Browse Prior Art Database

Method for CRT Brightness Control Within Beam Current Limit

IP.com Disclosure Number: IPCOM000107350D
Original Publication Date: 1992-Feb-01
Included in the Prior Art Database: 2005-Mar-21
Document File: 3 page(s) / 106K

Publishing Venue

IBM

Related People

Eagle, DJ: AUTHOR [+2]

Abstract

Described is a simple technique used to increase the range of user brightness adjustment available when a display is operating in beam current limit. The circuitry involved adds only around five extra components while offering improved user function.

This text was extracted from an ASCII text file.
This is the abbreviated version, containing approximately 52% of the total text.

Method for CRT Brightness Control Within Beam Current Limit

       Described is a simple technique used to increase the
range of user brightness adjustment available when a display is
operating in beam current limit.  The circuitry involved adds only
around five extra components while offering improved user function.

      Most CRT-based displays have circuitry that limits the maximum
average beam current that can flow, to limit X-ray emissions under
fault conditions and to reduce doming, a thermal effect that degrades
purity.  Maximum average beam current is typically set to around half
the peak beam current, but the problem is that the display's contrast
control is normally also used to adjust image brightness over the
complete range of minimum to maximum brightness. With, for example, a
full white screen, as the contrast control is turned up, the average
beam current flowing soon reaches the beam current limit and
adjusting the contrast control beyond this point has no effect on the
display. Thus, typically half the adjustment range on the control is
disabled. The above effect has become important as more software
packages operate in reverse video instead of forward video. With the
arrival of DOS 4.0 and Windows 3.0, users will notice the limitations
of the present way of controlling display brightness.

      Disclosed is a technique to enable the beam current limit to
become a variable, controlled by the user, accomplished by altering
how the contrast and beam current limit circuits operate.  A typical
beam current limit circuit, shown in Fig. 1, is the scheme used on
the 8515 display.  Here, a constant current source feeds into node A.
If the average current drawn by the EHT transformer is greater than
the current supplied by the constant current source, then the voltage
at node A falls, reducing the voltage at node B and reducing the
contrast signal.  A steady-state condition is rapidly achieved,
whereby the contrast signal settles at a level where the average
transformer current is equal to the output of the constant current
source, ignoring the small current drawn...