Abstract:
One or more current sensors are mounted externally of a heating member so as to simplify construction of the heating member. The sensors are particularly useful in safety control circuits for heating members using PTC wire as resistance heating elements.

Description:
[0001]    This application claims the benefit of U.S. Provisional Application Serial No. 60/318,999 filed Sep. 11, 2001.  
         FIELD OF THE INVENTION  
         [0002]    The present invention relates to electric heating blankets, pads and pillows.  
         DESCRIPTION OF PRIOR DEVELOPMENTS  
         [0003]    Present day warming blankets typically include a safety circuit for interrupting or cutting off electrical power to the blanket in the event of an electrical fault such as arcing, a short circuit or an open circuit.  
           [0004]    Conventional safety circuits measure voltage at the remote end of the heating wire, i.e., the end farthest from where the wire enters the blanket. This form of measurement requires a safety circuit module to be located inside the blanket or an additional pair of wires must be returned to the controller located on the exterior of the blanket. This requires the heating wire to complete a full circuit loop within the blanket and return back to the edge of the blanket where the resistance heating wire is connected to an external power cord. This connection requires an additional electrical connector pin or pins.  
           [0005]    It would be advantageous to be able to externally measure only the current entering a warming blanket or pad in order to detect an electrical fault in the resistance/heating wire. This would be particularly advantageous in the case of resistance wire fabricated from PTC (positive temperature coefficient) wire. In this case a complete looped circuit would not be required and a simplified and more economical blanket construction would be enabled.  
           [0006]    In any case, by measuring the current entering the blanket, a safety circuit can be located remotely and externally of the blanket.  
         SUMMARY OF THE INVENTION  
         [0007]    The invention has been developed to fulfill the needs noted above. A primary object of the invention is to provide an electric fault detector positioned externally from a heating blanket without the need for extending one or more wires back to the external controller. This object is achieved by locating most of the fault detecting electronics in an application specific integrated circuit (ASIC). This reduces both the safety circuit size and its cost.  
           [0008]    The resulting safety circuit can be located on a controller or near an electrical connector, i.e., near a wall plug, if desired. The combination of the safety circuit and the wall plug can be molded as a single compact module, if desired. By positioning the safety circuit at the wall plug and measuring electrical faults at this point, faults between the wall plug and blanket can be immediately detected, i.e., faults in the power cord. Prior safety circuits located at the blanket could not detect such faults.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]    [0009]FIG. 1 is a schematic circuit diagram of a safety circuit constructed in accordance with the invention;  
         [0010]    [0010]FIG. 2 is a view of an alternate sensor for use in the circuit of FIG. 1;  
         [0011]    [0011]FIG. 3 is a schematic top plan view showing a heating pad or blanket constructed in accordance with the Prior Art;  
         [0012]    [0012]FIG. 4 is an enlarged partial view of the portion of FIG. 3 enclosed in dashed lines; and  
         [0013]    [0013]FIG. 5 is a view similar to FIG. 3 showing a blanket constructed in accordance with the invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0014]    The present invention will now be described in conjunction with FIG. 1 which shows a safety circuit  10  including an application specific integrated circuit or ASIC  12 . In the example shown, the ASIC is a custom CMOS ASIC. If desired, discrete components can be mounted on a circuit board instead of using an ASIC. However, an ASIC is preferred because of its compact size and ability to be packaged as a small unobtrusive module or chip.  
         [0015]    Each power wire  16 ,  18  on power wall plug  20  extends through a small current sensor  22 ,  24 . The current sensors  22 ,  24  produce a voltage output when current flows through the respective power wires  16 ,  18 . These voltages, IN 1  and IN 2 , are input into the ASIC  12 . The current sensors are preferably coils, i.e., ferrite toroids, but can also be small resistors,  26 ,  28 , i.e., 0.1 ohm, as shown in FIG. 2. The voltage drop across the resistors produces a similar voltage input signal to that produced by the sensors  22 ,  24 .  
