Electric lamp and discharge devices: systems – Periodic switch in the supply circuit – Impedance or current regulator in the supply circuit
Reexamination Certificate
2001-09-05
2003-05-20
Philogene, Haissa (Department: 2821)
Electric lamp and discharge devices: systems
Periodic switch in the supply circuit
Impedance or current regulator in the supply circuit
C315S225000, C315S24100S, C315S22700A, C315SDIG007
Reexamination Certificate
active
06566822
ABSTRACT:
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 voltag
Schadhauser Klaus
Schmitt Harald
Bessone Carlo S.
Patent-Treuhand-Gesellschaft fuer Elektrische Gluehlampen mBH
Philogene Haissa
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