Abstract:
A thermo-sensitive heater and heater driving circuit are disclosed. The thermo-sensitive heater comprises a cord-shaped nylon thermistor that surrounds a heating element, such that the thermistor detects the temperature of the heating element and controls the driving current for a heating coil. Also, the present invention includes electromagnetic shielding material, which is formed by winding an electric conductor around the outer surface of the nylon thermistor, or formed as a wire mesh. This shielding material is advantageous for discharging the electric field radiated from inside of the heater to an external electric field, thus safely eliminating harmful electric fields. Also, the present invention further provides a driving circuit for safely driving the heater, which includes a temperature controller or an overheating prevention circuit.

Description:
CROSS REFERENCE TO RELATED ART 
     This application claims the benefit of Korean Patent Application Nos. 2001-32324 and 2001-45908, filed on Jun. 9, 2001 and Jul. 30, 2001, respectively, which are hereby incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates in general to an electrical heater, and more particularly to a thermo-sensitive heater used in various kinds of mats or blankets. 
     2. Description of the Related Art 
     Conventional electric products such as electric mats and electric mattresses include one or more temperature sensors arranged in a laminated mat having a heater. A temperature controller in the heater detects a heating temperature of the heater by the temperature sensor, compares the detected temperature with a preset temperature, and controls caloric power of the heater. The conventional electric product, designed to control the temperature of its heater as described above, has a problem that the price of the product is increased due to the use of the temperature sensors and sensor connecting wires. Such a conventional electric product also has a problem that the product does not meet the electromagnetic wave safety standards because electromagnetic waves are undesirably radiated from the lead wires extending between the sensors and the temperature controller. 
     The term “heating element”, “heating wire” or “heater” is intended to mean a cord-shaped heating material having flexibility, and coated with synthetic resins for protection, for being arranged in a heating product such as an electric mat, an electric blanket, an electric cushion, an electric bed, socks, and etc, and being used to perform the heating function of such products. 
     According to the prior art, a generally used non-magnetic heating wire is disclosed in Korean Utility Laid-open Publication No.97-64561. This electromagnetic wave attenuation heater has an insulation layer interposed between inner and outer coiled heating wires, with the ends of the wires connected to each other such that the directions of currents flowing in the conducting wires within a heating element are opposite to each other, and thereby the electronic waves from the wires can be offset. Consequently, the directions of circular magnetic fields surrounding the heating coils are also opposite to each other, and thereby the intensity of magnetic field from the heating coils can be decreased. However, even in a case of using the non-magnetic heating wire, there are generated electric fields, which fatigue the nervous system of a body. It is also common knowledge that magnetic fields prevent a person from sleeping soundly by affecting brain waves. Therefore, a method of eliminating the electric field in heaters must be devised. 
     Further, an electromagnetic wave removing apparatus has been proposed and used for discharging electromagnetic waves to the ground. An electromagnetic wave discharging apparatus is applied to various kinds of electric mats, as well as electric products having the electromagnetic wave attenuation heater. In the construction of such electromagnetic wave discharging apparatuses, an electromagnetic wave shielding element, such as a copper net and etc., is installed in an electric product such that the shielding element surrounds the heater inside the electric product. In such a case, the copper net used as the shielding element is connected to the ground. The installation of a copper net in an electric product for removing the electromagnetic waves from the product is problematic in that it wastes materials, complicates the production process, and increases the weight and cost of the product, thus deteriorating the competitive power and design flexibility of the product. 
     For the foregoing reasons, there is a need for a heater that reduces electromagnetic radiation without requiring increased amount of materials and cost of production. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention is directed to a thermo-sensitive heater and heating circuit that substantially obviates one or more of the problems due to limitations and disadvantages of the related art. 
     It is an object of the present invention to provide a thermo-sensitive heater having both a nylon thermistor and an electric field shielding coil within a cord-shaped heater and operates such that its temperature controller detects the temperature of the heating element, and controls the driving current for a heating coil. 
     It is another object of the present invention to provide a thermo-sensitive heater for controlling a heater driving current without a separate temperature sensor. 
