Browse Prior Art Database

Circuit for Reducing Doming in CRT Displays

IP.com Disclosure Number: IPCOM000108304D
Original Publication Date: 1992-May-01
Included in the Prior Art Database: 2005-Mar-22
Document File: 3 page(s) / 146K

Publishing Venue

IBM

Related People

Eagle, DJ: AUTHOR [+2]

Abstract

Described is a simple circuit to reduce the effects of unwanted doming in CRT-based displays while maintaining original brightness in non- doming applications. This beam current limiting circuit involves the addition of only three low-cost auto-insertable components compared with conventional circuits in current displays.

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Circuit for Reducing Doming in CRT Displays

       Described is a simple circuit to reduce the effects of
unwanted doming in CRT-based displays while maintaining original
brightness in non- doming applications.  This beam current limiting
circuit involves the addition of only three low-cost auto-insertable
components compared with conventional circuits in current displays.

      Use of reverse video applications can give rise to an effect
known as doming.  The symptoms are that patches of brightness loss
and sometimes color change are seen on the screen caused by localized
thermal expansion of the CRT shadow mask resulting in loss of purity
margin.  Expansion occurs in areas of high brightness where beam
current is high.  The worst patterns for doming are full brightness
white blocks on a dark background, with the block size so small that
the monitor does not go into 'beam current limit' (BCL).  Using DOS4
and Microsoft windows, the user has the capability to create screens
that are likely to cause doming and then assume that dark patches on
the screen are an indication that the monitor is faulty.

      It is standard practice to incorporate a beam current limit
circuit to limit the brightness when a full white screen is used, to
restrict the maximum average beam current over the frame to a level
to minimize X-rays and to prolong CRT life.   Also reduced is stress
on other parts of the display such as the power supply.  The
brightness of a display operating in BCL to reduce doming is
approximately half the brightness available for forward video
applications.  A typical beam current limit circuit is shown in Fig.
1 where the beam current limit is effectively set by the value of R1,
assuming voltage rails are fixed.  Time constant C1-R1 is long
compared to the frame refresh period. If the average current drawn by
the EHT transformer exceeds a certain level, then the voltage on C1
falls taking Q1 out of saturation and the voltage at node A is
reduced.  The input to the gain circuitry also falls so reducing
screen brightness and beam current.  A steadystate condition is
rapidly achieved.

      More complicated solutions monitor the cathode drive voltages
via R-C networks and reduced the effective beam current limit when
bright blocks were detected.  The R-C network is required to ensure
that the brightness is not reduced for forward video text
applications.  It is effective at detecting blocks likely to cause
doming but requires setting up on the monitor line due to variations
in the video drive efficiencies of CRTs.  A costly traditional
solution to doming is to change the material of the CRT shadow mask
to INVAR* which has a low coefficient of thermal expansion and used
out of necessity on FST CRTs.  A cheaper, less effective technique is
to coat the shadow mask with bismuth oxide to reduce thermal
expansion.

      The primary advantage of the new circuit described here...