The present invention relates to a focusing unit for controlling the focusing voltage of a cathode ray tube within a TV receiver by appropriately controlling the high voltage of the FBT (flyback transformer) and, more particularly, to a dynamic focusing unit capable of controlling the peripheral focus in the up and down and the right and left dimensions of the TV screen correctly and over a wider range.
Generally, the focusing unit is a device for focusing the electron beam, being an electrostatic lens formed between anode electrode and cathode electrodes by applying pertinent voltage to the focusing electrodes such that a clear image is displayed on a fluorescent screen by the electron beam emitted from the electron gun of the picture tube.
FIG. 8 is a schematic structural diagram of a picture tube with a spherical screen controlled by the focusing circuit of Korean Utility Model official publication gazette No. 89-8220, and wherein the electrons emitted from cathode 100 of the electron gun are made into an electron beam being radiated to a beam focusing point on a fluorescent screen 130. For focusing this electron beam, focusing electrode 110 is provided within the interior of the picture tube and then pertinent voltage is applied from the exterior. Thereby, the electron beam is focused by the electrostatic field of focusing electrode 110.
However, although the electron beam focused upon the fluorescent screen 130 of the aforementioned spherical picture tube does not produce deviation, in the case when the fluorescent screen 130 is flat as the picture screen 140, a focusing deviation 150 is produced at the peripheral regions of the flat screen.
Accordingly, since the conventional focusing device was a device designed for a spherical cathode ray tube, a focusing deviation has been produced at the peripheral region of the flat cathode ray tube.
In order to solve the above-described defect, a device has been developed in which focusing is correctly controlled at peripheral regions of cathode ray tubes by utilizing the horizontal flyback pulse signal and vertical output signal.
As shown in FIG. 9, a focusing device for controlling the electron beam to be focused on the flat screen of cathode ray tube 300 in response to the signal applied to focusing electrode 110 comprises a vertical signal integrating circuit 10 for integrating vertical pulse signal S.sub.1, thereby converting to a vertical parabolic waveform signal S.sub.2, a vertical amplifying circuit 20 for variably amplifying the output signal of said vertical signal integrating circuit 10, thereby controlling the up and down focusing on the screen, a horizontal signal integrating circuit 30 for integrating the horizontal pulse signal S.sub.4 inputted after being extracted from the flyback transformer, thereby converting to a horizontal parabolic waveform signal S.sub.5, a buffer circuit 40 for variably current amplifying the output signal of said horizontal signal integrating circuit, thereby controlling the right and left focusing on the screen, and an inversion amplifying circuit 50 for inversely amplifying the mixed vertical and horizontal parabolic waveform signals S.sub.6. The signal S.sub.7 outputted from said inversion amplifying circuit 50 is applied to focusing electrode 110 of picture tube 300 and also to anode 160 of picture tube 300. These elements are arranged so that the phenomenon of deviating focus at peripheral regions of the screen is prevented. The focusing unit serves a function which controls appropriately the high voltage coming out of the flyback transformer (FBT) thereby controlling the focus of the cathode ray tube within the TV receiving set.
However, there are many kinds of cathode ray tubes and different focusing voltages are required by each kind. In order to feed focusing voltage of the cathode ray tube for each kind, the wider the variable range of the voltage of the focusing unit is, the better.
A conventional focusing unit is shown in FIGS. 6 and 7 and comprises a knob 208 passing through hole 201 in the rectangular main body 200 of the focusing unit having annular ring 204 protruded with two rotation limiting protrusions 202, 203 extending inward, contact terminal fixing means 206 having a horseshoe shape to receive variable contact pin 205, knob body 210 structured with variable shaft means 207 formed integrally with said fixing means 206 as well as knob 208, and ceramic base board 230 provided with input and output voltage terminals 231, 232, circular printed portion 233 and bent printed portion 234.
However, in this conventional focusing unit, since variable contact pin 205 is moved along circular printed portion 233 around knob 208, the extent of the output voltage terminal 231 from the input voltage 25 terminal 232 is limited within a narrow range.
Therefore, since it has a simple structure that the first focusing voltage variable range 240 cannot cover by less than 10%-15% and also the second focusing voltage variable range 250 cannot cover by less than 2%-3%, there has been a problem of achieving a satisfactory variable voltage focusing range.