     It is still another object of the present invention to provide a driving circuit for safely driving the heater. 
     It is still another object of the present invention to provide a driving circuit having an overheating prevention circuit. 
     Additional features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings. 
     To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, a thermo-sensitive heater comprises a nylon thermistor arranged on a middle layer between a cord-shaped heating element and an electrical insulation coating layer for detecting a temperature of the heating element, and having a negative temperature characteristic. A current supplying terminal is connected to one of inner and outer surfaces of the nylon thermistor, and a temperature detecting terminal is connected to the other of the inner and outer surfaces of the nylon thermistor for controlling a driving current for the heating element by a temperature controller. 
     According to one aspect of the preferred embodiment of the present invention, the nylon thermistor is tubular and is formed on an outer surface of the cord-shaped heating element through an extrusion forming process and an inner side of the thermistor is connected to a heating coil which is also used in part as a temperature detecting terminal. 
     According to another aspect of the preferred embodiment, the thermo-sensitive heater employs a driving circuit. 
     In an alternative embodiment of the present invention, a thermo-sensitive heater having a heating element inside it, and having a coating layer with electric insulating and waterproofing means on its outside, comprises a cord-shaped nylon layer, as a thermo-sensitive device, that surrounds an entire heating element, a first electrode contacted with an inner surface of the nylon layer, a second electrode connected to an outer surface of the nylon layer, an electric insulation layer for surrounding the entire surfaces of the cord-shaped nylon layer, and a first shielding coil wound around entire surfaces of the electric insulation layer. 
     According to one aspect of the alternative embodiment, the first electrode is used as a heating coil and the second electrode is used as a second shielding coil where the heating element is a non-magnetic heating element. 
     According to another aspect of the alternative embodiment, the thermo-sensitive heater employs a driving circuit. 
     According to another aspect of the alternative embodiment, resistors within the circuit are arranged to heat a temperature fuse. 
     In another alternative embodiment of the present invention, wire meshes are used as electrodes and/or electric fields shields. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide a further explanation of the invention as claimed. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention. 
     FIG. 1 illustrates a partly broken perspective view showing a thermo-sensitive heater according to a first embodiment of the present invention; 
     FIG. 2 illustrates a circuit diagram of a heater driving circuit according to a first embodiment of the present invention; 
     FIG. 3 illustrates a detailed circuit diagram of a heater driving circuit according to a first embodiment of the present invention; 
     FIG. 4 illustrates a partly broken perspective view showing a thermo-sensitive heater according to a second embodiment of the present invention; 
     FIG. 5 illustrates a partly broken sectional view showing a thermo-sensitive heater according to a second embodiment of the present invention; 
     FIG. 6 illustrates a circuit diagram of a heater driving circuit according to a second embodiment of the present invention; 
     FIG. 7 illustrates a partly broken perspective view showing a thermo-sensitive heater according to a third embodiment of the present invention; 
     FIG. 8 illustrates a partly broken perspective view showing a thermo-sensitive heater according to a fourth embodiment of the present invention; and 
     FIG. 9 illustrates a partly broken perspective view showing a thermo-sensitive heater according to a fifth embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION 
     With reference to the drawings, and in particular to FIGS. 1-9 thereof, a thermo-sensitive heater and driving circuit embodying the principles and concepts of the present invention will be described. 
     FIG. 1 is a partly broken perspective view showing a thermo-sensitive heater  100  according to a first embodiment of the present invention. FIG. 2 is a view showing a circuit diagram of a heater driving circuit  101 . Referring to FIG.  1  and FIG. 2, the heater  100  according to the first embodiment of this invention comprises a nylon thermistor  11 , a current supplying terminal  13  and a temperature detecting terminal  12 . The nylon thermistor  11  is arranged on a middle layer between a cord-shaped heating element  20  and an electric insulation coating  23  for detecting a temperature of the heating element  20 . The current supplying terminal  13  is connected to the outer surfaces of the nylon thermistor  11 , and supplies a current during temperature detection. The temperature detecting terminal  12  is connected to the other end of the thermistor&#39;s inner surface, and detects a heating temperature of the heating element  20  when a temperature controller  14  controls the driving current for the heating element  20 . 
     Preferred specifications of the heater  100 , which is shown in FIG.  1  and FIG. 2, are provided in Table 1, below. 
     
