| 摘要 |
The present invention relates to a protective circuit for a fluorescent lamp (LA) with a first and a second lamp filament (W1, W2), a DC voltage source (UZ) with a positive and a negative pole, a half-bridge arrangement with a first and a second switch (S1, S2), the half-bridge arrangement being fed by the DC voltage source (UZ), and the first and the second switch (S1, S2) being interconnected to form a first reference point (A), the first reference point (A) being connected to the negative pole via a first resistor (R1), a decoupling capacitor (C2) that is arranged in a serial connection between the half-bridge arrangement and the first or the second lamp filament (W1; W2), the connection of the decoupling capacitor (C2) on the filament side forming a second reference point (B) that is connected to the negative pole via a second resistor (R2) , a comparator (VG) that has a first and a second input and an output, the first input being connected to the first reference point (A), and the second input being connected to the second reference point (B), the output being connected to the negative pole via a detection capacitor (C3), an evaluation circuit (AS) with the aid of which the voltage (UC3) dropping across the detection capacitor (C3) can be evaluated in order to deactivate the half-bridge arrangement upon overshooting of a predetermined voltage level, it further comprising a third resistor (R3) that bridges the decoupling capacitor (C2), and a fourth resistor (R4) that connects the first reference point (A) to the positive pole of the DC voltage source (UZ), the first, the second, the third and the fourth resistor (R1R2, R3, R4) being selected such that the first and the second reference point (A, B) are at the same potential without the fluorescent lamp (LA) inserted.
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