| 摘要 |
1,255,043. Supply systems for discharge lamps. WESTINGHOUSE ELECTRIC CORP. 9 April, 1969 [29 April, 1968], No. 18136/69. Heading H2H. [Also in Division G3] The current through a discharge lamp 10 is maintained at a desired average value by alternately supplying the lamp from a D.C. source 11 until the current reaches a predetermined value and then using the energy stored in an inductor 14 to pass current through the lamp (via a diode 18) for a predetermined fixed time, these alternate cycles producing a triangular shaped current waveform giving the desired average current, Fig. 2 (not shown). The lamp current is detected by a resistor R1 whose output signal is used to control a Schmitt trigger circuit formed by transistors Q2, Q3 and determining the ON/OFF times of a switching transistor Q1 controlling the supply to members 10, 14. During warm-up of the lamp, these ON/OFF times are controlled by a current peak detecting circuit R2, R4, D2, C1 which causes the control circuit to operate in a variable pulse-frequency mode. When steady-state lamp operation has been reached the voltage developed across a capacitor C2 is increased to exceed that across capacitor C1 and cause control to pass to an average-current-detecting circuit R10, R11, D6, C2, which thereafter operates the transistor Q1 at a fixed mark/space ratio corresponding to the desired average current. This current is automatically varied, to maintain constant power with supply voltage variations, by a network R12, R13, C3, D5 which controls the Schmitt trigger circuit to vary the mark/space ratio of transistor Q1 in accordance with supply voltage variations as sensed by the capacitor C3. The described control circuit also automatically compensates for variation of the lamp voltage with ageing to maintain constant lamp power, the theoretical basis of this aspect of the invention being discussed. Operation of the circuit on an A.C. supply is envisaged together with various modifications such as replacing resistor R1 by an impedance or a current transformer and obtaining the predetermined OFF time of transistor Q1 by time delay means other than the circuit C2, R11.
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