       
         
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
             
             
               
                 covered layer 23 
                 PVC with a width of approximately 0.7 mm 
               
               
                   
                 (extrusion forming) 
               
               
                 nylon thermistor 
                 nylon resin with a width of approximately 0.45 mm 
               
               
                 11 
                 (extrusion forming) 
               
               
                 shielding coil N3 
                 rolled copper wire formed by compressing a copper 
               
               
                   
                 wire with a diameter of approximately 0.23 mm to a 
               
               
                   
                 width of approximately 0.1 mm 
               
               
                 electric insulating 
                 Silicon rubber with a width of approximately 0.45 mm 
               
               
                 resin layer 22 
                 (extrusion forming) 
               
               
                 heating coil N2 
                 rolled copper wire formed by compressing a copper 
               
               
                   
                 wire with a diameter of approximately 0.18 mm to a 
               
               
                   
                 width of approximately 0.1 mm 
               
               
                 center support 
                 polyester filament yarn with a diameter of 
               
               
                 structure 21 
                 approximately 0.6 mm (2000 denier) 
               
               
                 heating coil N1 
                 rolled copper wire formed by compressing a copper 
               
               
                   
                 wire with diameter of approximately 0.18 mm to 
               
               
                   
                 a width of approximately 0.1 mm 
               
               
                   
               
             
          
         
       
     
