Browse Prior Art Database

CRT Phosphor Protection Circuit

IP.com Disclosure Number: IPCOM000051804D
Original Publication Date: 1981-Mar-01
Included in the Prior Art Database: 2005-Feb-11
Document File: 3 page(s) / 46K

Publishing Venue

IBM

Related People

Marple, WP: AUTHOR

Abstract

In conventional CRT displays, the grid voltage is typically grounded and the video amplifier input to the cathode is modulated from ground potential to approximately a +50-volt level. If a defect occurs to both the horizontal and vertical deflection amplifiers for the CRT, the electron beam will be focused on the center of the display face and can burn phosphor at the point of incidence. Alternately, if a video amplifier failure occurs so that the cathode of the CRT assumes a relatively negative potential with respect to the normal black level potential, the phosphor on the face of the tube can be damaged.

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CRT Phosphor Protection Circuit

In conventional CRT displays, the grid voltage is typically grounded and the video amplifier input to the cathode is modulated from ground potential to approximately a +50-volt level. If a defect occurs to both the horizontal and vertical deflection amplifiers for the CRT, the electron beam will be focused on the center of the display face and can burn phosphor at the point of incidence. Alternately, if a video amplifier failure occurs so that the cathode of the CRT assumes a relatively negative potential with respect to the normal black level potential, the phosphor on the face of the tube can be damaged.

The circuit disclosed herin is designed to prevent damage to the phosphor in the CRT by selectively applying a ground potential to the grid of the CRT during normal frames of the raster, applying a -14-volt blanking signal to the grid of the CRT during the blanking interval, and alternately applying a -80-volt potential to the grid to repel the electron beam in the event of the failure of both deflection amplifiers or the video amplifier, so as to protect the phosphor of the CRT display.

The figure illustrates the phosphor protection circuit. A detector DET has as its inputs a signal from;the horizontal and vertical deflection amplifiers indicating where the both deflection amplifiers are malfunctioning. A second input to the detector DET is connected to the video amplifier and indicates whether the video amplifier is defective. During normal operation, the input DD to the circuit from the detector DET is at a down level, and thus the device Q4 and the device Q3 are conductive. In this mode, if the blanking input BB is down, indicating a normal raster frame, the device Q5 is conducting and causes the ground potential at the cathode of the diode CR6 to be applied to the grid of the CRT.

When the signal on the blanking line BB is up, the device Q5 is off, causing the diode CR18 to be forward biased, thereby conducting the -15-volt power supply voltage through the diode CR10 to the grid of the CRT, thereby imposing the black blanking condition on the video output for the display.

When the detector DET determines that either both horizontal and vetical deflection amplifiers are malfunctioning or that the video amplifier is malfunctioning, the protection input line DD is up and turns off the devices Q4 and Q3. The charge which has accumulated on the capacitor C is applied through the diode CR13 and the diode CR10 to the grid of the CRT tube. During this period, isolation diodes CR3, CR5, CR18 and CR11 isolate the entire circuit so that...