1Field of the Invention
The present invention relates generally to electrical switches of the snap-action type and, more particularly to a novel snap switch which operates satisfactorily at very low current and voltage levels and which includes a contact which has a self-cleaning action.
2. Background Art
Snap switches are well known devices for switching the flow of electricity and are manufactured, for example by Westport Development & Manufacturing Company, Inc. The type of such switches to which the present invention relates have a one-piece spring blade with a contact carrying free end which is selectively moved between electrical terminals. The blade is of the snap-acting, over center type and has an operation point which, if sufficiently linearly moved beyond a prescribed limit, distorts the blade to effect the snap-action and a subsequent movement of the free contact end from one terminal to another. The movement of the blade is maintained within a range by two spaced apart stop members. The amount of movement of the operation point from one stop member until the blade shifts from one contact to the other is precisely determined and is generally maintained constant throughout a wide range of operating temperatures.
Such type of snap switches is described in U.S. Pat. Nos. 3,278,700; 3,609,269; and 4,017,699, issued to Robert R. Hellman, and assigned to the assignee of the present invention, the disclosures of all of which patents are hereby made a part hereof by reference. The first of the patents describes a snap switch having a bracket for preloading the blade and certain features for temperature compensation. The second of the patents describes such a switch disposed in a hermetically sealed housing and actuatable by means of the distortion of a sidewall of a tube forming part of the housing. The third of the patents describes a snap switch employing two snap switches mounted together so that both are actuated simultaneously.
While the switches described above have provided satisfactory service, they are somewhat limited in that there are lower limits of voltage and current below which they cannot be used, due to fouling of the flat surfaces contacted by the round contact ball at the free end of the blade. This fouling is due, in part, to the creation of polymer from ambient impurities by the contact ball as it only slightly wipes the surfaces. This slight wiping action is insufficient to clean the polymer thus formed from the surfaces. In addition, when the contact ball contacts a spherical surface on a terminal, there is a flattening of the contacting surfaces because of the wiping action. The lower limits of known such switches are about 10 milliamperes and 10 volts.
Another factor contributing to the limitations of the prior snap switches is that the blade itself is in the electrical circuit between two terminals, since the fixed end of the blade is mounted on a common terminal. This introduces additional resistance into the total circuit, since desirable spring materials are relatively poor electrical conductors. The lowest total switch resistance obtained with conventional switches has been on the order of about 60-80 milli-Ohms.
Furthermore, it is desirable that the current carrying components of such switches have the most efficient electrical characteristics, but that these be combined with lower cost materials for other components requiring other characteristics such as weldability and suitability for high temperature service.
Accordingly, it is an object of the present invention to provide a snap switch that is capable of use at lower voltage and current levels than previously known such switches.
An additional object of the invention is to provide such a switch that employs a self-cleaning contact.
Another object of the invention is to provide such a switch in which the blade thereof does not become part of the electrical circuit being switched.
A further object of the invention is to provide such a switch which is usable at higher temperatures than previously known such switches.
Yet an additional object of the invention is to provide such a switch that may be economically constructed.
Yet another object of the invention is to provide such a switch which has lower total electrical resistance than previously known such switches.
Other objects of the present invention, as well as particular features and advantages thereof, will be elucidated in, or be apparent from, the following description and the accompanying drawing figures.