     As described above, the heater  100  includes a nylon thermistor  11  for temperature detecting on the cord-shaped heating element  20 , such that the heater provides heating temperature information of the heating element  20  to the temperature controller  14  without using a separate temperature sensor. 
     Specifically, the thermistor  11 , formed on the outer surface of the cord-shaped heating element  20  through an extrusion forming process, is a tubular nylon thermistor of which the inner surface is connected to a heating element coil N 2 . The thermistor  11  is formed as a part of the cord-shaped heating element  20 , and the temperature controller  14  measures the temperature of the heating element  20  using the thermistor  11 . 
     Referring to FIG. 2, an alternating current (AC) supplying voltage is connected to a driving current input terminal T and neutral terminal N. A diode D is arranged between the heating coil N 1  and the terminal T. During a heating cycle with a positive AC voltage applied to the terminal T, the positive voltage is applied to the heating coils N 1 , N 2  in addition to the anode of an SCR (Silicon Controlled Rectifier, not shown) through the terminal T, thus driving the heating coils N 1 , N 2  and preparing for a trigger operation of the SCR. On the other hand, during a temperature detecting cycle with the positive AC voltage applied to the terminal N, the positive voltage is applied to the nylon thermistor  11  through the terminal N and the terminal  13  (or a shielding coil N 3 ). 
     The heating coil N 2  is connected to the inner surface of the tubular type nylon thermistor  11 . As a result of this, the heating coils N 1 , N 2  connected to each other in series are used as the temperature detecting terminal  12  during the temperature detecting cycle. 
     The shielding coil N 3  is wound around the outer surface of the nylon thermistor  11 . During the heating cycle with the driving current applied to the heating element  20 , the shielding coil N 3  absorbs the electromagnetic waves radiated from the heating element  20 , and radiates the absorbed electromagnetic wave to the neutral terminal N connected to the ground. 
     A heating resistor R is preferably arranged in parallel to the diode D in order to induce a temperature voltage left on the inner surface of the nylon thermistor  11  to the terminal  12  or the heating coils N 2  and N 1  when a positive voltage is applied to the tubular nylon thermistor  11  through the terminal N and the terminal  13 . 
     The temperature controller  14  detects the temperature voltage of the heating element  20  at the temperature detecting terminal  12  during a temperature detecting cycle, and controls the driving current for the heating coils N 1  and N 2 . 
     FIG. 3 is a view showing the driving circuit  101  of this invention in detail. Referring to FIG. 3, the temperature controller  14  according to first embodiment will be described in detail. 
     As shown in the drawing, an SCR is arranged between the heating coil N 2  and the terminal N so as to switch on/off the driving current for the heating coils N 1  and N 2 , which flows through the terminal T. 
     During a temperature detecting cycle, a temperature detector  31  detects a temperature voltage inducted to the temperature detecting terminal  12  arranged between the heating resistor R and the heating coil N 1 , amplifies the detected voltage, and outputs the amplified voltage to a temperature comparator  33  in a next heating cycle. 
     Referring to FIG. 3, a temperature setting unit  32  is installed to set a heating temperature of the heating element  20 . This temperature setting unit  32  is realized as a variable resistor receiving a constant voltage Vcc from a circuit voltage supplying unit  38 . Further, the temperature setting unit  32  is arranged to operate in conjunction with a switch “sw” used for switching on/off the driving current for the heating element. 
     The temperature comparator  33  compares a temperature (or voltage) detected by a temperature detector  31  during the heating cycle with the preset temperature(or voltage), outputs a “high” signal if the detected temperature is lower than the preset temperature and outputs a “low” signal if the detected temperature is higher than the preset temperature. 
     For power saving, a zero detector  34  is installed in the temperature controller  14 . The zero detector  34  detects a voltage at the terminal N, generates a “high” signal for a predetermined period of time on the basis of the time when the voltage at the terminal N is 0 V—in detail, for a time of {fraction (1/20)} of one AC cycle—and outputs a “low” signal for the remaining time of the AC cycle. 
     Further, a disconnection detector  35  for the shielding coil N 3  is arranged in order to cut off the driving current for the heating element  20  automatically, when the temperature rises excessively due to a disconnection of the shielding coil N 3 . The disconnection detector  35  is connected to one end of the shielding coil N 3  of which the other end is connected to the terminal N, such that the disconnection detector  35  generates a “high” signal if the shielding coil N 3  is not disconnected, and generates a “low” signal if the shielding coil N 3  is disconnected. 
