Electric lamp and discharge devices: systems – Periodic switch in the supply circuit – Impedance or current regulator in the supply circuit
Reexamination Certificate
2001-01-23
2002-08-13
Vu, David (Department: 2821)
Electric lamp and discharge devices: systems
Periodic switch in the supply circuit
Impedance or current regulator in the supply circuit
C315S106000, C315S219000, C315S276000
Reexamination Certificate
active
06433490
ABSTRACT:
BACKGROUND OF THE INVENTION
Field of the Invention
The present invention relates to an electronic ballast for a low-pressure discharge lamp having an inverter which is arranged to be connected to a direct current voltage source, a load circuit which is connected to the inverter and which is configured to contain a lamp and a series resonant circuit, and also including an evaluating circuit arrangement which reacts to different operating states of a lamp and in the case of a defect or removal of such lamp, generates corresponding signals for switching off the inverter and that has a circuit arrangement for identifying a lamp change or a lamp defect.
A ballast having such a circuit arrangement is known, for example, from EP 0 146 683 B1. The resonant capacitor of the series resonant circuit in this case is arranged between the two electrodes of the discharge lamp, thereby making it possible for the electrodes to be preheated before the lamp is ignited. Furthermore, the ballast has a bistable switching device with an operating state and an off-state, with the switching device, in the case of a non-igniting discharge lamp, tripping into the off-state and switching off the inverter. The function of this circuit arrangement is based on the fact that the amplitude of the current flowing by way of the load branch with the lamp in the case of a lamp that is not ignited is substantially greater than in the case of a lamp that is ignited. A holding-current circuit that is run by way of one of the electrodes of the discharge lamp then holds the bistable switching device in this off-state for so long until it is interrupted by the insertion of a new lamp, thereby automatically initiating a restart of the lamp.
A disadvantage of this circuit arrangement though lies in the fact that even after the ignition of the lamp a parallel current flows by way of the resonant capacitor and by way of the two coils of the lamp. During normal operation of the lamp this parallel current signifies lost energy and impairs the lamp's illuminating power or the degree of efficiency. Furthermore, in the case of this ballast it is not possible to regulate the heat output independently of the lamp current, something which can be regarded as being disadvantageous in particular during a dimmed operation of the lamp, since the reduction in current brought about by the dimming should be compensated for by the coil heating.
SUMMARY OF THE INVENTION
It is therefore the object of the present invention to specify an electronic ballast for a low-pressure gas discharge lamp in which in the switched-off state of the inverter the state of the lamp and in particular a change of the lamp is detected with the least possible outlay and which, in comparison with the prior art, renders possible better control of the heating of the lamp coils.
This object is achieved by means of a ballast which has a heating transformer for coils of a lamp and connected in the lamp load circuit, a primary winding of the heating transformer being connected in series with a switch to an output of the inverter and being arranged to be connected to a direct current voltage source when the inverter is switched off on account of a heating coil defect or removal of a lamp, the switch being clocked during its switched off phase and the evaluating circuit being arranged to evaluate current that flows through at least one winding of the heating transformer. The ballast in accordance with the invention is distinguished in that provided for the purpose of heating the coils there is a heating transformer, the primary winding of which is connected in series with a switch to the output of the inverter. The current in the primary winding is transmitted to two secondary windings which, in each case with one of the two coils, form a heating circuit. In this connection, the current flowing through the primary winding is detected by means of an evaluating circuit arrangement which in the case of a defect of at least one of the two coils or in the event of the removal of the lamp or in the case of a defect of the lamp detected by further evaluating circuit arrangements causes the inverter to be switched off. In this case, even in the switched-off state of the inverter the primary winding of the heating transformer is connected to a direct-voltage source, in this off-phase the switch that is connected in series with the primary winding is clocked, and by means of the evaluating circuit arrangement the current flowing through the primary winding and/or the secondary winding(s) of the heating transformer is evaluated. This current is substantially dependent upon whether a lamp is in the system or whether its two coils are intact. The heating transformer steps down the heating voltage towards the lamp to a great extent so that the levels of coil resistance for their part are stepped up towards the primary winding. Evaluation of the flow of the current accordingly does not only give information on whether a lamp is inserted, but in addition also on whether and, if this is the case, which coil is defective. If in the off-phase the defective lamp is replaced by a new one, this is identified by the evaluating circuit arrangement which then automatically initiates a restart of the lamp.
In comparison with the ballast of EP 0 146 683 B1, a substantially higher degree of efficiency is attained for the lamp, since by opening the switch the coil heating can be completely switched off after the lamp has been ignited and thus no leakage currents occur. furthermore, the heat output can be regulated by temporarily closing the switch.
Further developments of the invention are described and claimed herein. The current-valuation is effected most simply by measuring the voltage drop across a measuring resistor that is connected in series with the primary winding. Furthermore, the series circuit arrangement consisting of the primary winding and the switch can be connected to a charging/discharging capacitor, with the amplitude of the measured current of the resultant charging or discharging curves being evaluated in its time characteristic or at specific instants in order to detect the state of the lamp.
The flow of current in the heating transformer or the voltage drop across the measuring resistor respectively depends inter alia as well upon the direct voltage that is fed to the heating transformer. However, this can change quite easily over time—for example on account of mains fluctuations. In a further development of the invention therefore a second measuring resistor can be provided in a heating circuit which consists of a lamp coil and the pertinent secondary winding, with the voltage that drops across this measuring resistor likewise being evaluated. A comparison of the two voltages then permits a statement to be made on the state of the electrodes of the lamp independently of voltage fluctuations. This is effected, for example, by forming the differential voltage which is then compared with a rated value. As will be shown, this method allows a very simple, yet meaningful analysis to be made of the state of the lamp. Alternatively, however, the flow of current in the heating transformer at respective specific instants can also be compared with an earlier measured value or a reference value. In this case, just one single measuring resistor would be sufficient, with it being possible to evaluate the current selectively in the primary winding or in one of the two secondary windings.
The use of a heating transformer is already known from EP 0 707 438 A3 or from EP 748 146 A1 and DE 295 14 817 U1, in which here as well in each case there is mention of the coil heating being switched off after the ignition of the lamp. Furthermore, EP 0 707 438 A3 provides for the heating current to be evaluated in order to identify possible lamp defects. However, in none of the cases of the ballasts described in these specifications is it provided that the inverter be switched off and the change of a lamp be identified. The invention is also suitable for use for electronic ballasts which operate a pl
Koch Stefan
Marent Günter
Fitzpatrick ,Cella, Harper & Scinto
Tridonic Bauelemente GmbH
Vu David
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