US3504235A

US3504235A - Solid state limit switch

Info

Publication number: US3504235A
Application number: US777110A
Authority: US
Inventors: Art Lee
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1968-11-19
Filing date: 1968-11-19
Publication date: 1970-03-31
Anticipated expiration: 1987-03-31

Description

March 31, 1970 SOLID STATE LIMIT SWITCH Filed NOV. 19, 1968 2 SheetsfSheet l.

l ARUEE A. LEE 3,504,235

March 31, 1970 A. LEE

SOLIDSTATE LIMIT SWITCH 2 Sheets-Sheet 2 Filed Nov. 19, 196s United States Patent O 3,504,235 SOLID STATE LIMIT SWITCH Art Lee, Bloomington, Ill., assigner to General Electric Company, a corporation of New York Filed Nov. 19, 1968, Ser. No. 777,110 Int. Cl. H02b 1/04; H01h 9/00, .Z9/00 U.S. Cl. 317-112 15 Claims ABSTRACT OF THE DISCLOSURE A solid state limit switch for controlling energization of a load from a current supply includes a housing with a base and sidewalls defining a cavity and a barrier in the cavity opstanding from the base and extending across the cavity to divide the cavity into first and second isolated compartments. A magnetic reed switch is positioned in the iirst compartment and magnetic field producing means is located in the second compartment for movement relative to the reed switch for operating the reed switch. Operating means is positioned in the second compartment for effecting movement of the magnetic field producing means and actuating means is mounted on the exterior of the housing for actuating the operating means through an opening of the housing which opens into the second compartment. The first compartment also contains electrical circuit means including a semiconductive switch responsive to operation of the reed switch and electrical conductors which are connected to the circuit means extend through a second opening of the hou-sing opening into the first compartment. An insulating potting compound substantially completely fills the flirst compartment to embed the reed switch and the electrical circuit means.

BACKGROUND OF THE INVENTION Limit switches have previously been designed to include a magnetic reed switch which is operated under the influence of a movable permanent magnet which is moved by actuating means responsive to the arrival of a movable device, such as a machine tool part, at a particular point in its path of travel. A limit switch of this general type is disclosed in U.S. Patent 3,364,318. Limit switches are frequently employed in locations where they are subject to severe environmental conditions, such as dust, oil and metal shavings, and it is therefore highly desirable that limit switches be designed such that their operating parts are effectively sealed against such environrnental conditions so as to be capable of reliable and repetitive operation over long periods of service. While attempts have previously been made to design limit switches which are immune to environmental conditions, these attempts have not been entirely satisfactory inasmuch as they have resulted in limit switches of bulky, mullti-part and costly construction including complex mechanisms for moving the magnet relative to the reed switch.

Also, where the control function to be effected by the limit switch is the energization or deenergization of a motor, the reed switch contacts are ordinarily incapable of handling the motor current, 'which is generally well above the functional ampere current rating of the reed switch. Therefore, it has previously been necessary to provide in addition an electromagnetic relay which controls the motor and which includes an operating coil energized through the reed switch contacts. It is thus seen t be desirable to provide a reed switch type limit switch which has increased power switching capabilities so as to eliminate the need for an additional electromagnetic relay. While attempts have previously been -made to provide limit :switches with increased power switching capabilities, as far as can be determined, no single limit switch design has providedI for both increased power switching capability and immunity against environmental conditions in a compact, low cost and simplified construction.

OBJECTS OF THE INVENTION It is therefore a primary object of the invention to provide a novel and improved limit switch which is of compact and low cost construction and which is immune to environmental conditions.

It is a further object of the invention to provide such a limit switch which also has increased power switching capabilities.

It is another object of the invention to provide a novel and improved reed switch type limit switch of compact and inexpensive construction which is immune to environmental conditions, which has increased power switching capabilities, and which has a simplified and effective operating mechanism for moving the magnet relative to the reed switch.

It is still a further object of the invention to provide a limit switch as defined in the preceding object wherein the limit switch may be either of the normally open or normally closed type with only a very minor structural change required to provide one or the other type.