     An AND gate  36  is installed to logically combine the output signals from the zero detector  34 , the temperature comparator  33  and the disconnection detector  35 . The AND gate  36  outputs a driving signal for the heating coils N 1 , N 2  to a driving unit  37  when all of the output signals from the zero detector  34 , the temperature comparator  33 , and the disconnection detector  35  are “high”. 
     The driving unit  37  generates a driving signal of the SCR as a switching device for switching the heating coils N 1 , N 2  if the AND gate  36  outputs a “high” signal. 
     The temperature controller  14  as configured above is operated as follows. During the temperature detecting cycle with a positive voltage applied to the terminal N and a negative voltage applied to the terminal T, the negative voltage is applied to the anode of the SCR and the positive voltage is applied to the cathode of the SCR. Thus, the SCR is turned off to inactivate the heating coils N 1 , N 2 . The positive voltage applied to the terminal N is supplied to a temperature detecting current circuit, wherein the temperature detecting current circuit includes the current supplying terminal  13 , the nylon thermistor  11 , the heating coils N 1 , N 2 , the heating resistor R and the terminal T. A current detected by the temperature detecting current circuit is in inverse proportion to the resistance of the nylon thermistor  11  and in proportion to the temperature, and a voltage proportional to the current applied to the terminal  13  is taken at opposite sides of the heating resistor R. 
     During the heating cycle with a positive voltage applied to the terminal T and a negative voltage applied to the terminal N, the SCR is turned on and thus, a current of the diode D flows in a forward direction and the positive voltage at the terminal T is applied through the diode D to the heating coils N 1 , N 2  not to the resistor R. 
     However, even during the heating cycle, in a specific condition that the predetermined period of time set by the zero detector  34  is deviated from the restricted time, or the detected temperature is over the preset temperature, or the output of the AND gate  36  is “low” due to a detection of disconnection of the current supplying terminal  13 , the SCR is turned off, thus preventing the heating coils N 1 , N 2  from being driven. 
     An operation of preventing an excessive rise of the temperature of this invention is described as follows. If the nylon thermistor  11  is fused or damaged for any reason and then the shielding coil N 3  used as the current supplying terminal  13  is connected to the heating coil N 2 , the positive voltage at the terminal N is supplied to the heating coils N 1 , N 2  directly. In this case, a high current flows through a circuit, which starts from the terminal N and ends at the terminal T, via the shielding coil N 3 , the heating coils N 2 , N 1  and the heating resistor R. The resistor R is thus heated to a high temperature and then, the temperature fuse “tf” connected to the resistor R is cut. 
     Further, when the SCR is shorted, the current flows through the terminal N, the SCR, the heating coil N 2 , the heating coil N 1  and the resistor R. In this case, the heating resistor R is heated, and thus, the fuse “tf” is cut and the temperature controller  14  shown in FIG. 3 maintains a safe operation of the heater. 
     FIG. 4 is a partly broken perspective view showing a thermo-sensitive heater  200  according to a second embodiment of the present invention, and FIG. 5 is a cross-sectional view showing this embodiment of the present invention. Referring to FIG.  4  and FIG. 5, the construction and operation of the heater  200  are described in detail. 
     The heater  200  comprises a nylon layer  111 , a first electrode  112 , a second electrode  113 , a second electric insulation layer  114 , a first shielding coil  116 , and a coating layer  128 . Alternative to the nylon layer  111 , other suitable insulating layer may also be used. 
     The nylon layer  111  in the manner of a cord is a thermo-sensitive device arranged to surround an entire heating element  120  in order to get an electric resistance value of a thermistor corresponding to a temperature variation of the heating element  120 . 
     The first electrode  112  is contacted with an inner surface of the nylon layer  111  for applying a temperature measuring current to the nylon layer  111 , and is used as a heating element of the heating element  120 . The second electrode  113  for temperature detection is connected to an outer surface of the nylon layer  111  for detecting an electric resistance value of the nylon layer  111 , which is varied according to the temperature variation of the heating element  120 . 
