Patent GB-895568-A (GB)
Title: Improvements in or relating to capacitors
IPC: H01G2/14
Filed: 1960-10-24
Abstract: 895,568. Capacitors; voltage-dependent capacitors; printed circuits. TELEGRAPH CONDENSER CO. Ltd. (Sprague Electric Co.). Oct. 24, 1960, No. 36382/60. Class 37. A capacitor is constructed from a thin dielectric layer having a non-conductive low-permittivity coating on one face thereof, with an orifice within which conductive material bears on the dielectric layer, and a connecting electrode formed on the coating which is in control with the conductive material; the opposite face of the dielectric layer being provided with a further electrode which may be similarly constructed. A ceramic disc 10 which may be of barium titanate (Fig. 5) has a spot of conducting material deposited at the centre of one or both faces to form an electrode 12 whose contact area determines the required capacitance. The disc and electrode are covered with a vitreous enamel annulus 13 having a central orifice 14 extending inwardly to the electrode, and a large electrode 15 overlies a substantial area of the annulus and contacts the electrode 12. A mass of solder 16 overlies electrode 15 and secures a wire or tab 17 (Fig. 3) of cross-section sufficient to conduct away any heat developed in the capacitor. A similar electrode arrangement may be applied to the opposite face of the disc 10 with an electrode 12 of the same or of different size, or a plane electrode may be applied direct to the opposite face. The capacitor unit and its lead is covered with a hardenable resin for protection and insulation which cover may be wax impregnated after hardening. The electrode 12, annulus 13, and electrode 15 may be screened on to the disc 10 in succession, and the vitreous enamel of the annulus may be filled with, e.g., aluminium oxide. The binder material used for bonding the particles of the electrode material to the bases to which they are applied is desirably one in which the conducting particles do not float. In a modification (Fig. 6, not shown) the vitreous enamel annulus may be applied direct to the ceramic disc and a single electrode applied over the annulus and extending through the control orifice to the face of the disc. The lead wires and plastic coating may be omitted and the capacitor located in a button type mounting, which improves heat dissipation. The permittivity of the ceramic disc material is of at least one order of magnitude greater than that of the vitreous enamel. A capacitance constructed as above may be incorporated in a printed circuit (Figs. 7, 8, not shown) comprising a dielectric base layer which may be of ceramic material on which electrical components, e.g. terminals and resistors, may be mounted. A low permittivity, e.g. vitreous enamel coating is applied in two layers to the dielectric base to underlie the terminals and resistors, and a rectangular orifice is formed in the coating and is filled with a metal electrode contacting the dielectric layer, having an active area defined by the orifice. A conductive element overlying the enamel coating and the orifice therein connects the electrode to the printed circuit, and a corresponding electrode is applied to the opposite side of the dielectric base.