SUMMARY OF THE INVENTION In carrying out the invention in one preferred form thereof, the limit switch includes a housing having a base and sidewalls defining a cavity with a barrier located in the cavity upstanding from the base and extending across the cavity to divide the lcavity into first and second compartments. A magnetic reed switch is located in the first compartment and a permanent magnet is in the second compartment for movement relative to the reed switch for operating the reed switch. The second compartment contains operating means for effecting movement of the magnet, and actuating means is mounted on the exterior of the housing for actuating the operating means through an opening in the housing which opens into the second compartment. Electrical circuit means is located in the first compartment and includes a semiconductor switch having two main current carrying terminals and a gate terminal. The semiconductor switch normally exhibits a high impedance characteristic between its two main terminals, and exhibits a low impedance characteristic between the two main terminals in response to application of a control signal to the gate terminal. The two main terminals are connected to control the energization of a load from a current supply and the reed switch contacts are connected to apply a control signal to the gate terminal when the need switch contacts are closed. Electrical conductors are connected to the circuit means and extend through an opening of the housing which communicates with the rst compartment, and an insulating potting compound fills the first compartment to embed the reed switch and the electrical circuit means without entering the second compartment.

BRIEF DESCRIPTION OF THE DRAWlNG FIGURE 1 is a view in elevation of the limit switch constructed in accord with the invention;

FIGURE 2 is a view similar to FIGURE l with the cover plate removed to show the cavity with the first compartment thereof filled with potting compound;

FIGURE 3 is a view similar to FIGURE 2 with the actuating head removed and with the cover plate removed to show the electrical circuit means in the first compartment;

FIGURE 4 is an enlarged fragmentary view with parts in section showing particularly the barrier, the reed switch,

the magnet and the magnet operating means Within the housing cavity, the reed switch being in a normally open condition;

FIGURE 5 is a view similar to FIGURE 4 with parts removed showing the normally open reed switch of FIG- GURE 4 in an actuated closed condition;

FIGURE 6 is a view similar to FIGURE 5 showing the magnet repositioned to provide a normally closed reed Switch; y

FIGURE 7 is a view in section taken alo-ng the lines 7-7 of FIGURE 5;

FIGURE 8 is an exploded View in perspective showing the barrier and the parts of the magnet operating means;

FIGURE 9 is a view with parts in section showing the semiconductive switch and the heat sink block in which it is supported mounted on the base of the housing; and

FIGURE l0 is a schematic circuit diagram showing the reed switch and the electrical circuit connected thereto.

DESCRIPTION OF A PREFERRED EMBODIMENT Referring now to the drawing there is illustrated in FIGURE 1 a so-called track type limit switch incorporating the invention and including a housing 11 shown in FIGURE 1 and a cover 12 secured to the housing 11 as by screws 13. The housing 11 may be formed of an aluminum casting and contains operating parts of the limit switch which are actuated by mechanism within an actuating head 1-4 secured to the exterior of the housing 11 at one end thereof by screws 15. In order to retain the operating parts the housing 11 includes a cavity 16 defined by the base of the housing, by a pair of spaced

sidewalls

17 and 18 and by a part of spaced

end walls

19 and 20 connecting the sidewalls.

In accord with the invention a barrier or partition 21 is positioned in the cavity 16 to divide the cavity into rst and second isolated

compartments

22 and 23 which house respectively a switching element and electrical circuit means connected thereto, and simplified operating means for operating the switching element. The barrier 21 may assume a variety of forms and preferably comprises a plastic insert of generally channel-shaped configuration including a wall 25 with a pair of spaced

parallel lianges

26 and 27 extending transversely from the longitudinal edges of the wall 25 as best shown in FIGURE 8. The barrier 21 is positioned within the cavity 16 as shown in FIGURE 4 adjacent the end wall 19 with its wall 25 upstanding from the base of the housing 11 to extend transversely across the cavity between the

sidewalls

17 and 18, and with the longtitudinal edges of the

flanges

26 and 27 in engagement with the end wall 19. The flange 26 extends along the base of the housing 11 in engagement therewith Whereas the flange 27 is located adjacent the top of the cavity 16. The compartment 23 is thus defined by the barrier 21, the end wall 19 and adjacent portions of the

sidewalls

17 and 18 and the base, and is isolated from the compartment 22.

