Abstract:
Disconnection device for an electronic operating device for gas discharge lamps. The DC component is evaluated at a coupling capacitor (C 5 ) specifically via a filament. Disconnection is performed in the event of filament breakage. In addition, the AC component of the generator output (O) can be monitored via a second filament, and disconnection can therefore also be performed in the event of breakage of the second filament.

Description:
TECHNICAL FIELD 
     The invention relates to a circuit arrangement for operating one or more low-pressure discharge lamps. In particular, this is a circuit which detects breakage of a filament of a lamp and puts the circuit arrangement into a safe mode. 
     PRIOR ART 
     The service life of a low-pressure discharge lamp fitted with filaments is determined chiefly by the service life of the filaments. If the filaments are consumed, there is firstly an increase in lamp voltage in association with an undesired temperature increase in the filament region of the lamp. The lamp mostly also exhibits a rectifying effect in this stage. Finally, the filament breaks, and this can lead to destruction of the lamp operating device and to dangerous overheating of the lamp ends. Some disconnection devices are known for safe operation of the lamp and protection of the operating device: 
     The lamp voltage is frequently used in order to obtain a criterion for disconnecting the operating device, (for example, EP 0 809 923). However, even in normal operation, the lamp voltage is subject to strong fluctuations, and so it is impossible in many cases to specify a unique threshold at which disconnection is to be undertaken. The operating device mostly includes what is termed a coupling capacitor, which absorbs the direct component of the output voltage of the AC voltage generator included in the operating device. The voltage across the coupling capacitor is used in U.S. Pat. No. 5,493,181 to detect the abovementioned rectifying effect of the lamp. It is necessary in this case to arrive at a quantitative statement on the value of this voltage and compare it with a threshold. It is also valid here that the value of the voltage to be measured is subject in normal operation to strong fluctuations, and so it is frequently impossible to specify a unique threshold. Reliable disconnection is therefore impossible in many cases, or very complicated technically. 
     It has also emerged that monitoring the filaments with regard to breakage suffices in order to be able to ensure reliable operation of the system of lamp and operating device. In known solutions, it is detected whether a DC test current can flow through the filaments to be tested (DE 3805510). The disadvantage of this method is that the test current flows in addition to the current required for normal operation, and thus constitutes an additional load for the filaments. 
     During dimmed operation, in particular, the filaments are subjected to an additional heating current, over and above the current for the gas discharge. There are solutions for detection of filament breakage which monitor the presence of the additional heating current (EP 0 422 594). However, the additional heating current is frequently very small compared with the current for the gas discharge for which reason detection is complicated and unreliable. 
     SUMMARY OF THE INVENTION 
     It is the object of the present invention to provide a disconnection device for an operating device which accomplishes reliable disconnection of the operating device in the event of breakage of a filament, doing so with a low outlay. 
     This object is achieved in the case of a device having the features of the preamble of claim  1  by means of the features of the characterizing part of claim  1 . Particularly advantageous refinements are to be found in the dependent claims. 
     Many operating devices for gas discharge lamps contain an AC voltage generator which outputs at its output a voltage which has a direct component. A half-bridge circuit which includes two controlled switches connected in series can be used to implement the AC voltage generator. However, it is mostly lamps which ought not to conduct direct current which are operated with the aid of these operating devices. Consequently, in addition to other components, the lamp is connected as a rule to the AC voltage generator via what is termed a coupling capacitor. It is important for the disconnection of the operating device according to the invention that the current for the gas discharge of the lamp is fed at only one end of a filament. The coupling capacitor absorbs the DC voltage component of the AC voltage source. This DC voltage component can be filtered out via an averaging unit for the purpose of disconnecting the operating device in accordance with the invention. A simple design of the averaging unit is a first-order low pass filter which, in the simplest case, comprises only a resistor and a capacitor. The DC voltage component of the coupling capacitor is now fed to a circuit part (denoted by SD below) which is responsible for the disconnection and has a threshold characteristic at its input. It is important that this feeding takes place via a filament. In the event of filament breakage, the DC voltage component of the coupling capacitor is absent at the input of the circuit part SD. The threshold characteristic at the input of the circuit part SD need only be capable of detecting the DC voltage component of the coupling capacitor. This can be implemented very reliably without great outlay. However, it is to be noted that apart from the direct component of the coupling capacitor no further DC voltage component is fed to the input of the circuit part SD. 