     The second electric insulation layer  114  surrounds the entire surfaces of the cord-shaped nylon layer  111 . The first shielding coil  116  is wound around the entire surface of the second electric insulation layer  114  in order to discharge an electric field radiated from the heating element  120  to an external electric field. The coating layer  128  with electric insulating and waterproofing means surrounds the first shielding coil  116 . 
     Referring to FIGS. 4 and 5, the heater  200  as a non-magnetic field emitting heating element is described in detail. The non-magnetic heating element  200  comprises an electric insulation core wire  121 , a first heating coil  122 , a first electric insulation layer  123 , a second heating coil  124 , an end connection part  125 , and driving current connection terminals  126 ,  127 . 
     The first heating coil  122  is wound around the entire surfaces of the core wire  121 . The first electric insulation layer  123  is arranged in outer surface of the first heating coil  122 . The second heating coil  124  is wound around the entire surfaces of the first electric insulation layer  123 . The end connection part  125  is arranged to connect each one end of the heating coils  122 ,  124  to each other. The driving current connection terminals  126 ,  127  are arranged to apply the driving current to the other ends of the heating coil  122 ,  124  connected to each other. 
     In this case, the heating coils  122 ,  124  are copper wires without an insulation coating. 
     When the driving current flows into the driving current connection terminals  126 ,  127  of the non-magnetic heating element, the directions of currents flowing through the heating coils  122 ,  124  are opposite to each other. Thereby, the directions of circular magnetic fields formed around the heating coils  122 ,  124  are opposite to each other, thus decreasing the intensity of the total magnetic field from the heating element. 
     The thermo-sensitive heater applied to the non-magnetic heating element of this embodiment of the present invention comprises a nylon layer  111 , a first electrode  112 , and a second electrode  113 . The nylon layer  111  is arranged to surround the entire surfaces of the second heating coil  124  in the manner of a cord. The first electrode  112  is arranged to apply the temperature detecting current to the entire surfaces of an inner circle of the nylon layer  111 , and is used as the second heating coil  124 . The second electrode  113  is wound around the entire outer surfaces of the nylon layer  111  for detecting the electric resistance variation according to the temperature variation. 
     The first electrode  112  is driven as a heating coil  124 , and is connected to the entire inner surfaces of the nylon layer  111  in the shape of a coil and then operates as an electrode for applying the temperature detecting current to the nylon layer  111 . 
     Further, the electrode  113  for temperature detection is wound around the outer surface of the cord-shaped nylon layer  111  in the shape of a coil, thus enabling the temperature to be detected at the entire surface of the nylon layer  111 . Additionally, the electrode  113  is used as the second shielding coil  115  for radiating the electric field from the heating element to the external electric field due to its construction of surrounding the entire surfaces of the nylon layer  111 . 
     The nylon layer  111  as a thermo-sensitive device, arranged on the heating element  120  has a negative temperature characteristic of decreasing the electric resistance value as the temperature rises. 
     Consequently, in order to drive the heater, a heater driving circuit measures the temperature voltages at both the first electrode  112  and the second electrode  113 , processes an operation requiring with the measured voltages, and controls the heating temperature of the heater. 
     If being used as a second shielding coil  115 , the second electrode  113  is connected to the external electric field, such that the electric field radiated from the heating element can be discharged. 
     The first heating coil  116  always surrounds the heating element  120  in the shape of a spiral coil at the outer surface of the electric insulation layer  114 . In this case, the first shielding coil  116  is connected to an external electric field such as a ground or a neutral terminal of an AC power supply, such that the electric field radiated from the heating element can be charged to the external electric field. 
     Moreover, if the second electrode  113  is connected to the external electric field for using the second electrode  113  as the second shielding coil  115 , a dual-spiral shielding coil shields the electric field of the heating element to discharge it to the external electric field, thus enabling the electric field radiated from the heating element to be more perfectly eliminated. 
     Preferred specifications of this embodiment of the present invention, which is shown in FIGS. 4 and 5, are given in Table 1, below. 
     