The limit switch includes a switching element preferably comprising a reed switch 29 which is positioned within the compartment 22 as shown in FIGUR-E 4. The reed switch 29 is illustrated in the form of a single pole reed switch having a pair of deflectible magnetic reed contacts 29a and 29h enclosed within a glass tube 29C with terminals of the reed contacts extending outwardly of the glass tube. In order to support and accurately position the reed switch 29 and parts of the reed switch operating means, the invention provides support means comprising a specially shaped sleeve 30 which is preferably formed of plastic in a tubular configuration and which is positioned in the cavity 16 to extend generally parallel to the direction of spacing of the

end walls

19 and 20. The sleeve 30 preferably forms part of the barrier 21 and has a closed end 32 closed to the compartment 22 and facing the end wall 20, and an open end 33 opening into the compartment 23 and facing the end wall 19. The sleeve 30 projects through an opening 34 formed in the barrier wall 25 and includes an annular lip 35 at its open end which is secured as by ultrasonic bonding to the adjacent annular surface of the wall 25 which faces the end wall 19. The closed end 32 of the sleeve 30 includes a central recessed portion 36 projecting toward the open end 33 and which is spaced from the sidewall of the sleeve. The recessed portion 36 has a closed end 37 facing the end wall 19 and closed to the compartment 23- and an open end 39 facing the end lwall 20 and opening into .the compartment 22. The reed switch 29 is positioned within the recessed portion 36 by insertion through the open end 39 whereby it is held in position within the compartment 22.

In order to operate the reed switch 29 suitable magnetic field producing means is positioned in the compartment 23 for movement relative to the reed switch. In the preferred embodiment illustrated the magnetic field producing means is in the form of a toroidal, ceramic permanent magnet 40 which is magnetized in the axial direction and which is positioned within the sleeve 30 to surround a part of the recessed portion 36 with clearance so that the magnet 40 is movable axially in vertical directions as viewed in FIGURE 4 relative to the reed switch 29. The

reed switch contacts

29a and 29b are closed by mutual attraction in the presence of a magnetic flux component of predetermined strength directed along the elongated dimension of the contacts. In order to provide a normally open limit switch the arrangement is such that the magnet 40 is normally in its position illustrated in FIG- UR-E 4 wherein it is displaced axially upwardly of the area wherein the contacts 29a and 2911 overlap so that magnetic flux emanating from the magnet 40Y provides a longitudinal flux component having insufficient strength to cause closure of the reed contacts.

In order to urge the magnet 40 to its normal position shown in FIGURE 4 there is provided bias means shown in the form of a coil spring 41 in the sleeve 30 surrounding a part of the recessed portion 36 and between the closed end 32 and the confronting surface of the magnet 40. For the purpose of moving the magnet 40 between its axially spaced operating positions against the bias of the spring 41 the invention provides simplified operating means including a tubular spacer v43 within the sleeve 30 surrounding a part of tre recessed portion 36 and between the upper surface of the magnet 40 as viewed in FIGURE 4 and the free end 44 of a lever 45 having a pivoted end 46. The end 46 of the lever 45 is pivotally mounted in any suitable manner and in the illustrated embodiment the end 46 includes a hook 46a which is received in a recess formed in a projection `47 upstanding from the wall 25 of the barrier 21, the projection 47 extending transversely between the

lianges

26 and 27 as illustrated in FIGURE 8. The lever `45 extends between the sidewalls 17 and 18 with portions of the lever being urged against

surfaces

49 and 50 of the end wall 19 by action of the spring 41 when the lever 45 is in its normal undeflected condition as shown in FIGURE 4. The lever 45 is mounted for pivotal movement between its normal position shown in FIGURE 4 and its actuated position shown in FIGURE 5 wherein it displaces the spacer 43 and the magnet 40 downwardly as viewed in FIGURE 5 from their normal positions illustrated in FIGURE 4. The barrier 21 is provided with a transverse intermediate wall 52 including a central notch 53 in which a portion of the lever 45 is positioned and guided during its pivotal movement.

