HIGH-VOLTAGE RESISTOR AND HIGH-VOLTAGE SUPPLY COMPRISING SUCH A HIGH-VOLTAGE RESISTOR

Abstract: The invention relates to a high-voltage resistor comprising a substantially rectangular substrate of an electrically insulating material. This substrate is provided, at two ends, with two electrodes which are connected to each other by means of a meander-shaper resistance layer. In accordance with the invention, one or more recesses are formed in one side face or two opposite side faces of the substrate. Resistors of this type or high-voltage supplies comprising such a high-voltage resistor can be operated at higher voltages and/or demonstrate a longer service life.

Description: High-voltage resistor and high-voltage supply comprising such a high-voltage resistor The invention relates to a high-voltage resistor comprising a substantially rectangular substrate of an electrically insulating material which is provided, at two ends, with two electrodes which are connected to each other by means of a meander-shaped resistance layer. The invention also relates to a high-voltage supply comprising a transformer or a voltage multiplier as well as a high-voltage resistor, which are cast into an insulating mass. High-voltage resistors as well as high-voltage supplies in which such high-voltage resistors are incorporated are used, in particular, in monitors and display tubes. A high-voltage resistor of the type mentioned in the opening paragraph is described in the non-prepublished European patent application having publication no. 96203033.4 (PHN 16.059). The high-voltage resistor described in said document comprises an electrically insulating substrate of aluminium oxide on which a meander-shaped resistance layer of a sintered thick-film paste is provided by screen printing. Two electrodes of sintered AgPd paste are provided at the ends of the resistance layer, also by screen printing. The applicant has found that a number of important disadvantages occur when the above-described high-voltage resistor is operated at high voltages of 20 kV and more. If these resistors are used under such conditions, they are customarily cast into an insulating mass, for example an epoxy. Temperature cycling tests to which high-voltage resistors in such an insulating mass were subjected showed that breakdown may occur at the above-mentioned high voltages. In addition, these tests showed that, under said conditions, the insulating mass may be subject to undesirable crack formation. These undesirable effects form an impediment to a long service life of the resistor or the high-voltage supply. It is an object of the invention to obviate the above-mentioned drawbacks. The invention more particularly aims at providing a high-voltage resistor which, at the same dimensions and the same construction, can be operated at higher voltages and/or exhibits a longer service life. In addition, the formation of cracks in the epoxy should be reduced. A further object of the invention is to provide a high-voltage supply comprising a transformer or a voltage multiplier as well as a high-voltage resistor, which are cast into an insulating mass, which high-voltage supply can be operated at higher voltages and/or has a longer service life. These and other objects of the invention are achieved by a high-voltage resistor comprising a substantially rectangular substrate of an electrically insulating material which is provided, at two ends, with two electrodes which are connected to each other by a meander-shaped resistance layer, which high-voltage resistor is characterized in that one or more recesses are formed in a side face of the substrate. The invention is based on the experimentally gained insight that creep between the electrodes occurs to a substantial degree along the side face of the substrate. The creep distance along the side face of the substrate is increased substantially by providing one or more recesses. As a result, the risk of breakdown between the electrodes is reduced and higher voltages can be applied to the resistor. At an equal load, the measure in accordance with the invention enables the service life of the resistor to be increased. An advantageous embodiment of the high-voltage resistor in accordance with the invention is characterized in that the recesses extend to between the meander-shaped resistance layer. This measure results in a substantial further increase of the creep distance. Another interesting embodiment of the high-voltage resistor in accordance with the invention is characterized in that the recesses are provided at two opposite side faces of the substrate. It has been found that this measure results in a substantial reduction of the shortest distance between both side faces, thus causing the mechanical stresses in the insulating mask to be reduced. By virtue thereof, the risk of crack formation in the envelop- ing mass is reduced. This leads to a longer service life of the resistor. The invention also relates to a high-voltage supply comprising a transformer or voltage multiplier as well as a high-voltage resistor, which are cast into an insulating mass. In accordance with the invention, this high-voltage supply is characterized in that a high-voltage resistor as described hereinabove is employed. By increasing the creep distance and reducing the mechanical stresses in the insulating mass, the high-voltage supply in accordance with the invention can be operated at higher voltages and/or its service life extended. These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter. In the drawings: Fig. 1 shows a plan view of a first