         [0016]    The ASIC amplifies and processes the input signal(s) from the current sensors  22 ,  24  (or  26 ,  28 ) and determines or calculates an average sensor signal value based on the two inputs. However, if desired, only one sensor  16  or  18  (or  26  or  28 ) can be used to input a single voltage signal IN 2 . Two sensors are preferred for the ability to detect a short circuit.  
         [0017]    The resulting average (or single) voltage signal is preferably offset by a predetermined amount set by an external offset voltage  28  produced by a voltage divider  30  having resistors  29 ,  31 . The average (or single) voltage signal is also preferably amplified by a gain determined by, for example, an external 1% resistor  32 , for example. The resulting output control signal  34  is sent through signal wire  35  is in the form of an analog voltage signal which is proportional to the current in the line or power cords  16 ,  18 .  
         [0018]    The analog voltage signal  34  is sent to a microcomputer  36  in a conventional controller  38 . An analog to digital (A/D) converter  40  located in the microcomputer  36 , or elsewhere, receives the analog voltage signal  34  and converts it to a digital signal. The digital signal is processed by the microcomputer  36  to monitor the current in the power cords  16 ,  18  and check for abnormal current values which indicate a fault condition. A triac  50  in series with the line  18  allows the microcomputer  36  to control power to the heating pad or blanket. When a fault condition is detected, the controller switches off the power from power cords  16 ,  18  to the heating pad or blanket.  
         [0019]    The entire safety circuit  10  can be located in the controller  38  or externally of the controller  38  such as in power cord plug  20 . The safety circuit  10  includes the current sensors  22 ,  24 , the ASIC  12  (or equivalent discrete components), the gain resistor  32 , the offset voltage setting resistors  29 ,  31  and a small optional power supply  42  for powering the ASIC in the case where the ASIC is mounted on plug  20 .  
         [0020]    When the safety circuit  10  is mounted by the wall plug  20 , the safety circuit is preferably molded with the wall plug  20  into a single unitary plug assembly or module. In this case, three wires would extend from the plug module to the blanket controller  38 , i.e., the two power wires  16 ,  18  and the safety signal wire  35 .  
         [0021]    A major advantage of the present invention is the ability to use the same ASIC for controlling different sizes and types of heating pads, blankets, pillows, etc. each having different current flow characteristics. Instead of redesigning the ASIC, one need only change the value of the external gain resistor  32  and/or the values of the offset voltage resistor(s)  29 ,  31  in voltage divider  30  to adapt the ASIC for different control applications and to optimize signal  35  for processing over the full useful input range of controller  38 .  
         [0022]    Another advantage of the invention is the elimination of any electronic components in the pad or blanket, since the circuit  10  can be located completely externally of the pad or blanket. This facilitates blanket/pad assembly and removes bulky components from the blanket/pad.  
         [0023]    Another major advantage of the invention is the ability to eliminate a return signal wire or wires  35  from the end of the resistance wire which heats the blanket or pad. For example, as seen in FIG. 3, the prior art heating blanket  50  has a serpentine loop of PTC resistance wire  52  which terminates at an active safety circuit module  54 . When a predetermined voltage variation is detected in wire  52  by module  54 , a signal is sent to controller  38  via signal wire  35  to open switch  56  and terminate power to resistance wire  52  in connector housing  58 .  
         [0024]    As seen in FIG. 4, module  54  can include one or more resistors  60 ,  62  extending across power wires  16 ,  18  at the remote end of wire  52 . The base of transistor  64  is connected between the resistors to send a go or no-go signal to signal wire  35  to control the operation of switch  56 . The module  54  complicates the assembly of blanket  50  as does signal wire  35 .  
         [0025]    A much simpler to fabricate blanket assembly is shown in FIG. 5 wherein module  54  and return wire  35  are eliminated from the blanket construction and located remote from the blanket in accordance with the invention. The safety circuit  10  of FIG. 1 can be mounted on wall plug assembly or module  20 , or the safety circuit  10  can be mounted in controller  38 , which can be mounted adjacent or remote from the blanket  50 .  
         [0026]    Blanket  50  is shown for explanation purposes only. Blanket  50  can be any heating member, generally including a flexible, pliable, soft and compliant cover made of a fabric or sheet material, and a resistance wire held within the outer layers of the cover.