In order to pivot the lever -45 for effecting operation of the reed switch 29', suitable actuating means are provided including mechanism within the actuating head 14. Any suitable type of actuating 'head may be utilized and in the illustrated embodiment the actuating head is of the rotary type including a main shaft 55 journalled by spaced Walls of the head 14 and having a part 56 projecting externally of the head 14 to which a suitable roller lever (not shown) may be attached to be engaged by a movable machine part or the like. The shaft 55 is urged to its normal position by suitable spring biased means (not shown) and carries a cam 57 cooperating with a cam follower in in the form of a pin 58 secured between spaced arms 60 of a shaft 61 having a threaded recess which threadably receives a screw 63 having a protruding head 65 which lightly touches the lever 45 when the shaft 55 is in its normal position. The shaft 61 extends through a bushing 66 fit within an opening 67 of the end wall 19 which opens into the compartment 23 at a recessed portion of the end wall 19 which is bridged by the intermediate part of the lever 45. As s'hown in FIGURE 2, a sealing ring 70 formed of rubber or like material is positioned within a recessed section 71 of the end wall 19 and includes a central opening which surrounds a restricted portion 72 of the shaft 61. The ring 70 is rlirmly held between the end wall 19 and the actuating head 14.

In operation, rotation of the shaft 55 in response to engagement of a machine tool part or the like with a lever (not shown) lixed to the part 56 of the sha-ft 55 results in rotation of the cam 57 which is suitably shaped to engage the pin 58 and depress it together with the shaft 61 and screw 63 downwardly as viewed in FIGURE 4. This results in deflection of the lever 45 about its pivoted end 46 to the position shown in FIGURE 5 wherein the free end 44 of the lever is displaced downwardly to move the spacer 43 and the magnet 40 downwardly against the bias of the spring 41 to their positions illustrated in FIGURE 5. The magnet 40 is thereby moved to a position wherein it surrounds the overlapping ends of the reed contacts such that a longitudinal magnetic iiux component emanating from the magnet is suflicient to effect attraction 'and engagement of the reed contacts as seen in FIGURE 5. When the shaft 55 is released by withdrawal of the movable machine part, it returns to its normal position whereby the pin 58, shaft 61, lever 45, spacer 43 and magnet 40 are returned to their normal positions as shown in FIGURE 4 under the expansive force of the spring 41.

In order to provide a limit switch having power switching capabilities the reed switch 29 is connected to electrical circuit means including a semiconductive switching ldevice which is rendered conducting in response to closure of the reed switch contacts. In the preferred embodiment of the invention the. semiconductive switching device comprises a bi-directional triode P-N-P-N switch provided with two main current carrying terminals and a gate terminal. In semiconductive switches of this type current flow between two main current carrying terminals in either direction can be controlled by the application of a low voltage, low current pulse between the gate terminal and one of the main terminals. These devices are commonly referred to as Triacs and such a device is preferably employed in the present invention. However, any other type semiconductive device which has at least one gate terminal may be. utilized and it is only necessary that the device employed exhibit a high impedance characteristic in the absence of a signal at its gate, and a low impedance characteristic in the presence c-f a gate signal.

