US3296488A - Control of heatable discharge lamps - Google Patents

Description

Jan. 3, 1967 R. TAYLOR CONTROL OF HEATABLE DISCHARGE LAMPS Filed Dec. 15, 1963 United States Patent 3,296,488 CONTROL OF HEATABLE DISCHARGE LAMPS Raymond Taylor, Birstafl, England, assignor to Associated Electrical Industries Limited, London, England, a British company Filed Dec. 13, 1963, Ser. No. 330,432 1 Claim. (Cl. 315-116) This invention relates to the control of heatable discharge lamps, and aims at providing an improvement which makes it possible to maintain accurately the degree of brightness as is set by an input power control.

It has been known that discharge lamps, such as lowpressure sodium lamps, can be operated with greatly reduced power input in order to dim their brightness. even to less than 1% of their maximum light output, provided the vapour pressure in the lamp envelope is maintained such as by external heating. Usually an outer envelope of transparent material is provided to accommodate the lamp and an electric resistance heater. Such lamps are commonly known as oven lamps.

If the lamps are normally supplied from an AC. source, controlled oppositely poled, parallel connected rectifiers can serve for reducing the power input, while avoiding start failures due to insuflicient striking voltage, when the rectifiers are made conducting at a point of a sinusoidal voltage near zero voltage.

The actual light output from a gaseous discharge lamp depends on both the lamp efliciency and current magnitude. It has been found that the lamp efiiciency changes with the shape of the actual current wave, which depends upon the supply voltage, and the invention aims at overcoming this difiiculty, particularly where accurate dimming, say to 3%, and 30% of maximum brightness is required, as for visual light approach indicator systems on aerodromes.

Accordingly the present invention resides in an arrangement for supplying current to an electric discharge lamp from an AC. source comprising parallel connected oppositely poled rectifiers, switch means for inserting into or removing from the path between said source and lamp said rectifiers, also comprising a means for controlling the conducting periods of the rectifiers, and a voltage regulator maintaining constant the input voltage of said rectifiers.

In the accompanying drawings:

FIGS. 1 and 2 show how the shape of a rectified current impulse depends upon the voltage wave form of the alternating current from which it is derived; and

FIG. 3 shows diagrammatically and by way of example a circuit arrangement according to the invention.

Referring to FIG. 1, it is assumed that oppositely poled rectifiers become conductive at instants A1 and B1 during the positive and negative half cycles. For a given voltage wave v1, the current pulses are shaped as indicated at 01, 02. If the AC. voltage amplitude is reduced to the shape v2 of FIG. 2, a similar power input and mean current can be obtained by rendering the rectifiers conductive at instants A2, B2, nearer the wave peaks. However, the shape of the current waves 03, c4 is difierent from that of 01, 02, therefore the lamp efliciency changes Patented Jan. 3, 1967 and the light output of the lamp differs. Thus in spite of a consistent watt input the lumen output varies. The invention avoids this drawback and insures a consistent light regulation characteristic.

Referring to FIG. 3, a sodium discharge lamp 1 is supplied through terminals 3, 4, from a single-phase source 2. An envelope 5 of transparent material contains the lamp and a heating resistor 6, thus represents the oven. Oppositely poled rectifiers 7, 8 are connected through a voltage regulator 9 to the terminals 3, 4. A change-over switch 10 is provided to connect the lamp either to the terminals 3, 4 for maximum light output, or to the rectifier output for dimmed lighting. The rectifiers 7, 8 have control electrodes 11, 12 supplied from a control box 13 which provides in a conventional manner lamp trigger impulses. It also includes a means for regulating the amount of lamp heating, which increases as the input to the lamp decreases. The light output can be reliably controlled if the rectifier input voltage is maintained constant and the vapor pressure in the lamp is not allowed to drop unduly with the lamp current reduction. According to the invention as this vapor pressure tends to decrease, due to reduction of the rectifier conducting period, the external lamp heating is automatically increased, so as to achieve a desired dimming etfect. Thus a manual member 14 is provided to set at will the lamp input and adequate heating input for a desired degree of lamp brightness.

Modifications are possible without departing from the invention as defined by the appended claims. The regulator 9 can be any type known in the art to ensure an output voltage constant to a desired degree while the input voltage may vary within a predetermined range. Thyratrons and semiconductor devices can be readily controlled by triggering voltage or current pulses and can supply adequate power outputs. A gradual and continuous control, or settable discrete steps such as 3%, 10% or 30%, of a full rating can be provided. The switch 10 may be remotely controlled by means of relays, for instance.

What I claim is:

In the combination of an electric discharge lamp and means for controlling the brightness thereof, the improvement comprising parallel connected oppositely poled rectifiers, an AC. source, switch means for selectively con necting said rectifiers in the circuit between said source and said lamp, a voltage regulator to maintain constant input voltage to said rectifiers, an electric heater for the lamp and control means for controlling the conducting periods of said rectifiers and for increasing the heater input as the conducting periods of the rectifiers are reduced.

References Cited by the Examiner UNITED STATES PATENTS 2,765,416 10/1956 Beese et a1. 3l315 X 3,205,404 9/1965 Kurata et a1 315-194 JAMES W. LAWRENCE, Primary Examiner.

S. SCHNEEBERGER, Assistant Examiner.

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