Electronic ballast for discharge lamps having an EOL monitoring circuit

1. An electronic ballast for the AC operation of a discharge lamp (LA 1 )

having an EOL monitoring circuit (R 1 , R 2 , U 1 , R 9 , U 2 -A, U 2 -B, U 3 -A, AE) for detecting the end of life of the electrodes of the discharge lamp (LA 1 ),

which EOL monitoring circuit (R 1 , R 2 , U 1 , R 9 , U 2 -A, U 2 -B, U 3 -A, AE) responds to an asymmetrical power of the discharge lamp (LA 1 ),

characterized in that a current associated with the asymmetrical power and a reference current are fed to a current differential amplifier (U 1 ) in the EOL monitoring circuit (R 1 , R 2 , U 1 , R 9 , U 2 -A, U 2 -B, U 3 -A, AE).

2. The ballast as claimed in claim 1 , in which the current differential amplifier (U 1 ) has a current mirror circuit at the input.

3. The ballast as claimed in claim 1 , in which the current differential amplifier (U 1 ) has a voltage output.

4. The ballast as claimed in claim 1 , in which an output signal line of the current differential amplifier (U 1 ) is connected to a window comparator (U 2 -A, U 2 -B).

5. The ballast as claimed in claim 1 , in which the EOL monitoring circuit (R 1 , R 2 , U 1 , R 9 , U 2 -A, U 2 -B, U 3 -A, AE) has a low-pass filter (R 2 , C 2 ) for filtering out parasitic oscillations.

6. The ballast as claimed in claim 5 , in which the low-pass filter (R 2 , C 2 ) has a capacitor (C 2 ) between a measured current input of the current differential amplifier (U 1 ) and the internal reference potential of the ballast.

7. The ballast as claimed in claim 1 , in which an output signal line of the current differential amplifier (U 1 ) is connected to a microprocessor circuit (μP).

8. The ballast as claimed in claim 1 , in which the reference current is derived from a reference potential via a resistor (R 1 ).

9. The ballast as claimed in claim 8 , in which the reference potential is the internal supply potential of a high-frequency converter (T 1 , T 2 ) for generating the AC supply power for the discharge lamp (LA 1 ).

10. The ballast as claimed in claim 1 , in which a coupling capacitor (C 10 ) is provided between the discharge lamp (LA 1 ) and the internal reference potential of the ballast, the current differential amplifier (U 1 ) has a reference to the reference potential, and the current associated with the asymmetrical power is derived from a tap between the coupling capacitor (C 10 ) and the discharge lamp (LA 1 ) via a resistor (R 2 ).

11. The ballast as claimed in claim 1 , in which a coupling capacitor (C 10 ) is provided between the discharge lamp (LA 1 ) and an AC output of a high-frequency converter (T 1 , T 2 ) provided for generating the AC supply power for the discharge lamp (LA 1 ), the current differential amplifier (U 1 ) has a reference to the reference potential, and the current associated with the asymmetrical power is derived from a tap between the coupling capacitor (C 10 ) and the discharge lamp (LA 1 ) via a resistor (R 2 ).

12. The ballast as claimed in claim 1 , which is designed for a low-pressure discharge lamp (LA 1 ).

13. A lamp system comprising a discharge lamp (LA 1 ) and a ballast as claimed in claim 1 .

14. The ballast as claimed in claim 2 , in which the current differential amplifier (U 1 ) has a voltage output.

15. The ballast as claimed in claim 2 , in which an output signal line of the current differential amplifier (U 1 ) is connected to a window comparator (U 2 -A, U 2 -B).

16. The ballast as claimed in claim 2 , in which an output signal line of the current differential amplifier (U 1 ) is connected to a microprocessor circuit (μP).

17. The ballast as claimed in claim 3 , in which an output signal line of the current differential amplifier (U 1 ) is connected to a microprocessor circuit (μP).

18. The ballast as claimed in claim 2 , in which the reference current is derived from a reference potential via a resistor (R 1 ).

19. The ballast as claimed in claim 2 , in which a coupling capacitor (C 10 ) is provided between the discharge lamp (LA 1 ) and the internal reference potential of the ballast, the current differential amplifier (U 1 ) has a reference to the reference potential, and the current associated with the asymmetrical power is derived from a tap between the coupling capacitor (C 10 ) and the discharge lamp (LA 1 ) via a resistor (R 2 ).

20. The ballast as claimed in claim 2 , in which a coupling capacitor (C 10 ) is provided between the discharge lamp (LA 1 ) and an AC output of a high-frequency converter (T 1 , T 2 ) provided for generating the AC supply power for the discharge lamp (LA 1 ), the current differential amplifier (U 1 ) has a reference to the reference potential, and the current associated with the asymmetrical power is derived from a tap between the coupling capacitor (c 10 ) and the discharge lamp (LA 1 ) via a resistor (R 2 ).