     The threshold characteristic can be implemented by a transistor. If a voltage is present at its input, it prevents charging of a capacitor (denoted below by C 7 ) which is connected, for example, via its output terminals. If, in the event of filament breakage, there is no input voltage, the capacitor C 7  is charged up and triggers disconnection of the operating device. The capacitor C 7  is discharged when the operating device is taken into use. It thereby prevents an undesired disconnection during the starting operation of the lamp. The value of the capacitance of the capacitor C 7  must be selected so large that disconnection can be triggered only after the DC voltage component at the coupling capacitor has stabilized in event of an intact lamp. If the DC voltage component is established, this is also an indication that the lamp has started properly. The DC voltage component at the coupling capacitor can therefore also be used to detect “lamp burning”. 
     The disconnection of the operating device can be performed by a further controlled switch. When the further switch is triggered, the above-named AC voltage generator is turned off. This can be performed in various ways. Mostly, an auxiliary voltage is required to generate trigger signals in the AC voltage generator. With the aid of said further switch, the auxiliary voltage of the AC voltage generator can be suppressed, thereby achieving disconnection of the operating device. Some AC voltage generators have a separate input at which a signal must be present in order to disconnect the output signal of the AC voltage generator for safety purposes (safety disconnection signal). This safety disconnection signal can also be suppressed with the aid of said further switch for the purpose of disconnection. 
     The above-described circuit arrangement according to the invention for detecting filament breakage is suitable first and foremost for only one filament or for filaments of a plurality of lamps which are connected in parallel and are all at the same potential. If, additionally, filaments are to be monitored which are at a different potential, this can be done in a different way including using methods which are already known from the prior art. In order to be able to ensure absolutely safe operation of a lamp, it is necessary to monitor all the filaments, since it cannot be foreseen which filament will break first. Since the filaments belonging to a lamp are at very different potentials, particularly in the case of starting, it is not possible, as a rule, to apply cost-effective implementations of the filament monitoring to all the filaments simultaneously. In this context, filament monitoring according to the invention permits combination with other monitoring methods. Thus, for example, filaments which are not monitored according to the invention by detecting the DC voltage component at the coupling capacitor can be monitored in a different way. If the AC voltage generator requires an auxiliary voltage, this can be conducted via the filaments which have not as yet been monitored. In the event of breakage of these filaments, feeding of the auxiliary voltage is interrupted and the AC voltage generator is disconnected. 
     A further possibility for monitoring filaments not monitored so far consists in detecting the AC voltage components at a lamp terminal. As in the case of the detection of the DC voltage component, the current for the gas discharge of the lamp is fed only at one end of the filament to be detected. The AC voltage present at the other end of this filament is coupled out via a capacitor. If the filament breaks, the amplitude of the coupled-out AC voltage is substantially reduced. This can be utilized according to the invention in order to permit the capacitor C 7  to be charged to a value which, as described above, leads to disconnection of the operating device. This is preferably performed by disturbing the discharge of the capacitor C 7  by means of a further controlled switch. 
     In addition, the following requirement is frequently placed on the disconnection of an operating device: if the lamp is changed after completed disconnection, the aim thereby is to reverse the disconnection and permit operation of the new lamp. This is accomplished according to the invention by virtue of the fact that the charging current of the capacitor C 7  is conducted via one or more filaments. If the lamp is removed, the capacitor C 7  is discharged. If the voltage across the capacitor C 7  undershoots a prescribed value, the disconnection is reversed. 
