Time Delay Switch

Abstract

A switch which embodies a fail safe feature achieved by means for interrupting the switch circuit when the switch contacts remain closed for longer than a predetermined time period, said switch comprising a support having first and second terminals to one of which a bimetallic element is conductively connected, and third and fourth terminals to which respective ends of a heater element are connected, a conductive strip disposed in normally nonconductive relation to any of the terminals, and a contact member attached to the second terminal, with the bimetallic element and conductive strip being normally spaced out of electrical contact with one another, and with the conductive strip and contact member being normally in electrical contact with one another.

Description

BACKGROUND OF THE INVENTION

Switches have generally been utilized to complete or disrupt electrical circuits, and when necessary have been employed with a time delay relay to close an electrical circuit for only a selected time period. In some installations, for example, a switch in a time delay relay is closed to an electrical circuit which operates a mechanism for performing a specified function and after a selected time interval controlled by the time delay factor of the relay the switch is opened so that the mechanism will cease its function.

In certain installations it may be desirable to utilize a first circuit for closing a switch to complete a second circuit to a mechanism for a specified time interval, after which the first circuit is disrupted and the switch is opened so that the second circuit is broken to stop operation of the mechanism. However, it has been found that such devices sometimes malfunction. For example, the first circuit may fail to be disrupted at the end of the preset time cycle, whereupon the mechanism may continue to operate with what may be undesirable or even disastrous effects.

SUMMARY OF THE INVENTION

The present invention is directed to a novel switch having a built-in safety feature which avoids the referred-to disadvantages of and objections to conventional devices of this character. The invention is achieved by a switch which embodies a bimetallic member which carries one of a pair of normally open contacts the other of which is mounted on a conductive strip or blade. A heater located adjacent the bimetallic element is connected to an external circuit and when heated causes movement of the bimetallic element in a manner whereby the normally open contacts become closed. A second normally closed pair of contacts, one of which is fixed and the other of which is mounted on the conductive strip, are maintained in close condition to complete an external circuit when the first pair of contacts are closed.

After a predetermined time interval, controlled by the bimetallic element structure, continued heating of the element will cause such continued movement of the element as will move the conductive blade in a manner such as will open the normally closed contacts. Thus, the external circuit will be disrupted.

BRIEF DESCRIPTION OF THE DRAWING

Other objects and advantages of this invention will be apparent from the following description taken in connection with the accompanying drawings, wherein:

FIG. 1 is a side elevational view of a switch embodying the invention shown in inoperative condition; and

FIG. 2 is a side elevational side view of the switch of FIG. 1 showing the parts thereof in operative positions.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring more particularly to the drawings wherein like characters of reference designate like parts in the respective views, there is provided an elongated housing or casing 10 preferably of nonconductive material, although it is to be understood that the casing may be made of conductive material if desired, in which case the conductive parts which are mounted within the casing or which extend through the casing must be insulated therefrom.

Inside the casing 10 is a flat bimetallic strip element 12 which is made of two laminated materials having different coefficients of expansion so that when the element is heated it will curve in response to the different expansion characteristics. One end of the element is fixedly located such as by anchoring it with screws to a projection or shelf 14 formed on an inner end wall 15 of the casing as shown.

The lower surface of the element 12 carries near its free end one of a pair of contacts 16-18, the second of the contacts being mounted on a conductive metal strip or blade 20 one end of which is fixed to the end wall 15 of the casing 10 as by a screw or the like.

The other end of conductive blade 20 carries on its upper surface one contact of a second pair of contacts 22-24, the other of which is mounted upon a short metal plate or bracket 26 fixedly attached to the adjacent end wall 27 of the casing 10 as by a screw or the like.

Contacts 16-18 are normally maintained in spaced or open relationship while contacts 24-26 are normally maintained in closed or engaging relation as shown. The bimetallic element 12 is electrically conductive and is connected by a lead 28 to a terminal 30 which extends through the top wall 33 of the casing 10. The fixed plate 26 is similarly connected by a second lead 32 to a terminal 34 which also extends through the top 33 of the casing. Thus, when terminals 30 and 34 are connected into an external circuit, the circuit will be completed when contacts 16-18 are closed through terminal 30, lead 28, element 12, contacts 16-18, blade 20, contacts 22-24, bracket 26, lead 32 and terminal 34. When this occurs, the circuit may be utilized to perform any selected function such as to activate mechanism.