As shown in FIGURE 9, the Triac 74 includes a metallic cylindrical casing 75 which forms one of its main current carrying terminals, and the other main current carrying terminal 76 and the gate terminal 78 project from one end of theI casing 75. In order to dissipate the considerable heat that is generated within the casing 75 of the Triac when it is conducting current, the Triac is preferably mounted within a heat sink assembly. To this end the casing 75 includes a serrated portion 79 which is press fit within a cylindrical aperture 80 formed in a metallic heat sink block 81 which is mounted on the baseof the switch housing 11. Inasmuch as the casing 75 and also the block 81 comprisel a main current carrying terminal of the Triac, it is desirable to electrically insulate these parts from the switch housing 11 and for this purpose a thin sheet of insulating material 83 is disposed between the block 81 and the base of the switch housing 11. The block 81 is mounted on the base by a metal screw 84 which passes through an opening 85 in the block 81 and is received in a threaded opening 86 in the base. The screw 84 is insulated from the heat sink block by an insulating washer 87. With the described arrangement a path of low thermal resistance is provided between the heat sink block and the base so that heat from the Triac is rapidly dissipated through the heat sink block, the base and the remainder ofthe housing to atmosphere.

Referring now to the circuit diagram of FIGURE l0 there is shown the electrical -circuit means including the Triac 74 which is responsive to operation of the reed switch 29 for controlling a load. The circuit means of FIGURE l0 is positioned within the compartment 22 of the switch housing 11 and certain components shown in FIGURE l0 are also illustrated in FIGURE 3. As shown in FIGURE l0, the terminal 88 of the reed switch contact 29a is connected through a current limiting resistor 90 to the gate 78 of the Triac. The terminal 91 of the reed switch contact 29b is connected through a Wire 92 to a vterminal 93 which is plugged into an opening of theA heat sink block 81 to thereby connect the terminal 93 to the casing 75 of the Triac which consitutes one of the main terminals thereof. Also connected to the terminal 93 is a wire 94 which leads to one side of a capacitor 95 the other side of which is connected through a wire 96 to a terminal 97 connected to one side of a capacitor 98. The othoer side of capacitor 98 is conected through a resistor 99 and a wire 100 to a ground connection 101 which conveniently comprises a connection to the base of the switch housing 11. An inductance 102 is connected between the terminal 97 and the main terminal 76 of the Triac and a conductor 103 connected to the terminal 97 leads to the circuit which is to be controlled. A wire 104 which also leads to the circuit to be. controlled is connected to a terminal 105 which is plugged into an opening in the heat sink block so as to connect the wire 104 to the casing 75 of the Triac. The series connection of a current supply and a load (not shown) is connected between the

wires

103 and 104. Operation of the circuit means of FIGURE l() will now be describe-d.

Assume that the reed switch contacts are in their normally open position and that an alternating voltage is applied between the

conductors

103 and 104. For such conditions the Triac is in its high impedance state and will not conduct current. When a machine tool part or the like effects rotation of the shaft 55 from its normal position, such results in displacement of the magnet 40 to its position effective to close the reed switch contacts whereupon a signal is applied to the gate 78 of the Triac 74 through resistor 90 and the closed reed switch contacts to render the Triac conducting. While the Triac is conducting, the load (not shown) is energized and a very low voltage appears between the main terminals of the Triac. Therefore the voltage. between the main terminal provided by the casing 75 and the gate 78 is also very low so that the voltage across the reed switch contacts is very low whereby the Triac provides contact protection for the reed switch contacts. Such contact protection greatly extends the life of the reed switch as compared to its life in conventional applications wherein arcing between the reed switch contacts occurs when they open during the application of higher voltages to the contacts. The reed switch contacts when closed carry a very minute current which also contributes to an extended life of the reed switch. When the reed switch contacts subsequently open due to withdrawal of the. machine part from the limit switch, the Triac will continue to conduct until the occurrence of the next zero point on the alternating current wave of the voltage source at which time the Triac will stop conducting.

In order to prevent false gating of the Triac 74 due to voltage surges or transients occurring in the power lines, a suppressor network comprising the capacitor and the inductance 102 is provided. High frequency transient oscillations in the power circuit are by-passed around the Triac 74 through the capacitor 95. Further, such bypassing is assured inasmuch as any attempt of the transients to pass through the Triac will be delayed by the inductance 102. A series RC network comprising capacitor 98 and resistor 99 forms an internal switching suppressor by balancing high frequency pulses generated by an inductive load against the stray capacitance between the Triac, heat sink block and the base of the housing. This balancing network lwill prevent false firing of other solid state limit switches by one which is being triggered, or by a refiring due to an inductive feedback from the load being controlled. The limit switch of this invention can be employed in parallel or in series with other similar limit switches or with conventional mechanical limit switches, and can be operated n close proximity to other similar limit switches.