     Implementing this inventive idea requires a distinction to be made between AC voltage generators which are externally excited and those which are self-excited. Externally excited AC voltage generators have for the purpose of triggering the circuit breaker an oscillator which requires an auxiliary voltage. In order to detect breakage of a filament in a way not performed by detecting DC voltage across the coupling capacitor, it is possible, as described above, for said auxiliary voltage to be conducted via the filament to be checked. The charging of the capacitor C 7 , whose voltage is used for the disconnection, can also be performed via the same filament. Firstly, in the event of breakage of this filament, the oscillator is turned off, and thus the operating device is disconnected; secondly, in the event of a change of lamp, charging of the capacitor C 7  is interrupted and the disconnection is thereby reversed. 
     Self-excited operating devices do not have a separate oscillator. The trigger signal for the circuit breaker is obtained from the load circuit. Consequently, there is no possibility of disconnecting the oscillator by means of interrupting the auxiliary voltage in the event of filament breakage. According to the invention, in this case, in the event of breakage of the filament which is not being monitored by the DC voltage level across the coupling capacitor, disconnection can be performed by means of the above-explained detection of the AC voltage component. However, this filament should not then bear the charging current of the capacitor C 7  on its own. The breakage of the filament would then certainly be detected, but the subsequent charging of the capacitor C 7  would be interrupted, for which reason no disconnection would come about. Consequently, according to the invention, both lamp filaments are used in order to provide the charging current for the capacitor C 7 . Irrespective of which filament breaks, it is therefore ensured that a charging current which leads to disconnection is provided for C 7 . This AND operation of the filament currents is achieved by virtue of the fact that the lamp terminals which are not fed by the AC voltage generator are connected in each case to the capacitor C 7  via a diode. 
     In this context, it is necessary to mention another aspect of the operating device with self-excited AC voltage generator. In particular, in the case of the AC voltage generator with a half bridge, importance attaches as to which state of charge the capacitors have on the occasion when the circuit breaker is first closed. The capacitors must be charged such that this first closing of a circuit breaker effects a flow of current which brings about the self-excitation of the AC voltage generator. The charge relationships of the capacitors can be displaced before starting the AC voltage generator by means of the two said diodes for AND operation. If appropriate, it is necessary to modify the starting circuit whose task is to close one of the two half-bridge switches once. This modification can be such that it is no longer the lower half-bridge switch, but the upper half-bridge switch which is first closed. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The aim below is to explain the invention in more detail with the aid of a plurality of exemplary embodiments. In the drawing: 
     FIG. 1 shows a circuit diagram of an operating device for a gas discharge lamp with disconnection according to the invention in the event of breakage of one of the two filaments, with an externally excited AC voltage generator, and 
     FIG. 2 shows a circuit diagram of an operating device for a gas discharge lamp with disconnection according to the invention in the event of breakage of one of the two filaments, along with a self-excited AC voltage generator, and 
     FIG. 3 shows a circuit diagram of an operating device for a gas discharge lamp with disconnection according to the invention in the event of breakage of one of the two filaments, along with an externally excited AC voltage generator, and an increased interference immunity. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Capacitors are denoted below by the letter C, resistors by R, inductors by L, transistors by T and diodes by D, followed by a number in each case. 
     The operating device in FIG. 1 is designed for operating on an AC voltage network. The system voltage of, for example, 230 Veff is connected to the terminals AC 1  and AC 2 . D 1 , D 2 , D 3  and D 4  form a full-wave rectifier which makes available at its outputs P (positive) and M (frame) a DC voltage which is termed supply voltage below. The capacitor C 1  is connected between P and M in order to smooth the supply voltage. An AC voltage generator G draws its energy via P and M. The AC voltage generator G makes available at the output O an AC voltage with a direct component for operating a gas discharge lamp. The AC voltage generator G requires an auxiliary voltage H. The auxiliary voltage H is derived directly from the supply voltage via R 1  only for starting purposes. For operation the auxiliary voltage H is generated via C 3 , which is connected at the terminal J 2  of the filament W 1 . D 5 , D 6  and C 2  serve to rectify and stabilize the AC voltage fed in via C 3 . The lamp inductor L 1  connects the output O of the AC voltage generator G to the lamp filament W 1  at the terminal J 1 . The circuit for the gas discharge current through the lamp Lp is connected to frame M by the filament W 2  at the terminal J 3  via the coupling capacitor C 5 . On the side of the lamp not connected to the AC voltage generator G, the resonance capacitor C 4  is connected to the filament W 1  at the terminal J 2  and to the filament W 2  at the terminal J 4 . 