       
         
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
             
             
               
                 121 core wire 
                 glass fiber wire with a diameter of approximately 0.5 
               
               
                   
                 mm (1500 denier) 
               
               
                 122 first 
                 rolled copper wire formed by compressing a copper 
               
               
                 heating coil 
                 wire with a diameter of approximately 0.23 mm to a 
               
               
                   
                 width of approximately 0.1 mm 
               
               
                 123 first electric 
                 silicon rubber with a width of approximately 0.35 mm 
               
               
                 insulation layer 
                 (tubular extrusion forming) 
               
               
                 124 second 
                 rolled copper wire formed by compressing a copper 
               
               
                 heating coil 
                 wire with a diameter of approximately 0.23 mm to a 
               
               
                   
                 width of approximately 0.1 mm 
               
               
                 111 nylon layer 
                 nylon resin with a width of approximately 0.3 mm 
               
               
                   
                 (tubular extrusion forming) 
               
               
                 112 first electrode 
                 rolled copper wire formed by compressing a copper 
               
               
                   
                 wire with a diameter of approximately 0.23 mm to a 
               
               
                   
                 width of approximately 0.1 mm 
               
               
                 113 second 
                 rolled copper wire formed by compressing a copper 
               
               
                 electrode 
                 wire with a diameter of approximately 0.23 mm 
               
               
                   
                 to a width of approximately 0.1 mm 
               
               
                 114 second elect- 
                 silicon rubber with a width of approximately 0.35 mm 
               
               
                 ric insulation 
                 (tubular extrusion forming) 
               
               
                 layer 
               
               
                 116 first 
                 rolled copper wire formed by compressing a copper 
               
               
                 shielding coil 
                 wire with a diameter of approximately 0.23 mm 
               
               
                   
                 to a width of approximately 0.1 mm 
               
               
                 128 coating layer 
                 PVC with a width of approximately 0.7 mm (tubular 
               
