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Beam Current Measurement

IP.com Disclosure Number: IPCOM000041419D
Original Publication Date: 1984-Jan-01
Included in the Prior Art Database: 2005-Feb-02
Document File: 2 page(s) / 51K

Publishing Venue

IBM

Related People

O'Rourke, R: AUTHOR [+2]

Abstract

The following method of beam current measurement used for cathode ray tubes (CRTs) has several advantages over the moving coil ammeter method currently in use. Measurements are not affected by static charge, test gear is not directly connected to high voltage circuits, measurement of peak current is possible, and the apparatus should not require frequent re-calibration. Measurements of the very low beam currents such as are found in the cathode-anode circuits of CRTs, are usually accomplished by connecting a moving coil meter in series with the anode lead. The anode of a color CRT operates typically at about 25 KV and, therefore, great care must be exercised in such measurements.

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Beam Current Measurement

The following method of beam current measurement used for cathode ray tubes (CRTs) has several advantages over the moving coil ammeter method currently in use. Measurements are not affected by static charge, test gear is not directly connected to high voltage circuits, measurement of peak current is possible, and the apparatus should not require frequent re-calibration. Measurements of the very low beam currents such as are found in the cathode-anode circuits of CRTs, are usually accomplished by connecting a moving coil meter in series with the anode lead. The anode of a color CRT operates typically at about 25 KV and, therefore, great care must be exercised in such measurements. Also, as the measurements required are in the range of 10 to 500 mA, moving coil meters operating at these low current and high 'above-ground' voltage conditions tend to suffer from adverse static charge effects, frequently causing inaccuracies. The use of a conventional Hall-effect current probe (such as a Tektronix AM503) connected around the anode lead is not practicable for two reasons. The size of the anode lead itself will not fit in the probe jaws. The maximum sensitivity of the current probe is only: 1 mA = (10 mV)/division However, the method described as follows permits the use of a current probe by using a coil having 100 turns of very fine wire connected in series with the anode lead, around which the current probe is clamped. This arrangement multipl...