     A circuit part SD including the following components serves the purpose of disconnection: T 3 , R 2 , D 7 , T 4 , C 7 , R 5  and R 6 . The base of T 4  is connected to the input EDC of SD. The emitter of T 4  is connected to frame M. C 7  is connected between the emitter and collector of T 4 . The voltage at the collector of T 4  is fed to the gate of T 3  via a Zener diode D 7 . D 7  points with the cathode to T 4 . T 3  is connected with the source to frame M. The gate of T 3  is connected to frame M via R 2 . The drain of T 3  is connected to the auxiliary voltage terminal H of the AC voltage generator G. R 5  and R 6  form a voltage divider. The voltage divider is connected to frame M at the end of R 6 . The collector of T 4  is connected at the connecting point of R 5  and R 6  and therefore so is C 7 . The charging current for C 7  is fed into the end of R 5  of the voltage divider. This is performed via the filament W 1  and R 7  from the positive pole P of the supply voltage. In normal operation, the potential at the input EDC of SD is so large (&gt;0.7 V) that T 4  is in the conducting state. Consequently, C 7  remains discharged and the potential at the collector of T 4  is so low that the Zener diode does not conduct in the reverse direction. If the potential of EDC drops so far (&lt;0.7 V) that T 4  goes over into the blocking state, C 7  is charged via R 7 , the filament W 1  and R 5 . As soon as the voltage across C 7  is so high that D 7  starts to conduct in the reverse direction, T 3  is triggered and goes over into the conducting state. This short-circuits the auxiliary voltage H of the AC voltage generator G, and thereby disconnects the operating device. 
     The input EDC of SD is controlled from the connecting point of R 3  and R 4 . The other terminal of R 4  is connected to frame M and the other terminal of R 3  is connected to the terminal J 4  of the filament W 2 . C 6  is connected in parallel with R 4 . This circuit arrangement comprising R 3 , R 4  and C 6  acts as a low pass filter. The DC voltage component of the voltage present at C 5  is therefore conducted via the filament W 2  to the input EDC of SD. Consequently, in normal operation the potential at the input EDC is so high that the operating device is not disconnected. If the filament W 2  breaks, there is no longer a DC voltage at the terminal J 4  of the filament W 2 , the potential at the input EDC drops below the threshold at which T 4  is still in the conducting state, and the operating device is disconnected. In the case of a change of lamp, the charging current of C 7  is interrupted because of the lack of the filament W 1 . The potential at the collector of T 4  drops, T 3  blocks and the AC voltage generator is resupplied with the required auxiliary voltage (H) for restarting purposes. 
     In the event of a breakage of the filament W 1 , the auxiliary voltage H required to operate the AC voltage generator G and fed via C 3  is interrupted, and the operating device is thereby disconnected. 
     FIG. 2 shows an exemplary embodiment of the disconnection according to the invention by means of detecting filament breakage in the case of an operating device with a self-excited AC voltage generator G. The device is supplied with a DC voltage via the terminals DC+ and DC−. This corresponds to the supply voltage of FIG.  1 . The series circuit of two semiconductor switches T 6  and T 7 , which are designed here as MOSFETs, is connected between DC+ and DC−. The connecting point between transistors forms the output O of the half bridges implemented by the semiconductor switches T 6  and T 7 . The load current led off at the output O is detected by a feedback arrangement FB and fed respectively to a trigger circuit DR 1  and DR 2  for the semiconductor switches T 6  and T 7 . The trigger circuits DR 1  and DR 2  are respectively connected between the gate and source of the semiconductor switches T 6  and T 7 , and alternately effect closing and opening of these semiconductor switches, as a result of which an AC voltage affected with reference to DC− by a DC voltage component is present at the output O of the half bridge. The circuit elements R 20 , D 20 , D 21  and C 20  serve the purpose of starting the half-bridge oscillation for the first time. The series circuit of R 20  and D 20  is connected between DC+ and the half-bridge output O. The diac D 21  is connected to the connecting point. The other end of the diac D 21  is connected to the gate of the upper half-bridge transistor T 6 . C 20  is charged via R 20  when the device is started. If the voltage across C 20  exceeds the trigger voltage of the diac D 21 , the upper half-bridge transistor T 6  is triggered and the oscillation of the half bridges is started. Discharging of C 20  during operation is ensured via D 20 . 
