Patent Publication Number: US-6707370-B2

Title: Thermal switch and heater

Description:
FIELD OF THE INVENTION 
     This invention relates to a thermal switch and, more particularly, to a thermal switch with integrated over protection circuitry. 
     BACKGROUND OF THE INVENTION 
     Heating elements, such as cartridge heaters, are used for various applications, including heating of liquids. Typically, the heater is wired in a control circuit that regulates operation of the heater to maintain a desired temperature. This may be done with a separate temperature or thermal switch. 
     A thermal switch, such as a thermostat, has a control contact wired in series with the heater to control operation of the heater. The thermostat could be mounted to a vessel holding the liquid. Alternatively, the thermostat could be submerged in the liquid. However, such a thermostat and heater are not protected against malfunction such as high current surges, electrical shorts and overheating of the liquid caused by the heater. 
     The present invention is directed to improvements in thermal switch assemblies. 
     SUMMARY OF THE INVENTION 
     In accordance with the invention there is provided a thermal switch including an integral control circuit. 
     Broadly, there is disclosed herein a thermal switch comprising an elongate tubular sealed housing. A control circuit is mounted in the sealed housing. The control circuit comprises a thermostat, a fuse and a thermal overload switch electrically connected in series. An electrical connector is mounted to the housing and is connected to the control circuit. 
     It is a feature of the invention that the housing comprises a metal bushing housing the thermostat and the thermal overload switch. A flexible tube is secured to the metal bushing. The bushing and the flexible are filled with epoxy. 
     In accordance with one aspect of the invention, a heater is connected to the housing and wired in series in the control circuit. The heater may comprise a cartridge heater brazed to a metal bushing of the housing. The cartridge heater is filled with magnesium oxide. 
     In accordance with another aspect of the invention, a sealed thermal switch is provided for controlling liquid temperature. The switch comprises an elongate tubular submersible sealed housing. A control circuit mounted in the housing comprises a thermostat, a fuse and a thermal overload switch electrically connected in series. An electrical connector is sealed to the housing and connected to the control circuit. 
     Further features and advantages of the invention will be readily apparent from the specification and from the drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a plan view of a thermal switch in accordance with one embodiment of the invention; 
     FIG. 2 is a plan view of the switch of FIG. 1 with a bushing shown in section and other parts removed for clarity; 
     FIG. 3 is a plan view of a thermal switch in accordance with an alternative embodiment to the invention including an integral heater; and 
     FIG. 4 is a view similar to FIG. 2 for the thermal switch and heater of FIG.  3 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIG. 1, a thermal switch  10  in accordance with the invention is illustrated. The thermal switch  10  is adapted to not only control temperature but also to protect components that it is controlling from high current surges, electrical shorts and over temperature conditions. The thermal switch  10  is described below for controlling operation of an external heater. As is apparent, the thermal switch could be used for controlling other types of load devices. 
     The thermal switch  10  includes an elongate tubular sealed housing  12 . Particularly, the housing  12  is adapted to be submersible. The housing  12  consists of a metal bushing  14  and flexible tube  16 . In the illustrated embodiment of the invention the bushing  14  is brass. The bushing  14  defines an interior space  18 , see FIG. 2. A coupling nut portion  20  is disposed between a sleeve  22  and an NPTF threaded portion  24 . An opposite side of the threaded portion  24  includes a narrower diameter sleeve  26  closed by an end wall  28 . The flexible tube  16  comprises a convoluted tube which may be formed of, for example, PTFE, FEP or PFA. The tube  16  is flexible to allow bends to be formed in the tube, in use, for aligning connections. 
     The housing  12  encloses a control circuit  30 , see FIG.  2 . The control circuit  30  includes a thermostat  32 , a thermal cut off switch  34  and a fuse  36 . The thermostat  32  is factory preset to open and close an internal contact responsive to sensed temperature. The thermal cut off switch  34  incorporates an encapsulated contact that permanently opens under high temperature conditions. The thermal cut off switch  34  has a trip point higher than the thermostat setting and prevents overheating if, for example, the thermostat  32  fails. 
     The thermostat  32  is connected between a pair of leads  38  and  40 . The first lead  38  is connected via a connector  42  to the thermal cut off switch  34 . The opposite side of the thermal cut off switch  34  is connected via a connector  44  to a lead  46 . The opposite end of the lead  46  is connected via a connector  48  to the fuse  36 . The opposite end of the fuse  36  is in turn connected using a connector  50  to a lead  52  having an exposed opposite end  53 . The opposite thermostat lead  40  is connected via a connector  54  to a lead  56  having an exposed opposite end  58 . As such, the fuse  36 , the thermal cut off switch  34  and the thermostat  32  are connected in series between the exposed lead ends  53  and  58 . The exposed lead ends  53  and  58  are electrically connected to an electrical connector  60  for connection to an external circuit. 
