| 摘要 |
A lamp ballast connectable with a d.c. source comprises a regulating device having a voltage coil and a load coil. The voltage coil is connected in a circuit in parallel with the series-connected load coil and lamp. Alternate charge and discharge of a capacitor connected with the voltage coil impresses an alternating voltage thereacross. The capacitor is charged by current through the voltage coil and is discharged through a resonant commutating circuit comprising a thyristor triggered from an oscillator pulse circuit at a frequency of several KHz. The commutating circuit, which has a resonant frequency about twice that of the oscillator, further comprises a commutating reactor having substantially lower impedance than the load and voltage coils, a back current diode, and a resistance-capacitance dV/dt clamp that reduces back voltage spikes across the thyristor to safe rates of rise. Interaction between the load coil and the commutating reactor and/or voltage coil ensures adequate current limiting when resistance across the lamp terminals is low but permits high enough voltage for ignition when that resistance is effectively infinite. Preferred component values are specified for ballasts useable with mercury and low pressure sodium lamps rated at 100W and under.
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