     The circuit elements L 1 , C 4 , C 5 , C 6 , C 7 , J 1 , J 2 , R 2 , R 3 , R 4 , R 6  and D 7  are connected identically to those in FIG.  1 . By comparison with FIG. 1, T 3  is designed as a bipolar transistor. The collector of T 3  is connected to the gate of the lower half-bridge transistor (T 7 ) via the diode D 26 . If T 3  is triggered, a current which suppresses the triggering of T 7  flows via D 26 . The resistor R 5  is not, as in FIG. 1, connected directly to the terminal J 2  of the filament W 1 . Rather, it is connected both to J 2  and to the terminal J 4  of the filament W 2  with the aid in each case of a series circuit of a resistor and a diode (R 21 , D 22 , R 22 , D 23 ). The above-described AND operation of the charging current of C 7  is implemented thereby. 
     The AC voltage input EAC of the circuit part SD is also connected to J 2  via C 21 . C 21  conducts only the AC voltage component of the potential at J 2  to EAC. Downstream thereof is a voltage divider composed of the resistors R 25  and R 26  between EAC and DC−. The anode of D 25  and the cathode of D 24  is connected to the connecting point of R 25  and R 26 . The anode of D 24  is at the low potential of the supply voltage (DC−) and is required in order to evaluate the negative component of the AC voltage at EAC. The cathode of D 25  is connected to the capacitor C 22 . The other terminal of C 22  is at the low potential of the supply voltage (DC−) and C 22  serves to integrate the AC voltage rectified by D 24  and D 25  and present at EAC. The voltage present at C 22  is fed to a voltage divider, formed from the resistors R 27  and R 28 . The connecting point of R 27  and R 28  is connected to the base of transistor T 5 . Otherwise than in FIG. 1, in FIG. 2, the emitter of transistor T 4  is not connected to the low potential of the supply voltage (DC−) directly but via the collector-emitter path of T 5 . In the event of the absence of an AC voltage at EAC, T 5  and thus also T 4  are no longer triggered, as a result of which C 7  can be charged and disconnection is triggered. 
     A variant of the circuit diagram of FIG. 1 is illustrated in FIG.  3 . The signal from the coupling capacitor C 5  is occasionally subjected to substantial interference. The cause of this interference is frequently the sporadic contact which a filament already broken per se keeps remaking. This interference is counteracted by the extension in FIG. 3 with reference to FIG.  1 . The connection between the capacitor for averaging C 6  and the base of T 4  is no longer direct, but via the series circuit of R 31  and the emitter-collector path of the transistor T 31 . The collector of T 31  is connected to the base of T 4  and, for the purpose of further suppression of interference, to frame (M) via the parallel circuit of R 34  and C 31 . The base of T 31  is connected to frame (M) via R 33  and to the positive pole (P) via R 32  and R 35 . This circuit is used to evaluate only signals at the coupling capacitor C 5  which, with reference to the voltage at the positive pole (P), exceed a value set by the resistance values R 3 , R 4 , R 5 , R 6 , R 32 , R 33 , R 35 . If it is not desired for the evaluated signals to be a function of the voltage at the positive pole (P), a Zener diode between C 6  and the base of T 4  also suffices instead of the transistor T 31 . 
     A further variation in FIG. 3 with reference to FIG. 1 is the terminal of R 5 . It is not, as in FIG. 1, connected to the terminal J 2  of the filament W 1 , but to the positive pole (P), via R 35 . As a result, the disconnection is not reversed upon exchange of the lamp, but only in the event of a system interruption.