A normally open limit switch has been described which includes a normally open reed switch which is closed to trigger the Triac 74 in response to actuation of the limit switch. In accord with the invention a normally closed limit switch may be provided by effecting a very minor change in the structure of the normally open switch previously described such that only a minimum number of parts need be manufactured and stocked to provide either a normally open or a normally closed limit switch. In the present invention in order to provide a normally closed switch design it is only necessary to provide means for repositioning the magnet 40 relative to the reed switch 29 so that the magnet produces flux effective to maintain the reed switch contacts closed when the shaft 55 and thus the levers 45 are in their normal positions as shown in FIGURES 2 and 4. In the preferred embodiment of the invention the magnet 40 is repositioned by the mere addition of a spacer 107 which is located between the magnet 40 and the adjacent end of the spacer 43 as shown in FIGURE 6. Such arrangement effectively repositions the magnet 40 downwardly as viewed in FIGURE 6 from its position shown in FIGURE 4 to surround the reed switch at an area wherein its contacts overlap so that the magnet produces a longitudinal component of magnetic flux which is sufficient to maintain the contacts closed. With such arrangement deflection of the lever 45 to a position shown by dotted lines in FIGURE 6 as a result of actuation of the limit switch effects movement of the

spacers

43 and 107 downwardly as viewed in FIGURE 6 to move the magnet 40 to a position shown by dotted lines wherein the longitudinal magnetic flux component produced by the magnet 40 is insufiicient to maintain the reed switch contacts closed with the result that they open. Accordingly, the Triac is normally conducting current which is interrupted subsequent to opening of the .reed switch contacts resulting from actuation of the limit switch.

In assembling the limit switch the heat sink assembly is initially mounted on the base of the housing 11 and the sub-assembly comprising the barrier 21 including the sleeve 30 attached thereto, the magnet 40, the spring 41, the spacer 43 (and the spacer 107 if used) and the lever 45 is next inserted in the housing. A wedge 108 shown in FIGURE 3 is positioned between the closed end 32 of the sleeve and a portion of the housing to maintain the sub-assembly in position. A suitable sealant 109 is then applied to at least the exposed peripheral edges of the barrier 21 and adjacent surfaces of the sidewalls 17 and 1'8 and the end wall 19 of the housing. This seals the barrier 21 in the housing and effectively isolates the

compartments

22 and 23. Thereafter, the reed switch 29 is inserted within the recessed portion 36 and the circuit elements shown in FIGURE l are connected to the reed switch and the Triac. The cover 12 is next secured to the housing 11 and the compartment 22 is then filled with a suitable insulating potting compound which is inserted through the opening 111 in the end wall 20 through which the

conductors

103 and 104 extend. The

potting compound embeds the various parts of the electrical circuit means, the heat sink assembly and the reed parts in the compartment 22 are very effectively sealed against environmental conditions not only by the potting compound and the cover 12, but also by being iso-lated from the compartment 23. Moreover, the machanism within the compartment 23 for moving the magnet 40 is extremely simple in that it consists of only the lever 45 and the spacer 43.

Although the invention has been described with reference to certain specific embodiments thereof, numerous modifications are possible and it is desired to cover all modifications falling within the spirit and scope of the invention.

