A method and a coupling for decreasing the detrimental radiation caused by a cathode-ray tube

Abstract: The object of the disclosure is a method and a coupling for decreasing the alternating current field produced by a cathode-ray tube in the environment. The said field or radiation is at least partly due to the capacitances (C1, C2) of the anode (A) with respect, on the one hand, to the deflection coil (DY) and, on the other hand, to ground. These capacitances produce, mainly because of the return pulse of the deflection coupling, a line rate noise voltage to the anode. According to the invention, a back-off voltage (U 2 ) opposite to the noise voltage is fed to the anode (A) of the tube either directly or via a graphitizing (G). In cases where the same high-voltage transformer (TR) functions for both the high voltage of the anode and the horizontal deflection, the winding (L) of this transformer can also be used in generating the back-off voltage (U 2 ). The back-off voltage can thus be coupled to the anode (A) via the high-voltage capacitor (C3) of the transformer.

Description: The present invention relates to a method for decreasing the detrimental radiation caused by a cathode-ray tube in its surroundings, the radiation being at least partly due to the capacitance between an anode and a deflection coil. This capacitance creates line rate noise voltage for the anode. The invention also relates to coupling devices for carrying out the said method. Display units provided with cathode-ray tubes are increasingly being used in office work, in computer settings, process control, in laboratories etc. Therefore, special attention has begun to be paid to the detrimental electric and magnetic fields caused by visual display units and monitors especially in continuous and long-term use. The official recommendations will most likely determine limit values for both the magnetic fields and the electric fields which are in the low-frequency zone about 0 to 2 kHz (band 1) and in the high-frequency zone about 2 to 400 kHz (band 2). The present invention has particular, but not exclusive, reference to the provision of a method and a coupling for decreasing the variable electric fields in the high-frequency zone, ie. band 2. The invention is based on the observation that the cathode-ray tube produces the said type of an electrical field generally in three different ways. First, the horizontal deflecting windings of the deflection coils of the tube and the lead coupled thereto create around them an electric field proportional to the voltage in them. This voltage is known to be high, that is, a pulse-like voltage of about 1000 volts. Secondly, the capacitance between the deflection coil and the anode of the tube provides the anode with a voltage pulse proportional to the voltage in the coils of the deflection coil, to the capacitance between the anode and the deflection coil, and to the capacitance between the anode and ground. Thus a capacitive voltage division is created as the result of which the above-mentioned voltage pulse provides the anode with a voltage pulse according to band 2 and thus an electric field around the anode. Thirdly, while the tube is functioning, an electron beam discharges the capacitance between the anode and ground, which capacitance is recharged during each return pulse. Thus a line rate voltage resembling a saw-tooth wave is created for the anode. The voltage in turn creates an electric field of the said type according to band 2. The electric field created by the deflection coil and the leads coupled thereto can be eliminated in a relatively easy manner by packaging the cathode-ray tube in an electrically-conductive housing of metal or coated plastic. Thus the electric field forward of the anode remains the only problem. This field can be attenuated by installing a conductive, light-permeable panel in front of the screen and by grounding it. The said method is a very expensive solution, however, another alternative is to increase the capacitance between the anode and the ground by a separate, high-capacitance high-voltage capacitor. This solution is also expensive and makes the high-voltage transformer impractically bulky. AU-A-473214 discloses a method and a coupling for decreasing the radiation produced by a cathode-ray tube in its environment, as set out in the pre-characterising portion of claims 1 and 7. The purpose of the present invention is to provide a solution which also effectively attenuates the forward electric field while being both inexpensive and simple to realize. The invention is defined in the appended claims. The invention and the other features thereof will now be described in more detail, in an exemplary way, with reference to the appended single figure of drawing which partly schematically represents the coupling realizing the method according to the invention. In the drawing, the capacitance between anode A of the cathode-ray tube and deflection coil DY is indicated by reference C1 and the capacitance between the anode and the ground is indicated by C2, respectively. Capacitances C1 and C2 cause the above-mentioned voltage division resulting in line rate noise voltage coming to the anode circuit producing an alternating-current field. This voltage is compensated according to the invention by capacitively coupling to the anode a back-off voltage having an opposite polarity via the graphitizing G of the tube. In the implementation according to the drawing, the back-off voltage is generated by using the high-voltage transformer TR of the cathode-ray tube. To be precise, one winding L of the high-voltage transformer is coupled to the output of operational amplifier OA. Voltage U 1 is brought to the input of the operational amplifier, the voltage being dependent on the electron beam I A . This circuit further includes integrating portion R, C4 approximating the discharge of the capacitance C2 between the anode and the ground. The circuit produces in the output thereof a back-off voltage U 2 proportional to the saw-tooth voltage of the anode, only it is negative. The voltage is then fed via winding L and the high-voltage capacitor C3 to the graphitizing G of the tube with the intent of nullifying the above-mentioned saw-tooth voltage. In addition, winding L creates a voltage proportional to the negative return pulse which nullifies the voltage produced by the deflection coil. Thus in the case according to the drawing, the existing high-voltage transformer TR and the high-voltage capacitor C3 of the cathode-ray tube is used for producing the said back-off voltage. Such a solution is appropriate in the present case where the same high-voltage transformer TR works for both the high voltage of the tube and its horizontal deflection. In addition to the high-voltage transformer, the circuit includes, in the case according to the drawing, transistor T, capacitor C5 and diode D2. It is naturally also possible to produce the back-off voltage by a separate circuit. This is especially so in the case where the high voltage and the horizontal deflection of the anode are operated by different circuits. In accordance with the embodiment of the invention described the back-off voltage in the opposite direction to the return pulse is fed to the anode capacitively via graphitizing G of the picture tube.

