance L 1  is connected to the second electrode of the fluorescent lamp. 
     According to the invention, a capacitance C 1  and, parallel to it, the primary coil N 1  of the transformer T 2  are connected between the second poles of the fluorescent lamp electrodes. According to the invention, the switching element S 3 , which is controlled by the control block Ctrl, is connected to the secondary coil of the transformer T 2 . The inductance L 1  and the capacitance C 1  form a series-resonant circuit with which the voltage required for igniting the fluorescent lamp can be generated. In the embodiment of FIG. 2, the second pole of the second electrode in the fluorescent lamp is connected to the point between the capacitances C 2  and C 3 . The second poles of the capacitances connected in series are connected to the operating voltage and zero potential. 
     The operating principle of the control gear of the invention is that when the gear is switched on, a power unit supplies a voltage having a suitable frequency in comparison with the resonance frequency of the inductive component L 1  and the capacitance C 1  in the control gear. During preheating, the switching element S 3  of the transformer T 2  secondary coil is closed so that the transformer loads the lamp circuit and a preheating current, which is considerably higher than in normal use, flows through the lamp electrodes, and no voltage resonance can generate in the resonant circuit. The preheating stage of the lamp, i.e. the time during which the switching element is kept closed, lasts approximately one second to allow the temperature of the electrodes to rise sufficiently high for the generation of a thermal electron emission. 
     When the preheating stage is over, the switching element S 3  connected to the secondary coil of the transformer T 2  is opened, whereby a resonance voltage is generated in the resonant circuit formed by the inductive component Li and the capacitance C 1  and the fluorescent lamp connected to the lamp circuit can be ignited. The transformation ratio of the transformer T 2  is designed in such a manner that the number of coil turns N 2  in its secondary coil is considerably smaller than the number of coil turns N 1  in the primary coil. This way, a switch having a low voltage tolerance can be used as the preheating switch, i.e. the switching element S 3 . 
     After the preheating switch has opened, the state of the fluorescent lamp connected to the lamp circuit can be monitored by measuring the voltage in the secondary coil of the preheating transformer T 2 . Due to the transformation ratio of the transformer, the voltage measured from the secondary coil is low. In the embodiments of the figures, the voltage data is transmitted to the control electronics which perform the necessary control action on the basis of the transmitted data. It is easy to determine the working condition of the fluorescent lamp being controlled from the level of the voltage measured from the secondary coil of the transformer T 2 . When the measured voltage exceeds a predefined value, the lamp does not ignite. If the voltage is zero, either the heater circuit of the lamp is broken or there is a break in the wiring of the lamp. In both above-mentioned cases, the chopper supplying the voltage can be switched off. 
     It is obvious to a person skilled in the art that the basic idea of the invention can be implemented in many different ways. Thus, the invention and its embodiments are not restricted to the examples described above, but may vary within the scope of the claims.