What I claim as new and desire to Secure by Letters Patent of the United States is:

1. A limit switch for controlling energization of a load from a voltage source comprising in combination,

a housing having a pair of spaced end walls, a pair of spaced sidewalls connecting said end walls, and a base defining a cavity,

a barrier in said cavity upstanding from said base and extending across said cavity between said sidewalls to divide the cavity into first and second isolated compartments spaced in the direction of spacing of said end walls, said housing having first and second openings which open respectively into said first and second compartments,

a magnetic reed switch in said first compartment having a pair of contacts which are relatively movable between contact open and contact closed conditions under the influence of a magnetic field,

magnetic field producing means in said second compartment movable relative to said reed switch between spaced positions for operating said reed switch between its contact open and contact closed conditions,

operating means in said second compartment for effecting movement of said magnetic field producing means between its spaced positions,

actuating means mounted on the exterior of said housing for actuating said operating means through said second opening,

electrical circuit means in said first compartment responsive to operation of said reed switch,

electrical conductors connected to said circuit means and extending through said first opening, and

an insulating potting compound filling said first compartment to embed said reed switch and said electrical circuit means.

2. A limit switch as defined in claim 1 including a sleeve within said cavity having a closed end closed to said 4first compartment and having an open end opening into said second compartment, said closed end having a recessed portion closed to said second compartment and i opening into said first compartment, said reed switch being positioned Within said recessed portion, said magnetic field producing means comprising a toroidal permanent magnet within said sleeve surrounding a part of said recessed portion.

3. A limit switch as defined in claim 1 wherein said electrical circuit means includes a semiconductive switch having a metallic casing, a metallic heat sink block having an aperture in which said casing is received, said block being mounted on said housing electrically insulated therefrom and with low thermal resistance between the block and the housing Such that heat developed in said semiconductive device is rapidly transmitted through said casing and said block to said housing to be dissipated by said housing.

4. A limit switch as defined in claim 1 wherein said barrier comprises a plastic insert having a barrier wall and a pair of spaced parallel flanges projecting transversely from said barrier wall, said insert being positioned in said cavity with its barrier wall perpendicular to said base in spaced confronting relation with one of said end walls and with its flanges extending parallel to said base into engagement with said one end wall.

5. A limit switch as defined in claim 2 including spring means positioned within said sleeve between said closed end and said magnet to urge said magnet towards said open end, said operating means including a hollow spacer between said magnet and said open end.

6. A limit switch for controlling energization of a load from a voltage source Vcomprising in combination,

a housing having a pair of spaced end walls, .a pair of spaced sidewalls connecting said end walls and a vbase defining a cavity,

a barrier in said cavity upstanding from said base and extending across said cavity between said sidewalls to divide the cavity into first and second isolated compartments spaced in the direction of spacing of said end walls,l said barrier including .a sleeve extending generally parallel to the direction of spacing of said end walls, said sleeve having a closed end closed to said first compartment and having an open end opening into said second compartment, said closed end having a recessed portion closed to said second compartment and opening into said first compartment, said end walls having first and second openings which open respectively into said first and second compartments,

a magnetic reed switch having a pair of contacts which are relatively movable between contact open and contact closed conditions under the influence of a magnetic field, said reed switch being positioned within said recessed portion,

a toroidal permanent magnet position within said sleeve to surround a part of said recessed portion for axial movement relative to said reed switch between spaced positions for operating said reed switch -between its contact open and contact closed positio-ns,

spring means positioned within said sleeve between said closed end and said magnet to urge said magnet towards said open end,

operating means in said second compartment for eecting movement of said magnet between its spaced positions,

actuating means mounted on the exterior of said housing at the end wall adjacent said second compartment for actuating said operating means through said second opening,

an insulating potting compound filling said first compart-ment to embed said reed switch and said electrical circuit means.

7. A limit switch as defined in claim 6 wherein said operating means includes a lever having a first pivotally mounted end and having a second free end, and a hollow spacer having a first end engaged yby said second free end and having a second end within said sleeve in engagement with said magnet, a portion of said lever being positioned adjacent said second opening.

8. A limit switch as defined in claim 6 wherein said electrical circuit means includes a semiconductive switch having two main current carrying terminals and a gate terminal, said switch normally exhibiting a high impedance characteristic between said two main terminals and exhibiting a low impedance characteristic between said two main terminals in response to application of a control signal to said gate terminal, said two main terminals being connected to control the energization of a load from a voltage source, said reed switch contacts -being connected to apply a control signal to said gate terminal when said contacts are closed.

