GB1487584A - Lamp control circuit - Google Patents

Abstract

1487584 Automatic control of lamps XEROX CORP 12 Nov 1974 [19 Nov 1973] 48869/74 Heading G1A [Also in Divisions G3 and H2] Lamp control apparatus Fig. I comprises: a long multi-vapour arc lamp 2; a chopper 40 exposing a photo-cell 12 to light alternately from opposite ends of the lamp 2, and providing a reference signal at the frequency of alternation; means 10, 21, 22 responsive to the photo-cell to provide phaseopposed signals representing non-uniformity of lamp brightness; and means 50 responsive to the phase-opposed signals and the reference signal for providing an output representing the amount and sense of the non-uniformity. A 15" metal-halide vapour lamp 2, Fig. 1 in a photo-electrophoretic copier may undergo cataphoresis resulting in an end-to-end imbalance in its brightness, remediable by introducing a D.C. component into its A.C. supply. Light from opposite ends is guided by light pipes 11 to a rotary or vibratory chopper 40 which alternately presents the light-pipe outputs at 1KHz to a photodiode 12. Any A.C. component in the photodiode output is shaped by a tuned amplifier 21 of bandwidth 80- 100 Hz and applied to a phase-splitter 22 providing signals ° 1 , ° 2 . A 1 KHz signal from the chopper drive circuit 40 is filtered and applied to a phase converter 48 which provides a reference signal adjustable for phase conformity with the amplifier 21 output. The reference signal is squared and split at 47 to provide REF and REF signals which gate alternate pairs of amplifiers in a phase detector 50 receiving the signals ° 1 , ° 2 , whereby there is provided a train of full-wave rectified pulses representing in magnitude and polarity the light imbalance. The detector 50 output, smoothed in a timebase generator 70, controls the rise gradient of a sawtooth determining a variable first interval between pulses of a train, this variable first interval alternating with a fixed second interval (typically 5 ms) controlled by a monostable. These pulses are sorted by switching logic 80 into trains of α and # pulses which occur at the beginnings of the variable and fixed intervals respectively. In a lamp switch circuit 90, Fig. 6, to which the claims of Specification 1487585 are directed (see Division H2), pairs of SCRs 91/93 and 92/94 supply currents of opposite polarities to the lamp 2 in response to triggering pulses derived from the trains α and # which alternate in a variable duty cycle. Commutating capacitors Cl, C2 ensure that when one pair of SCRs is triggered on, a pulse is applied to the opposite pair to reverse its polarity and so turn it off. To avoid momentary shortcircuits, inductors L1-L4 are interposed between each lamp terminal and respective corresponding junctions of a capacitor with an SCR.