Ballast having a dimming device

1. An electronic ballast having a dimming device for controlling brightness of a low-pressure discharge lamp configured for operation with a periodically modulated lamp current, comprising;

means for operating the dimming device to adjust the lamp current to control the lamp brightness such that with decreasing brightness levels, both a maxima and a minima of an envelope of the lamp current become smaller;

wherein with further decreasing brightness levels, the periodic modulation of the envelope of the lamp current is superimposed by a lower limit of an amplitude of the lamp current such that periodic modulation of the envelope of the lamp current occurs in lamp current pulse packets with lamp current amplitudes above the lower limit; and

wherein with further decreasing brightness levels, widths of the pulse packets decrease and intervals between the pulse packets with limited lamp current amplitudes increase.

2. The electronic ballast as claimed in claim 1 , wherein the dimming device is configured such that the lower limit corresponds to a positive value for the envelope of the lamp current.

3. The electronic ballast as claimed in claim 1 , wherein the dimming device is configured such that the envelope of the lamp current is restricted by a maximum value at a high and maximum brightness level.

4. The electronic ballast as claimed in claim 1 , wherein the dimming device is configured such that respectively rising edges of the periodic modulation are steep compared with the respectively falling edges.

5. The electronic ballast as claimed in claim 4 , wherein the dimming device is configured to modulate the envelope of the lamp current in saw-tooth form or rounded saw-tooth form for controlling the brightness level.

6. The electronic ballast as claimed in claim 1 , wherein the dimming device further comprises a signal generator for generating a periodic signal for modulation of the lamp current and a circuit arrangement for restricting the periodic signal corresponding to, if appropriate, a maximum boundary and the minimum boundary.

7. The electronic ballast as claimed in claim 1 , wherein the dimming device includes a signal generator for generating a periodic signal for modulation of the lamp current and a device for synchronizing the periodic signal with a supply voltage of an inverter for producing the lamp current, the output signal from the signal generator being synchronized with a phase angle of the supply voltage of the inverter which is fluctuating at a low frequency.

8. The electronic ballast as claimed in claim 1 , further comprising:

an inverter for producing the lamp current;

a measuring device for measuring the lamp current or a variable dependent on the lamp current and for producing a controlled variable; and

a regulator controlled by the dimming device for controlling the inverter.

9. The electronic ballast as claimed in claim 8 , further comprising:

a device for preventing interruption of gas discharge;

wherein the device is configured to measure resistance of the lamp and to convert the lamp resistance into an additional controlled variable.

10. A low-pressure discharge lamp having an integrated electronic ballast as claimed in claim 1 .

11. A method for controlling brightness of a low-pressure discharge lamp by means of an electronic ballast having a dimming device configured for operation with a periodically modulated lamp current, comprising:

adjusting the lamp current such that with decreasing brightness, both a maxima and a minima of the envelope of the lamp current become smaller;

superimposing by a lower limit of an amplitude of the lamp current, with further decreasing brightness levels, a periodic modulation of an envelope of the lamp current such that periodic modulation of the envelope of the lamp current occurs in lamp current pulse packets with lamp current amplitudes above the lower limit; and

decreasing widths of the pulse packets, with further decreasing brightness levels, and increasing intervals between the pulse packets with limited lamp current amplitudes.

12. The method as claimed in claim 11 , wherein the method is implemented via the electronic ballast.

13. The method as claimed in claim 1 , wherein the lower limit substantially corresponds to zero.

14. The method ballast as claimed in claim 11 , wherein the lower limit substantially corresponds to zero.