9. A limit switch as defined in claim 8 wherein said semiconductive switch has a metallic casing, a metallic heat sink block having an aperture in which said casing is received, said block being mounted on said base electrically insulated therefrom and with low thermal resistance between said block and said base such that heat developed in said semiconductive switch is rapidly transmitted through said casing and said block to said base to be dissipated by said housing.

10. A limit switch as defined in claim 6 wherein said barrier comprises a plastic insert having a barrier wall and a pair of spaced parallel flanges'projecting transversely from said barrier wall, said insert being positioned in said cavity with its barrier wall perpendicular to said base in spaced confronting relation with one of said end walls and with its flanges extending parallel to said base into engagement with said one end wall.

11. A limit switch as defined in claim 10 wherein said plastic insertis sealed in position by a sealant applied to at least its exposed peripheral edges and adjacent surfaces of said one end wall and said sidewalls.

12. A limit switch for controlling energization of a load from a voltage source comprising in combination,

a housing having a pair of spaced end walls, a pair of spaced sidewalls connecting said end walls and a base defining a cavity,

a barrier in said cavity upstanding from said base and extending across said cavity between said sidewalls to divide the cavity into first and second isolated compartments spaced in the direction of spacing of said end walls, said barrier including a sleeve extending generally parallel to the direction of spacing of said end walls, said sleeve having a closed end closed to said first compartment and having an open end opening into said second compartment, said closed end having a recessed portion closed to said second compartment and opening into said first compartment, said end walls having first and second openings which open respectively into said first and second compartments,

a toroidal permanent magnet positioned within said.

sleeve to surround a part of said recessed portion for axial movement relative to said reed switch between spaced positions for operating said reed switch between its contact open and contact closed positions,

a hollow spacer having a first end within said sleeve in engagement with said magnet and having a second end projecting beyond said open end,

a lever in said second compartment having a pivotally mounted first end and having a second free end engaging the second end of said spacer, a portion of said lever being positioned adjacent said second opening,

actuating means mounted on the exterior of said housing at the end lwall adjacent said second compartment for actuating said lever through said second opening,

electrical circuit means in said first compartment responsive to operation of said reed switch, said circuit means including a semiconductive switch having two main current carrying terminals and a gate terminal, said switch normally exhibiting a high impedance characteristic between said two main terminals in response to application of a control signal to said gate terminal, said two main terminals being connected to control the energization of a load from a voltage source, said reed switch contacts being connected to apply a control signal to said gate terminal when said contacts are closed,

a metallic heat sink block having an aperture and mounted on said base electrically insulated there# from, said semiconductive switch having a metallic casing which is received in the aperture of said block, said block being mounted on said base with low thermal resistance between the block and the base such that heat developed in said semiconductive switch is rapidly transmitted through said casing and said block to said base to be dissipated by said housing,

electrical conductors connected to said circuit means and extending through said rst opening, and

an insulating potting compound lling said first compartment to embed said reed switch, said electric circuit means and said heat sink block.

13. A limit switch as dened in claim 12 wherein said barrier comprises a plastic insert having a barrier wall and a pair of spaced parallel flanges projecting transversely from said barrier wall, said insert being positioned in said cavity with its barrier wall perpendicular to said base in spaced confronting relation with one of said end walls and with its flanges extending parallel to said base into engagement with said one end wall.

v of said one end wall and said sidewalls.

References Cited UNITED STATES PATENTS 3,164,696 1/1965 Pusch 335-205 X 3,252,345 5/ 1966 Russel 200-47 X 3,283,274 11/1966 DeFalco 335-207 X 3,364,318 1/ 1968 Bulliet 200-47 LEWIS H. MYERS, Primary Examiner GERALD P. TOLIN, Assistant Examiner U.S. C1. X.R.

ZOO-47; 335-205