Abstract:
A safety wire for use in a heating blanket has a core, a heating wire wrapped around the core, and a guard wire spaced apart from the heating wire. In a normal mode of operation, the guard wire is not physically or electrically coupled to the heating wire, but in a breakdown mode of operation, the guard wire is electrically coupled to the heating wire. A TRIAC may be operatively coupled to the heating wire so that when the guard wire is electrically coupled to the heating wire, the TRAIC is turned on so that current flows from the heating wire to a fuse.

Description:
CLAIM OF PRIORITY 
       [0001]    This application claims priority to U.S. Provisional Patent Application Ser. No. 61/312,518, filed Mar. 10, 2010, the entire disclosure of which is incorporated by reference herein. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates generally to heating blankets. More particularly, the present invention relates to a heating blanket protection circuit for use with guard conductors. 
       BACKGROUND 
       [0003]    The present invention recognizes and addresses disadvantages of prior art constructions and methods of heating blankets, and it is an object of the present invention to provide an improved heating blanket protection circuit for use with guard conductors. 
         [0004]    Various combinations and sub-combinations of the disclosed elements, as well as methods of utilizing same, which are discussed in detail below, provide other objects, features and aspects of the present invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0005]    A full and enabling disclosure of the present invention, including the best mode thereof, to one of ordinary skill in the art, is set forth more particularly in the remainder of the specification, including reference to the accompanying drawings, in which: 
           [0006]      FIG. 1  is a schematic diagram of a control circuit for prior art heating blankets; 
           [0007]      FIG. 2  is a schematic diagram of a new control circuit in accordance with one embodiment of the present invention for use with prior art heating blankets; 
           [0008]      FIG. 3  is a top view of a heating blanket wire having a guard in accordance with one embodiment of the present invention; 
           [0009]      FIG. 4  is a top view of termination connectors in accordance with one embodiment of the present invention for connecting the heating blanket wire of  FIG. 1  to a control circuit; 
           [0010]      FIG. 5  is a top plan view of the termination connector of  FIG. 4  in use on the heating blanket wire of  FIG. 3 ; 
           [0011]      FIG. 6  is a top view of a heating blanket wire having a guard in accordance with one embodiment of the present invention; 
           [0012]      FIG. 7  is a top view of a heating blanket wire having a guard in accordance with one embodiment of the present invention; 
           [0013]      FIG. 8  is a schematic view of a protection circuit for use with any one of the heating blanket wires disclosed in  FIGS. 3 ,  6  and  7 ; and 
           [0014]      FIG. 9  is a schematic diagram of a control circuit for use with the protection circuit of  FIG. 8  and with any one of the heating blanket wires disclosed in  FIGS. 3 ,  6  and  7 . 
       
    
    
