Browse Prior Art Database

Zero Crossing Electric Field Emissions Discharge for Two-Wire Systems

IP.com Disclosure Number: IPCOM000110880D
Original Publication Date: 1994-Jan-01
Included in the Prior Art Database: 2005-Mar-26
Document File: 2 page(s) / 83K

Publishing Venue

IBM

Related People

Beeteson, JS: AUTHOR [+2]

Abstract

Described is a method of eliminating low frequency electric field emissions (ELEF) in two-wire (double insulated) monitors supplied with mains power from an unbalanced source. The need to connect a discharge path for an ESF coating can re-introduce noise emissions. The disclosed solution, suitable for both balanced and unbalanced sources, is a circuit which switches the connection of the ESF coating to a discharge path at a point in the mains cycle when that path is 'close to ground' and free of AC voltages which could increase ELEF noise.

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Zero Crossing Electric Field Emissions Discharge for Two-Wire Systems

      Described is a method of eliminating low frequency electric
field emissions (ELEF) in two-wire (double insulated) monitors
supplied with mains power from an unbalanced source.  The need to
connect a discharge path for an ESF coating can re-introduce noise
emissions.  The disclosed solution, suitable for both balanced and
unbalanced sources, is a circuit which switches the connection of the
ESF coating to a discharge path at a point in the mains cycle when
that path is 'close to ground' and free of AC voltages which could
increase ELEF noise.

      Electric field emission due to the power mains is very high in
two-wire double-insulated equipment.  Standard supply MPR-II
requirements are 25V/m, while some double insulated monitors can
measure 250V/m.  Solutions employing an active power cord utilizing
the property of most distribution systems of having the neutral line
either near earth potential, or a very low voltage, may use the
neutral as a shield round the line cord, and connected to any
internal metalwork, to eliminate the power cord generated electric
emission and so satisfy MPR-II.  A requirement also exists to provide
a discharge path for the ESF coating on the CRT screen.  Existing
two-wire monitors connect the secondary zero-volts (and hence the ESF
coating) to a center point of the mains rectifier bridge, through a
high impedance circuit, as shown in Fig. 1.  For normal unbalanced
mains distribution systems this point has a half-wave rectified mains
voltage signal which increases the ELEF from the monitor.  One
monitor measured a decrease of 40V/m when this network was removed.
To overcome this, the previous shielded neutral solution would
connect the ESF screen to the detected neutral line - and...