Means for closing contacts 16-18 is in the form of a heater 36 which is a resistive wire coiled around or beside the bimetallic element in spaced relation with it, the ends of the wires being connected to respective spaced terminals 38 and 40 extending through the casing top wall 33. Terminals 38 and 40 are adapted to be connected to an external circuit which supplies electrical current to the heater coil 36 for the purpose of generating heat therein. Such heat will cause the bimetallic element 12 to bend or flex as shown in FIG. 2.

It is well known that a bimetallic strip such as element 12 will bend or flex in the direction of the side having the smaller coefficient of expansion. Therefore, such flexure of element 12 will cause movement of contact 16 on element 12 into engagement with contact 18 on blade 20, thus completing an operating circuit through normally closed contacts 22-24 as described.

The heater coil may continue to be heated for a predetermined time after which it is intended that the circuit be broken to the mechanism which is being operated.

It will be apparent that subsequent interruption of the external circuit to heater coil 36 will cause the coil to cool and will result in the return of the bimetallic element 12 to its initial shape. This will, of course, cause contacts 16-18 to open. The operating circuit to the external mechanism will thus be disrupted and the mechanism will cease functioning in the absence of other means for continuing its operation. However, it has been found that the heater circuit may not be interrupted at the desired time for any one of many reasons, and therefore the circuit to the mechanism will remain closed and the mechanism will continue to operate with what may be disastrous results.

The present invention, therefore, includes a safety feature which provides for the required disruption of the circuit to the mechanism at the end of a predetermined time interval. This is achieved by the structures of the bimetallic element 12 and the blade 20. The rate at which a bimetallic element of specified shape moves is well known and easily computed. The tension of a metal strip such as blade 20 is also easily determined. Therefore, in accordance with this invention the heater coil 36 will remain energized and the bimetallic element will continue to be heated. Such additional heating of the element 12 will cause it to bend or flex still more against the inherent tension exerted by the blade 20 until, at the end of a predetermined time period, the blade 20 will be forcibly bent of flexed by the pressure of element 12, and the normally closed contacts 22-24 will open. This will interrupt the circuit and the mechanism will cease functioning.

When the heater coil 36 is thereafter deenergized by any suitable means, the parts will return to their normal positions as shown in FIG. 1 and the device is ready for a repetitive operation.

It will be apparent that the time delay which is built into the switch of this invention may be precisely controlled by the characteristics of the heater coil 36, the structures and compositions of the bimetallic element 12, and the metal blade 20. Since this represents conventional well-known engineering, no examples of specific sizes, shapes or compositions are believed to be necessary here.

The switch embodying this invention can be utilized where precise control of equipment or mechanism is desired for only selected time intervals and the safety feature built into the switch provides a novel and highly desirable fail safe element.

It will also be apparent that various changes and modifications in the structure shown and described may be made by those skilled in the art without departing from the spirit of the invention as expressed in the accompanying claims. Therefore, all matter shown and described is to be interpreted as illustrative and not in a limiting sense.

Claims

We claim:

1. A fail safe switch comprising a support having first and second spaced terminals, a heater element for connection to an external source of heater current, third and fourth terminals on the support and connected to respective ends of the heater element, a bimetallic element adjacent said heater element and connected to said first terminal, a conductive strip adjacent said bimetallic element and mounted on said support in normally nonconductive relation to any of said terminals, and a contact member adjacent said strip and connected to said second terminal, said bimetallic element and conductive strip being normally spaced out of electrical contact with one another, and said conductive strip and contact member being normally in electrical contact with one another.

2. A fail safe switch comprising a casing having first, second, third and fourth spaced electrical terminals, a contact member anchored to said casing and connected to said first terminal, an elongated bimetallic element having one end anchored to the casing and connnected to said second terminal, the opposite end of the bimetallic element extending within the casing toward said contact member and movable along a predetermined path when heated, an elongated flexible conductive strip anchored at one end to the casing in normally nonconductive relation to any of said terminals and with its opposite end disposed normally in electrical contact with said contact member, a midportion of said strip being disposed in the path of movement of the bimetallic element and normally spaced out of electrical contact therewith, and heating means adjacent said bimetallic element and connected at its respective ends to said third and fourth terminals for initially heating the element to cause its movement into electrical contact with the conductive strip and for continued subsequent heating of the element to cause it to forcibly flex the conductive strip in a manner to disrupt the electrical contact between the strip and contact member.

3. A switch as set forth in claim 2 wherein said bimetallic member carries a fixed contact on its movable end, and the conductive strip carries two spaced fixed contacts one of which is positioned for engagement by the fixed contact on the bimetallic element and the other being positioned for engagement with said contact member.

4. A switch as set forth in claim 2 wherein said bimetallic element is structured so that it will bend sufficiently to move the conductive strip out of engagement with the contact member at the end of a predetermined time period.