Claims: A method for decreasing the detrimental radiation produced by a cathode-ray tube in its environment, when the radiation is at least partly due to the capacitance (C1) between an anode (A) and a deflection coil (DY), the capacitance generating a line rate noise voltage to the anode, wherein a back-off voltage (U 2 ) opposite to the said noise voltage is capacitively coupled to the anode (A), characterized in that the said back-off voltage (U 2 ) is capacitively coupled to the anode (A) via the graphitizing (G) of the tube. A method according to claim 1, characterized in that the back-off voltage (U 2 ) is, with respect to its amplitude, a negative voltage proportional to the return pulse and/or an electron beam current (I A ). A method according to either of the preceding claims, characterized in that the same transformer (TR) functions both by forming the high voltage of the anode (A) and by forming the return pulse, and in that this high-voltage transformer (TR) is also used to generate the back-off voltage (U 2 ). A method according to claim 3, characterized in that the back-off voltage (U 2 ) is produced by the transformer (TR) and a high-voltage capacitor (C3). A method according to claim 1, characterized in that a separate transformer is used for generating the back-off voltage (U 2 ). A method according to any of the preceding claims 1 to 5, characterized in that a part of the back-off voltage (U 2 ) is formed dependent on the electron beam current (I A ). A coupling for decreasing the detrimental radiation caused by a cathode-ray tube in the environment, comprising circuit elements (OA, L) for forming an output signal, namely a pulse-like back-off voltage (U 2 ) proportional to the return pulse, the output signal of the circuit being capacitively coupled to the anode (A), characterized in that the said back-off voltage (U 2 ) is capacitively coupled to the anode (A) via the graphitizing (G) of the tube. A coupling according to claim 7, where the same high-voltage transformer (TR) is operable in forming both the anode voltage and the return pulse, characterized in that one winding (L) of the high-voltage transformer (TR) is operable to generate the said back-off voltage. A coupling according to claim 8 comprising an amplifier (OA) coupled to the winding (L), a signal (U 1 ) dependent on the intensity of the electron beam being coupled to the input of the amplifier whereby to generate the said back-off voltage. A coupling according to claim 9, characterized in that the back-off voltage (U 2 ) is coupled from the said winding (L) to the anode via the high-voltage capacitor (C3).

Inventors: HAAPAKOSKI HANNU AULIS, HAAPAKOSKI, HANNU AULIS
Applicants: NOKIA DISPLAY PRODUCTS OY
Application year: 1992
Main classification: H01J29
IPC codes: H04N3, H01J29