Electrolytic capacitor vent
[451 Apr. 15, 1975 United States Patent Ando 3,204,156 8/1965 Moresi et 317/230 3221.217 11/1965 317/230 3280381 10/1966 Ayer 317/230 3.688.162 8/1972 Willy et a]. 317/230 [22] Filed: July 23, 1973 Primary Examiner.10hn Zazworsky Assistant ExaminerWi11iam D. Larkins Appl. No.: 381,807  
 [57] ABSTRACT A sealed capacitor having a casing including a top wall having portions of reduced thickness so that excessive [51] Int. 1  
  pressure developed within the casing wi11 rupture the [56] uNlTE g g z xfr s gg rENTs portions of reduced thickness.  
 2,274,327 2/1942 Georgiev et a1. 317/230 1 Claim, 3 Drawing Figures 1 ELECTROLYTIC CAPACITOR VENT This invention relates to an electrolytic capacitor and more specifically to an improved pressure vent structure thereof.  
  When an inverse voltage. continuous excessive voltage or extraordinary pulse is applied to an electrolytic capacitor. an excessive current flows through the electrolyte and causes electrolysis to produce a gas. Moreover. the internal pressure is raised by Joules heat due to the internal resistance and sometimes the capacitor explodes by the excessive pressure. The same phenomenon may be caused by deterioration of the capacitor element. Although there is some variance in the magnitude of the breaking pressure depending upon the construction and material of the capacitor. it is about 30 killograms/cm in those having a cylindrical casing of about millimeters in diameter. Therefore. the explosion of the capacitor is sometimes quite dangerous.  
  According to prior techniques. safety means have been used by forming a through-hole in the sealing plate or terminal plate and closing the hole with a thin rubber plate. so that the rubber plate is broken by the EHICI&#39;IIHI pressure. However. it is difficult to obtain a constant breaking pressure because the strength of the rubber plate depends upon various parameters such as thickness. hardness. kind of rubber and the like which is difficult to control. Moreover. the rubber plate may deteriorate with a lapse of time and the pressure vent function may become unreliable.  
  Therefore. an object of this invention is to provide an improved pressure vent structure for an electrolytic capacitor. which is simple and more reliable than prior art vents.  
  According to this invention. the top of the casing has a portion which is made thinner than the remainder of the top. More specifically. the top face of the casing has a plurality of domains which are defined by boundaries including a plurality of radial lines intersecting at the center of the top face.  
  This invention will be described in detail hereinunder with reference to the accompanying drawings.  
 In the drawings:  
  FIG. 1 is a cross sectional view representing an example of an electrolytic capacitor including a prior art pressure vent structure;  
  FIG. 2 (A) is a plan view of an embodiment of the electrolytic capacitor including the pressure vent structure according to this invention; and  
  FIG. 2 (B) is a cross sectional view of the capacitor of FIG. 2 (A).  
  Throughout the drawings, like reference numerals are used to denote like structural components.  
  Referring now to FIG. 1, there shown is an electrolytic capacitor provided with a pressure vent structure according to the prior art. The capacitor comprises an aluminium casing 2 containing a capacitor element 1 which is coaxially arranged therein. and a terminal plate 3 which air-tightly closes the open end of the easing 2 by being fixed thereto by swaging the open edge 7 as shown. The terminal plate 3 has a pair of rivets 5a and 5b passing therethrough. Terminals 4a and 4b of the capacitor element 1 are fixed to the inner ends of the rivets and terminal leads 6a and 6b are fixed to the outer ends. The terminal plate 3 consists of a plastic disc 31! and a rubber disc 3b and the plastic disc 3a has a through hole 8 in the center.  
  When the internal pressure of the capacitor of FIG. I is raised above a specific level. the rubber disc 3b is broken and the internal gas is exhausted through the hole 8. However. the pressure at which the rubber disc is broken is dependent almost entirely upon the strength of the rubber disc which is difficult to precisely control. and the reliability ofthe pressure vent function is unavoidably reduced. Although the above construction of pressure vent structure is given merely by way of illustration. there are many variations of constructions. though it is common to adopt a rubber layer as a safety valve and the same disadvantage also occurs in the variations.  
  Referring next to FIGS. 2A and 28 there shown is an embodiment of electrolytic capacitor provided with a pressure vent structure according to this invention. The capacitor comprises a capacitor element 1. an aluminium casing 2 and a terminal plate 3. The terminal plate 3 consists of a single rubber disc and is fixed to the open end of the casing 2 by swaging the open edge 7 and forming a circumferential groove 10 as shown. A pair of terminal leads 6a and 6b pass through the terminal plate 3 and are welded directly to terminals 40 and 4b of the capacitor element 1. Due to the simplified structure of terminal plate 3. simple lead-out means can be adopted.  
  Referring especially to FIG. 2(A). there shown is a top face of the casing 2 of this invention. including a flower-like pattern 91:. The pattern 9a has a contour including four diametral lines 11 intersecting at the center. According to this invention. the inside portion 9a of the flower-like pattern has a thickness which is substantially less than that of the outer portion 9h. Such a pattern can be formed by a coin-pressing process. When the internal pressure of the casing 2 is raised. the internal stress of the material of the top wall of the easing is collected at the center and the top wall is broken typically along two orthogonal chain lines 12. As the strength and thickness of the top wall of the metal casing can be controlled precisely. the pressure at which the casing is broken can be maintained substantially constant and&#39;the pressure vent function of the capacitor is therefore reliable.  
  As an example. the pressure vent pattern of FIG. 2(A) was used on aluminium casings having a diameter of 10 millimeters a height of 13 millimeters and a wall thickness of 0.4 millimeter. The inner portion 9a was arranged with a thickness of 0.2 millimeters. while the outer portion 9b had a thickness of 0.4 millimeters. The internal pressure of each casing was raised in 2 killograms/cm steps. keeping each pressure constant for 1 minute at each step. The internal pressures at which the casings were broken and the number of casings broken at each pressure are given in the following table.  
 INTERNAL PRESSURE NUMBER OF BROKEN CASINGS (Kg/em&#34;) l4 l6 I6 29 IX 4 10 I thereof were broken within a range as narrow as 2 killograms/cm? This variance of the actuating pressure is significantly less than that of the prior art structure. which may be about killograms/cm&#34; or higher.  
  It should be noted that the above description is only for illustrative purposes and various modifications can be made within the scope of this invention. For example. the number and shape of petals 9a and 9b can be arbitrarily selected in accordance with the desired actuating pressures. Moreover. other shapes of patterns such as circles. squares. crosses and the like can he adopted in place of the flower-like pattern. though they may be somewhat inferior in comparison with the flower-pattern.  
  The pressure vent structure of this invention not only provides improved reliability of the pressure vent function but also simplifies the construction and manufac- 4 turing process of the capacitor.  
 What is claimed:  
  1. An electrolytic capacitor comprising a cylindrical housing. a wall integrally formed with said housing and closing one end thereof, an electrolytic capacitor within said housing and having terminals extending therefrom. and means closing the other end of said housing with the terminals extending therethrough. said wall having a plurality of wedge-shaped sectors angularly spaced one from the other and radiating outwardly from a common center. the wall portions forming said sectors each having a thickness less than the remaining portions of said wall whereby said sectors form a precise pressure release means should the pressure exceed a predetermined magnitude within said housing.