               
                   
                 extrusion forming) 
               
               
                   
               
             
          
         
       
     
     FIG. 6 is a circuit diagram of a heater driving circuit  201  for driving and controlling the heater of this embodiment of the present invention. 
     The heater driving circuit  201  includes a switching SSCR, a temperature detecting resistor RT 1 , a temperature detector  131 , a temperature setting unit  132 , a comparator  133 , a zero detector  134 , a disconnection detector  135 , an AND gate  136 , an amplifier  137 , a diode DD, and a heating resistor RT 2 . 
     Referring to FIG. 6, the switching SSCR is arranged in serial to the heating element  120  so as to switch on/off the driving current applied to the heating element  120  during a driving cycle with a positive voltage applied to a neutral terminal NT of AC power supply. 
     The temperature detecting resistor RT 1  is arranged to apply the positive voltage to the second electrode  113 , bypass the positive voltage through the nylon layer  111  and the first electrode  112 , and output a voltage difference between both ends of the resistor RT 1  as a temperature voltage, during a temperature detecting cycle when a positive voltage is applied to the hot terminal HT and the SSCR is turned off. 
     The temperature detector  131  detects and amplifies the temperature voltage induced at the second electrode  113  through the second electrode  113  during the temperature detecting cycle, and outputs the detected temperature voltage to the comparator  133  during the driving cycle. 
     The temperature setting unit  132  sets a driving temperature of the heating element by a variable resistor, and outputs the set temperature as a temperature setting voltage corresponding to the set temperature to the comparator  133 . 
     The comparator  133  compares the detected temperature voltage with the temperature setting voltage, and outputs a logic “high” signal if the detected temperature voltage is lower than the temperature setting voltage while outputting a “low” signal if the detected temperature voltage is higher than the temperature setting voltage, during the driving cycle. 
     The zero detector  134  detects a voltage at the neutral terminal NT, and sets a trigger point of time of the SSCR—for example, a time of {fraction (1/20)} of one AC cycle—around 0 V. 
     The disconnection detector  135  detects a disconnection of the second electrode  113 , and outputs the detected result to the AND gate  136 . 
     The AND gate  136  logically combines the output signals from the zero detector  134 , the temperature comparator  133 , and the disconnection detector  135 , and outputs the combined signal. 
     The amplifier  137  amplifies the output signal of the AND gate  136 , and provides the amplified signal to a gate of the SSCR as a SSCR driving signal. 
     The diode DD is arranged to be connected to both ends of the heating element  120  in forward direction to a positive voltage applied to the hot terminal HT for preventing the driving current from flowing through the heating element  120  by the positive voltage of the hot terminal HT if the SSCR is damaged. 
     The heating resistor RT 2  is arranged to cut the temperature fuse TF when a current flows in the forward direction through the diode DD. 
     Referring to FIG. 6, the SF is a current fuse, SW is a power supply on/off switch, and RD is a disconnection detecting resistor. Further, the heating resistors RT 2  and the temperature detecting resistor RT 1  are arranged to heat the temperature fuse TF. 
     Hereinafter, the operation of the heater driving circuit of this embodiment of the present invention is described in detail referring to FIG.  6 . 
     First, when the driving temperature of the heating element is set by the temperature setting unit  132  and the switch SW is turned on while the positive voltage is applied to the neutral terminal NT, if the SSCR is turned on, the heating element  200  is activated, while if the SSCR is turned off, the heating element  120  is inactivated. On the other hand, while the positive voltage is applied to the hot terminal HT, a reverse voltage is applied to the SSCR, thus stopping the flow of driving current through the heating element  120  to inactivate it. 
     When the AND gate  136  outputs a logic “high” signal, and the amplifier  137  amplifies the output signal of the AND gate  136 , and then the logic “high” signal from the amplifier  137  is applied to a gate of the SSCR, the SSCR is turned on. 
     Here, the conditions of outputting a “high” signal by the AND gate  136  are described. First, the zero detector  134  outputs a logic “high” signal during the driving cycle, however, a logic “low” signal not during the driving cycle. Then, the trigger point of time of the SSCR is around 0 V of the AC power supply. 
     Further, the comparator  133  compares the detected temperature with the set temperature, outputs a logic “high” signal if the detected temperature is lower than the set temperature while outputting a “low” signal if the detected temperature is higher than the set temperature. 
     The disconnection detector  135  checks a state of the second electrode  113  for temperature detecting, outputs a logic “high” signal if the second electrode  113  is in normal state, while outputting a “low” signal if disconnection of the electrode  113  is detected. 
     If the SSCR is damaged, the positive voltage of the hot terminal HT is applied to the heating element  120 . However, the positive current according to the positive voltage is applied to the diode DD as a forward directional voltage while heating the heating resistor RT 2 . Then, the forward directional voltage is bypassed to the neutral terminal NT, thereby preventing the heating element  120  from overheating. 
     If the positive voltage of the hot terminal HT is applied to the heating resistor RT 2  and the resistor RT 2  is heated, the temperature fuse TF is cut and the driving circuit is powered off. 
     In case that the nylon layer  111  is melted, or the second electrode  113  is electrically connected to the second heating coil  124  by any reasons, the positive current of the hot terminal HT flows into the neutral terminal NT through the second electrode  113  and the heating coil  124 , thus overheating the heater. In this case, the resistor RT 1  used as a temperature detecting resistor is heated and the fuse TF is cut, and thus preventing the heater from being overheated. 
     Further, the first or second shielding coil  116  or  115  is connected to the neutral terminal NT, thereby enabling the electric field radiated from the heating element  120  to be eliminated by bypassing it. 
     FIG. 7 is a partly broken perspective view showing a thermo-sensitive heater  300  according to a third embodiment of the present invention. The heating coil  124 , which is also the first electrode  112 , in FIG. 4 is replaced by a wire mesh  212 , as shown in FIG.  7 . Consequently, the wire mesh  212 , which acts as a heating coil and as a first electrode, eliminates the need for a first shielding coil (as depicted by element  116  in FIG. 4) located on the outer surface of the second electric insulation layer  114  because of its ability to effectively reduce an electric field radiated from the heating element  120  to an external electric field. The end connection part  125  is arranged to connect each one end of the heating coil  122  to the wire mesh  212 . 
     FIG. 8 is a partly broken perspective view showing a thermo-sensitive heater  400  according to a fourth embodiment of the present invention. The shielding coil  115 , which is also a second electrode  113 , in FIG. 4 is replaced by a wire mesh  213 , as shown FIG.  8 . Similar to the embodiment described by FIG. 7, the wire mesh  213 , which acts as a shielding coil and a second electrode, eliminates the need for a first shielding coil (as depicted by element  116  in FIG. 4) located on the outer surface of the second electric insulation layer  114  because of its ability to effectively reduce an electric field radiated from the heating element  120  to an external field. 
     FIG. 9 is a partly broken perspective view showing a thermo-sensitive heater  500  according to a fifth embodiment of the present invention. The first heating coil  122  that is wound around the entire surfaces of the core wire  121  in FIG. 4 is replaced by a plurality of wires  222  that surrounds the surface of the core wire  121 , as shown in FIG.  9 . Acting as a heating coil, the plurality of wires  222  generates an electric field such that a need for a first shielding coil (as depicted by element  116  in FIG. 4) located on the outer surface of the second electric insulation layer  114  is eliminated. In addition, the heating coil  124  or the shielding coil  115  could be substituted by a wire mesh, as exemplified in FIGS. 7 and 8, to further shield from electric fields generated by the heater. 
     The heaters shown in FIGS. 7 to  9  may also be used with the driving circuits shown in FIGS. 2,  3  and  6 . 
     It will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the spirit or scope of the invention. Thus, it is intended that the present invention cover the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.