Browse Prior Art Database

Automatic CRT Heater Control

IP.com Disclosure Number: IPCOM000038613D
Original Publication Date: 1987-Feb-01
Included in the Prior Art Database: 2005-Jan-31
Document File: 3 page(s) / 56K

Publishing Venue

IBM

Related People

Beanlands, P: AUTHOR [+2]

Abstract

To extend CRT life, a cathode heater is initially run at a low voltage and increased as cathode emission decays. Screen brightness or beam current is sensed to provide an input to a digital controller to automatically control the heater voltage supplied by a voltage regulator. An adaptive algorithm operates at intervals to determine the optimum heater voltage to minimize cathode decay. The maximum brightness of all cathode ray tubes reduces with life. The primary reason for this is the decay in emission capability of the cathodes. If a cathode is run within its normal limits of current density, then the life is determined primarily by the cathode temperature. This is because the most important constituent of the universal oxide cathode is free barium, which is volatile.

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Automatic CRT Heater Control

To extend CRT life, a cathode heater is initially run at a low voltage and increased as cathode emission decays. Screen brightness or beam current is sensed to provide an input to a digital controller to automatically control the heater voltage supplied by a voltage regulator. An adaptive algorithm operates at intervals to determine the optimum heater voltage to minimize cathode decay. The maximum brightness of all cathode ray tubes reduces with life. The primary reason for this is the decay in emission capability of the cathodes.

If a cathode is run within its normal limits of current density, then the life is determined primarily by the cathode temperature. This is because the most important constituent of the universal oxide cathode is free barium, which is volatile. Consequently, small increases in temperature greatly increase the rate of barium evaporation and hence the rate of current emission decay. A minimum temperature must also be maintained to ensure good electron emission and prevent cathode poisoning by free oxygen remaining after evacuation/gettering. CRT life may be dramatically improved by running at a low voltage, particularly in early life, before any shift of emission characteristic has occurred. This article describes an adaptive system that can achieve this major improvement in tube life, to the point where CRTs need no longer be considered as degrading components. The system is particularly suited to products incorporating microprocessors where the necessary algorithm and control may be implemented, adding only a small extra hardware cost. The major elements of the system are detailed in Fig. 1. A controlled voltage regulator drives the CRT heaters. The heater voltage is then slowly reduced (perhaps at a rate of 0.1 V/minute), under the control of an algorithm, while the beam current is monitored. Since the CRT is operating on an emission plateau, there would be no discernable brightness change to the user. The algorithm senses when current had fallen by a predetermined amount, determined by the evaluation of life test data, and maintain the voltage at this level. The three cathodes all change together, and so it would be possible to monitor either individual gun currents or a composite. This automatically optimizes the heater voltage for CRT life. This algorithm operates at intervals during tube life, maintaining th...