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GROUND ISOLATION DIODE FOR CRT SPARK GAP AND VIDEO GROUNDING

IP.com Disclosure Number: IPCOM000025726D
Original Publication Date: 1987-Aug-31
Included in the Prior Art Database: 2004-Apr-04
Document File: 2 page(s) / 143K

Publishing Venue

Xerox Disclosure Journal

Abstract

Referring to the drawing Figure, the spark gap grounding of a Cathode Ray Tube (CRT) circuitry 5 display presents several design problems that affect circuitry reliability and electromagnetic emissions. To minimize electromagnetic emission interference (EM) at high video bandwidths, it is desirable to ground the exterior CRT conductive coating 6 directly to the chassis 7. This is most effectively done by using a CRT bulb which has the conductive coating sprayed up to and over the mounting hardware, usually a bonded shell or a tension band. Unfortunately, although this may reduce EMI related emissions, it provides an undesirable return path through the video circuitry and the chassis for CRT random arcing.

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XEROX DISCLOSURE JOURNAL

GROUND ISOLATION DIODE FOR CRT SPARK GAP AND VIDEO GROUNDING Charles R. Nail

Proposed Classification
U.S. C1.328/8 Int. C1. H02h 7/20

+ 18KV

Referring to the drawing Figure, the spark gap grounding of a Cathode Ray Tube (CRT) circuitry 5 display presents several design problems that affect circuitry reliability and electromagnetic emissions. To minimize electromagnetic emission interference (EM) at high video bandwidths, it is desirable to ground the exterior CRT conductive coating 6 directly to the chassis 7. This is most effectively done by using a CRT bulb which has the conductive coating sprayed up to and over the mounting hardware, usually a bonded shell or a tension band. Unfortunately, although this may reduce EMI related emissions, it provides an undesirable return path through the video circuitry and the chassis for CRT random arcing.

Arcing is common and is normal for CRT technology. Arcs occur between the interior anode which is charged to + 18kV or more and the more negatively charged gun components. To enable a short circuit path for arc energy back to the conductive coating, an external bypass spark gap 9 with spark gap ground

Volume 12 Number 4 JulyIAugust 1987 18 5

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GROUND ISOLATION DIODE FOR CRT SPARK GAP AND VIDEO GROUNDING (Cont'd)

10 is provided for each of the pins entering the neck of the CRT. Depending upon the design of the video circuitry, the spark gap ground 10 may or may not be attached to the video ground 11 at the neck of the CRT. To minimize EMI problems and to provide a ground path from the video ground 11 to the spark gap ground 10 before the spark gap 9 has fired, it is desirable to have the two grounds connected. However, it is not desirable to have a return path from the spark gap 9 through the chassis ground after the spark gap has fired.

To solve the foregoing problem a ground Isolation Diode 12 is placed between the video ground 11 and the spark gap ground 10.

Under normal operating conditions the CRT anode and interior conductive coating are about +18KV or more with respect to ground. Frequently conditions inside the CRT tube itself will cause an arc to jump between the internal conductive coating and the more negative gun compo...