embodiment of the high-voltage resistor in accordance with the invention, Fig. 2 shows a plan view of a second embodiment of the high-voltage resistor in accordance with the invention, Fig. 3 schematically shows a high-voltage supply in accordance with the invention. It is noted that, for clarity, the Figures are not drawn to scale. Fig. 1 shows a high-voltage resistor (1) which comprises a substantially rectangular substrate (2) of an electrically insulating material. In this case, use was made of aluminium oxide. The dimensions of the substrate are 25.4 mm by 12.7 mrr. by 1.0 mm. Electrodes (3, 4) are provided at two ends of this substrate by screen printing using an AgPd paste. A meander-shaped resistance layer (5) is situated between the electrodes (3, 4). This layer is also provided by screen printing. For the resistance material use was made of a lead- ruthenate-containing paste. The substrate is also provided with a coating (6) of a synthetic resin, for example a silicon polymer. In accordance with the invention, three recesses (7) are formed in a side face of the substrate situated between the ends of the substrate on which the electrodes are provided. The recesses can be formed by punching, cutting, etc., after the manufacture of the substrates or, of course, during the manufacture of said substrates. As shown in the Figure, the recesses preferably extend to between the meander-shaped resistance layer. The formation of these recesses (7) causes the creep distance along the side face of the substrate between the electrodes (3, 4) to be substantially increased. By virtue thereof, the risk of breakdown between these electrodes is reduced and higher voltages can be applied to the resistor. Fig. 2 shows a preferred embodiment of the high-voltage resistor in accordance with the invention. In Figs. 1 and 2, like reference numerals refer to like parts. In this embodiment, the recesses are formed at two opposite side faces of the substrate. This measure has the additional advantage that the shortest distance between both side faces is reduced, which leads to a reduction of the mechanical stresses in the insulating mass. By virtue thereof, the risk of crack formation in the enveloping mass is reduced. In a number of comparative experiments, the properties of a number of known high-voltage resistors A (without recesses in the substrate) were compared to high- voltage resistors B in accordance with the first embodiment (recesses in a side face of the substrate) and to high-voltage resistors C in accordance with the second embodiment (recesses in two opposite side faces of the substrate). A series of 20 high-voltage resistors of each of the types A, B and C were cast into an insulating mass of an epoxy. Subsequently, at a direct voltage of 20 kV, these resistors were subjected to a temperature cycling test. In this test, the resistors were maintained at a temperature of, successively, 40 °C and 100 °C for 3 hours. The percentage of rejects was determined after 11, 20, 28 and 36 temperature cycles. Ex. 11 20 28 36 A 100 85 60 35 B 100 100 90 75 C 100 100 100 90 Table 1 shows the results of the experiment. The indicated percentages represent the number of resistors that passed the test after the indicated number of cycles. Table 1 also shows that the presence of the recesses has a favorable effect on the service life of a high-voltage resistor. The greatest effect is achieved if the recesses are formed in two opposite side faces of the substrate. Fig. 3 schematically shows a high-voltage supply which, in this case, comprises a transformer 11 as well as a high-voltage resistor 12. Both elements and a screen/focus potentiometer 13, which is known per se, are cast into an insulating mass of an epoxy material. The housing of the epoxy material is schematically indicated by means of a dotted line 14. In accordance with the invention, a high-voltage resistor as described hereinabove is employed in said housing. The transformer 11 steps up the initial voltage via six layers of windings. Across the high-voltage resistor 12, there is applied 4/6 Λs of the total stepped-up voltage. The Gl and G2 electrodes of a display tube 15 can be driven via the screen/focus potentiometer. In summary, it is concluded that the invented high-voltage resistor and high-voltage supply comprising such a high-voltage resistor can be operated at higher voltages and/or demonstrate a longer service life.

Claims: CLAIMS: 1. A high-voltage resistor comprising a substantially rectangular substrate of an electrically insulating material which is provided, at two ends, with two electrodes which are connected to each other by means of a meander-shaped resistance layer, characterized in that one or more recesses are formed in a side face of the substrate. 2. A high- voltage resistor as claimed in claim 1, characterized in that the recesses extend to between the meander-shaped resistance layer. 3. A high-voltage resistor as claimed in claim 1 or 2, characterized in that the recesses are provided at two opposite side faces of the substrate. 4. A high-voltage supply comprising a transformer or a voltage multiplier as well as a high-voltage resistor, which are cast into an insulating mass, characterized in that use is made of a high-voltage resistor as described in any one of the preceding claims.

Inventors: HASPESLAGH HAROLD ROGIER MICHE, HASPESLAGH, HAROLD, ROGIER, MICHEL
Applicants: KONINKL PHILIPS ELECTRONICS NV, PHILIPS SVENSKA AB, KONINKLIJKE PHILIPS ELECTRONICS N.V., PHILIPS AB
Application year: 1998
Main classification: H01C7
IPC codes: H01C7, H01C17