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Browse Prior Art Database

Bias Detection Technique

IP.com Disclosure Number: IPCOM000091870D
Original Publication Date: 1968-Jun-01
Included in the Prior Art Database: 2005-Mar-05
Document File: 2 page(s) / 32K

Publishing Venue

IBM

Related People

Johnson, HW: AUTHOR

Abstract

The circuit monitors the buildup of a bias voltage for a device such as a CRT and prevents connection of the device to the bias voltage until it is safe to do so. Bias voltage for the cathode of CRT 1 is generated at the wiper of potentiometer 2 and is connected to such cathode when relay 3 is closed. Capacitor C serves as a noise filter and also maintains cathode bias voltage in the event of a failure of V1 for a long enough period of time to enable detection of the failure of V1 for a long connect the cathode from its bias before excessive beam current can damage CRT 1. However, C presents a difficulty during power-up sequences, as the sensing of V1 is then not adequate to insure that the cathode bias voltage is up since additional time is needed to charge C.

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Bias Detection Technique

The circuit monitors the buildup of a bias voltage for a device such as a CRT and prevents connection of the device to the bias voltage until it is safe to do so. Bias voltage for the cathode of CRT 1 is generated at the wiper of potentiometer 2 and is connected to such cathode when relay 3 is closed. Capacitor C serves as a noise filter and also maintains cathode bias voltage in the event of a failure of V1 for a long enough period of time to enable detection of the failure of V1 for a long connect the cathode from its bias before excessive beam current can damage CRT 1. However, C presents a difficulty during power-up sequences, as the sensing of V1 is then not adequate to insure that the cathode bias voltage is up since additional time is needed to charge C.

To overcome this difficulty, C is connected such that its charging current is the base current of Q2. Q1 is off since V1 is not yet sensed and thus the And condition necessary to close relay 3 is not satisfied. The charging current of C causes Q2 to turn on in the saturation condition and hold the And condition off even though V1 is sensed. Q2 remains on until the charging current of C diminishes to the point where the gain of Q2 can no longer maintain it in the saturation condition. As Q2 turns off, the And condition is satisfied, relay 3 is closed, and Q1 is turned on to provide a ground return for C in the filtering mode.

Proper selection of R enables delay of relay 3 closure un...