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Cathode Ray Tube Phosphor Protection Circuit

IP.com Disclosure Number: IPCOM000094533D
Original Publication Date: 1965-Feb-01
Included in the Prior Art Database: 2005-Mar-06
Document File: 2 page(s) / 34K

Publishing Venue

IBM

Related People

Beetle, DH: AUTHOR

Abstract

This circuit protects the phosphor of a cathode ray tube by interrupting the beam when the charge applied to the phosphor during an unblanking interval exceeds a preset level.

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Cathode Ray Tube Phosphor Protection Circuit

This circuit protects the phosphor of a cathode ray tube by interrupting the beam when the charge applied to the phosphor during an unblanking interval exceeds a preset level.

Transistor T2 cooperates with resistor R3 to form a current source that is adjustable according to intensity control voltage V2 to control brightness. Transistor T3 is connected to conduct the current i in response to a blanking control voltage V1 to suppress beam current during blanking. Capacitor C is connected to conduct in series between the current source and the cathode of the CRT to provide a voltage across its terminals A and B that is proportional to the charge applied to the CRT. Transistor T1 is connected to conduct the beam current and to turn off and interrupt the beam when the voltage across C becomes greater than 6 volts. During blanking, diode D3 clamps the CRT cathode to a point of appropriate potential to prevent transistor breakdown.

During blanking, diode D1 limits terminal A of C to a voltage of +6 volts and T3 conducts in saturation to give terminal B of C also +6 volts. Thus, during the unblanking interval the voltage across C starts at zero volts and is directly proportional to the charge applied to the CRT phosphor.

The voltage at the CRT cathode is the value established by the volt-ampere characteristic of the CRT for the selected beam current and A is approximately +6 volts. Thus, as C charges, B goes more negative. Aft...