     Prior to installation of an electrical connector  60 , the bushing interior space  18  and the convoluted tube  16  are filled with an epoxy  62 . The epoxy seals all of the components and secures the convoluted tube  16  to the bushing sleeve  22 . Thereafter, the connector  60  is electrically connected to the lead ends  53  and  58  and mechanically secured onto the convoluted tube  16  with a water tight seal in a conventional manner. 
     In use, the thermal switch  10  is wired in series with an external heater. For example, the electrical connector  60  is connected between a power source and an external heater. As a result, the thermostat  32 , thermal overload or cut off switch  34  and the fuse  36  are in series with the external heater. The rating of the thermostat  32  and the proper temperature setting are user selected. The thermostat  32  is thus operable, in use, to selectively energize an external heater and thus control heat. The fuse  36  is selected with a slightly higher rater than the amperage of the heater or other device that is controlled. The fuse  36  eliminates premature failures of the heater or device that are due to voltages causing surge currents or due to short circuit conditions. The thermal cut off switch  34  is selected to have an over temperature cut out (OTC) to prevent and protect the device being heated from damage due to over heating. If this occurs, the thermal cut off switch  34  will open causing the unit to shut down. Once the thermal cut off switch  34  or the fuse  36  trips, then the thermal switch  10  must be replaced. Thus the thermal switch  10  acts as a safety device. 
     As is apparent, the particular type and lengths of leads and types of connectors can be selected according to desired specifications. 
     In accordance with the invention, the thermal switch  10  is adapted to control temperature and to protect the components being controlled from high current surges, electrical shorts and over temperature conditions. 
     Referring to FIGS. 3 and 4, a temperature controlled heater  70  is illustrated. The temperature controlled heater  70  comprises a cartridge heater integrally formed with a thermal switch that is generally similar to the thermal switch  12  of FIG.  1 . 
     The temperature controlled heater  70  includes an elongate tubular sealed housing  71 , a heater  72 , and an electrical connector  78 . Particularly, the housing  71  is adapted to be submersible. The sealed housing comprises a metal bushing  74  and a convoluted tube  76 . 
     The heater  72  comprises a cartridge heater which may be similar to that described in Rysemus, U.S. Pat. No. 5,486,682, assigned to the assignee of the present application, the specification of which is incorporated by reference herein. Such a cartridge heater includes a cylindrical swaged sheath  80  housing the heating element (not shown) in the form of an electrical resistance wire having exit leads  82 . The sheath  80  is filled with magnesium oxide. 
     The bushing  74  includes a coupling nut portion  84  connected to an NPTF threaded portion  86 . The bushing  74  includes a through opening  88  with an internal shoulder  90  connecting a counter bore  92 . The through opening  88  receives the cartridge heater  72  which is connected thereto as by brazing. A stainless steel sleeve  94  is received in the counter bore  92  and abuts the shoulder  90  and is secured to the bushing with a subsequent epoxy fill. As is apparent, the sleeve  94  could be integral with the bushing  74  as in the embodiment of FIG.  1 . 
     The temperature controlled heater  70  includes a control circuit  96  enclosed in the housing  71 . The control circuit  96  includes a thermal cut off switch  72 , a thermostat  74  and a fuse  76 . The thermal cut off switch  72  is generally similar to the thermal cut off switch  34  of FIG.  2 . Likewise, the fuse  76  is generally similar to fuse  36  of FIG.  2 . The thermostat  74  is generally similar to the thermostat  32  of FIG. 2, albeit being of a longer and narrower configuration in the illustrated embodiment. The control circuit  96  further includes a lead  98  having an exposed end  100 . An opposite end is connected via a connector  102  to the thermal cut off switch  72 . The opposite side of the thermal cut off switch  72  is connected via a connector  104  to one of the heater leads  82 . The opposite heater lead  82  is connected via a connector  106  to a first lead  108  of the thermostat  74 . Another thermostat lead  110  is connected via a connector  112  to a lead  114 . The lead  114  is connected via a connector  116  to the fuse  76 . The fuse  76  is connected to a lead  118  having an exposed end  120 . The lead exposed ends  100  and  120  are electrically connected to the electrical connector  78 . As with the embodiment of FIG. 1, the interior space of the convoluted tube  76 , sleeve  94 , bushing  74  and the internal end of the heater  80  are filled with epoxy to seal the housing  71  and secure the convoluted tube  76 , the sleeve  94  and the bushing  74  together. 
     In the described embodiment of the invention, the magnesium oxide of the cartridge heater  80  acts as a barrier between the heating element and the thermostat  74 . As such, the thermostat  74  is operable to sense temperature of the fluid surrounding the bushing  74 . As is described above, the temperature controlled heater  70  uses similar components as the thermal switch  10  of FIG. 1 with the exception that the control circuit  96  is an integral element with the heater  72 . Temperature and current ratings are selected similarly to the thermal switch  10  of FIG.  1 . 
     Thus, in accordance with the invention, there is provided a thermal switch, with and without an integral heater, adapted not only to control temperature but protect components it is controlling from high current surges, electrical shorts and over temperature conditions.