1. Field of the Invention
The present invention relates to a large display apparatus having a matrix of a number of discharge lamps, for displaying various types of images including characters by controlling the light-on of the discharge lamps. The present invention relates also to a discharge lamp load circuit in the large display apparatus for controlling the light-on of the discharge lamps for an intentional image formation.
2. Description of the Related Art
The large display apparatus called a large image display apparatus or a large color electric display board has been used in a stadium, for example. The display apparatus has a matrix of a great number of discharge lamps. To display various of images and characters, the light-on and light-off of the lamps and/or intensities of lights emitted from the lamps must be controlled intentionally.
The discharge lamp load circuit, by convention, includes a high frequency power source, switching means coupled between each the lamps and the high frequency power source, and control means for turning on/off the switching means. To light on a lamp, the switching means is turned on by the control means to allow current to the lamp. Particularly, to control an intensity of light from a lamp, viz., a gradation of the lamp, a pulse width of a signal applied to the switching means is controlled by the control means. During a period of time as defined by the controlled pulse width, the switching means allows current to be fed to the lamp. The pulse width control is systematically applied to all of the lamps to form a desired image.
Various types of large display apparatuses and discharge lamps load circuits contained therein have been proposed. Some specific examples of them are enumerated: Published Unexamined Japanese Patent Application Nos. 59-180586 (corresponding to U.S. Pat. No. 4,635,052), 60-70490, 61-15194 (U.S. patent application Ser. No. 749,435 and its continuation-in-part Application No. 207,932), 61-158398, and 62-14695, and U.S. patent application Ser. No. 595,532.
The high frequency power source may be constructed with a full-bridge inverter. When the full-bridge inverter is used, a capacitor is coupled with the power source. Provision of the capacitor is for obtaining an improved power factor against a variation of a load including a current-limiting impedance constituted by inductances and a resistive element constituted by discharge lamps or display elements.
When a capacitance of the capacitor is small, the influence by the inductive load is great at the rated load point, and hence the power source efficiency is degraded. When the capacitor has a large capacitance, the power source efficiency is satisfactory at the rated load point, but where the load is small or the number of lighting lamps is small, the influence by the capacitive load is great and the power source efficiency is degraded.