Electric lamp and discharge devices: systems – Periodic switch in the supply circuit – Periodic switch cut-out
Patent
1997-08-12
1999-03-16
Kinkead, Arnold
Electric lamp and discharge devices: systems
Periodic switch in the supply circuit
Periodic switch cut-out
315209R, 315307, 315205, H05B 4129
Patent
active
058834729
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The invention concerns a circuit arrangement for the operation of a discharge lamp.
In the operation and upon ignition of a discharge lamp, high voltages can occur in a defective lamp and towards the end of a lamp's service life, which can lead to destruction of the corresponding electronic ballast if no suitable countermeasures have been provided. Customarily, a cutoff circuit is used to monitor lamp voltage. As soon as lamp voltage has been higher than a particular value for longer than a predetermined time, the cutoff intervenes and the lamp generator, which is generally a half bridge generator, is stopped.
Electronic ballasts with a cutoff circuit from the OSRAM GmbH company (product designation QT EC) are already known. A circuit arrangement corresponding to this state of the art is shown in FIG. 1. In this arrangement, the cutoff device is formed from a bistable flip-flop circuit consisting of two transistors T1 and T2 (thyristor equivalent circuit). The lamp voltage U.sub.L is fed via a voltage divider R4, R3 and a time-lag device R4, Ct to the base of one of the transistors of the bistable flip-flop circuit, which forms the trigger input of this circuit. When the bistable flip-flop circuit trips to its conducting or operation state, a cutoff transistor T3 stops the lamp generator, which is a half bridge generator in the example described, by suppressing the base drive of the lower transistor Tu of the half bridge generator via a diode D1. A starting capacitor Cs, which is located parallel to the switching path of the lower transistor Tu of the half bridge generator via a diode D4, is short-circuited via a diode D2 so that renewed starting attempts by the inverse rectifier are prevented.
This known circuit has two disadvantages in particular. In order that the oscillation of the half bridge generator can be dependably interrupted, the base of transistor Tu of the half bridge circuit must be connected with low impedance to ground. However, since the base voltage of transistor Tu of the half bridge generator can also show negative values, the base of this transistor Tu of the half bridge generator is connected via a diode D1 to the collector of the cutoff transistor T3. In this way, however, dependable disconnection is made more difficult, since the base of the lower transistor Tu is not directly connected to ground upon disconnection, but only via the diode path from D1 via T3.
A further disadvantage of the known circuit arrangement is that an energy source U.sub.v must be made available to provide the base current for the cutoff transistor T3. In known circuit arrangements, an electrolytic capacitor is provided for this purpose, which is loaded either using the trigger signal or via a pump circuit.
Therefore, the purpose of the present invention is to make a circuit arrangement available for the operation of a discharge lamp, in which the aforementioned problems are avoided.
SUMMARY OF THE INVENTION
The invention as shown in FIGS. 2 and 3 concerns a circuit for operating a discharge lamp with a half-bridge generator. The energy source for the base current of a turn-off transistor (T3) is formed by starter capacitor (C3) which starts the half-bridge generator. The realization of the circuit arrangement according to invention according (FIG. 2) has the advantage that no additional source of energy is required for the cutoff process, but rather that this energy source is provided by the already present starting capacitor C3. In the previously mentioned known circuit, the starting capacitor is discharged when the half bridge generator is in operation, via a diode D4 which is located parallel to the switching path of transistor T2 of the half bridge generator.
By insertion of a Zener diode D9, the starting capacitor C3 remains loaded at the value of the Zener voltage of the Zener diode and is used in this way as an energy source. If the bistable flip-flop circuit T5, T4 becomes conductive, not only is the lamp generator stopped, but a restart by discharge of the starting c
REFERENCES:
patent: 4554487 (1985-11-01), Nilssen
Bessone Carlo S.
Kinkead Arnold
Patent-Treuhand-Gesellschaft Fuer Elektrische Gluehlampen MBH
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