Browse Prior Art Database

Cancellation of Very Low Frequency Magnetic Field

IP.com Disclosure Number: IPCOM000035261D
Original Publication Date: 1989-Jun-01
Included in the Prior Art Database: 2005-Jan-28
Document File: 2 page(s) / 58K

Publishing Venue

IBM

Related People

Funakoshi, A: AUTHOR [+2]

Abstract

Disclosed is a method for operating a cancellation coil during the retrace period of a CRT display device, which cancels a magnetic field of very low frequency generated by horizontal deflection yoke.

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Cancellation of Very Low Frequency Magnetic Field

Disclosed is a method for operating a cancellation coil during the retrace period of a CRT display device, which cancels a magnetic field of very low frequency generated by horizontal deflection yoke.

Previous cancellation coils were operated during both the display period and retrace period, so that degradation of displayed image and a loss of power arose. This problem is solved in this article.

The cancellation coil operated during the retrace period suppresses the worst-time derivative of magnetic flux density (MB/Mt) from the horizontal deflection yoke without image quality degradation, since MB/Mt is in proportion to the time derivative of yoke current, as below: MB/Mt dI/dt I: horizontal yoke current

(Image Omitted)

and dI/dt has the maximum value in the retrace period of beam deflection.

In Fig. 1a, electron beam from the gun of a cathode ray tube is deflected toward the left side on the screen in period t4 and toward the horizontal yoke current.

Periods t2 and t3 are the retrace periods during which the electron beam goes back to the left edge from the right edge on the screen, and the display image does not appear.

In Fig. 1b, transistor TR1 is turned on during the period t1 and turned off during the period t2, t3 and t4. When TR1 is on, yoke current passes through path t1, and when TR1 goes off, yoke current flows into C1 and returns to the horizontal deflection yoke (HDY) in resonance mode. At the momen...