     DETAILED DESCRIPTION 
       [0015]    Reference will now be made in detail to presently preferred embodiments of the invention, one or more examples of which are illustrated in the accompanying drawings. Each example is provided by way of explanation of the invention, not limitation of the invention. In fact, it will be apparent to those skilled in the art that modifications and variations can be made in the present invention without departing from the scope or spirit thereof. For instance, features illustrated or described as part of one embodiment may be used on another embodiment to yield a still further embodiment. Thus, it is intended that the present invention cover such modifications and variations. Additional aspects and advantages of the invention will be set forth in part in the description that follows and, in part, will be obvious from the description, or may be learned by practice of the invention. 
         [0016]    Referring to  FIG. 1 , a prior art control circuit  10  is shown having an electric blanket heating circuit  16  connected across terminals H 1  and H 2 . Power is connected to control circuit  10  across terminals L 1  and L 2 . A fuse  20  is operatively coupled to L 1  and two Triodes for Alternating Current (TRIACS)  12  and  14 . The gate of each TRIAC  12  and  14  is operatively coupled to a CPU  18 . The use of two TRIACS  12  and  14  is to guard against a single TRIAC failing permanently by short and causing overheating to occur. In the dual TRIAC configuration, both TRIACs would have to fail for overheating to occur, thus the chance of this happening is greatly reduced. However, it is theoretically possible that a hardware failure in CPU  18  could result in signals where the TRIACS could both turn on. For example, a failure that would connect the CPU  18  to L 2  could theoretically result in unintended signals to the gates of TRIACS  12  and  14  resulting in sufficient gate currents to turn on both TRIACS. If this condition were allowed to persist, overheating of heating blanket  16  may occur. 
         [0017]    Referring to  FIG. 2 , with new control circuit  10 , TRIAC  14  is connected so as to open the fuse if the TRIAC is ever turned on. If the theoretical failure described above occurs, both TRIACs would be turned on, and the fuse would therefore open before overheating could occur. In addition, CPU  18  monitors current via voltage  28  across current sensing resistor  30 . Also, if TRIAC  12  shorts, U 1  can determine that current is flowing when TRIAC  12  should be off. In this case, U 1  commands TRIAC  14  to turn on, which in turn causes fuse  20  to open. Thus, the concept of guarding against the shorting of TRIAC  12  is still intact, and the new control circuit configuration prevents unintended operation of heating blanket  16  if TRIAC  12  shorts. 
         [0018]    Referring to  FIG. 3 , a new safety wire is shown for use in a heating blanket. 
         [0019]    The safety wire has an inner layer  102  made from a polyester core, a middle layer having two conductors: the first conductor H 1  formed from heater alloy (Percon-19) and the second conductor G 1  formed from guard alloy (304 stainless steel) and an outer layer  104  formed from PVC insulation. The conductors are wound in a helical fashion and do not connect to one another electrically or physically in normal operation. The spacing between wires H 1  and G 1  may vary depending on the voltage across the heating wire of the blanket and the most advantageous manufacturing practices. In all cases, the spacing should be sufficient to prevent any electrical connection between heating wire H 1  and guard wire G 1 . Provision is made at the ends of the assembly for connection to the individual windings. Current through the heater winding produces heat for the blanket. 
         [0020]    The guard winding is used to sense leakage current or breakdown between the heater winding H 1  and the guard winding G 1 . When overheating occurs, the outer coating will melt, allowing migration of the wires so that the heating wire comes into electrical contact with the guard wire or sufficiently close for dielectric breakdown. The guard winding does not vary predictably with temperature, and no attempt is made to measure the resistance of or the voltage across the guard winding. The guard wire requires connection at one point, and thus the wire from the controller to the blanket comprises three conductors, not four as required by prior art designs. A three wire design is more flexible and is lower in cost than other safety wire designs. It should be noted that for additional reliability, the guard wire may be connected at both ends. 
         [0021]    Referring to  FIG. 4 , a printed circuit board connector  110  is shown having two main conductors  106  and  108 . Main conductor  106  defines three trace conductors  106   a  that are formed at an angle with respect to main conductor  106 . Similarly, main conductor  108  also defines three trace conductors  108   a  that are formed at an angle with respect to main conductor  108 . The angle of trace conductors  106   a  with respect to main conductor  106  is complementary to the helical angle of guard wire G 1 . Moreover, the angle of trace conductors  108   a  with respect to main conductor  108  is also complementary to the helical angle of heating wire H 1 . 
         [0022]    Referring to  FIG. 5 , safety wire  100  is shown in one preferred embodiment mounted to printed circuit board connector  110  by clamping the stripped wire  100  to trace conductors  106   a  and  108   a  with a bolt-down fixture that holds the wire in place. In the embodiment shown, the trace conductors are shown in a flat top view. The angle and spacing of the trace conductors match the helical angle and spacing of the wires. After stripping the outer insulation from the wire, the ends must be trimmed and oriented so as to make proper contact with the respective traces conductors. The printed circuit board connector will hold the wire in place and ensure proper electrical contact. The same termination should be made at both ends of heat wire H 1  and guard wire G 1 . Connecting at both ends gives protection even if a break were to occur somewhere along the guard wire within the blanket since the entire length of the guard will still connect to the gate of TRIAC  14 . 
         [0023]    Referring to  FIG. 6 , in yet another preferred embodiment a safety wire  200  is shown having an inner layer  202  formed from a polyester core, a second layer  204  of heater alloy (Percon-19), a third layer  206  of PVC insulation, a fourth layer  208  of guard alloy (304 stainless steel) and a fifth layer  210  of PVC insulation. Guard winding  208  does not measure temperature but detects leakage current from heater winding  204  caused by guard wire  208  and heater wire  204  coming close together as a result of overheating or physical damage. The guard wire requires connection at one point, so the wire from the controller to the blanket comprises only three conductors, not four as required by prior art designs to reduce cost. In one preferred embodiment, the helical windings are set at ten windings per inch. 
         [0024]    Referring to  FIG. 7 , in still another preferred embodiment a safety wire  300  has an inner layer  302  formed from a stranded guard conductor ( 304  stainless steel), a second layer  304  of PVC insulation material, a third layer  306  formed from a helical winding of heater alloy (Percon-19) and a fourth layer  308  formed of PVC insulation. With respect to inner layer  302 , the guard wire does not need to vary predictably with temperature, and the heating wire from the controller to the blanket comprises three conductors, not four as required by prior art designs. 
         [0025]    As described with reference to  FIGS. 3 ,  6  and  7 , the safety wire can have various constructions, but common to all constructions is a safety shield or guard wire. With reference to  FIG. 8 , a protection circuit is shown for use with the safety wires of  FIGS. 3 ,  6  and  7 . This protection circuit can be physically located either on the same printed circuit board as the rest of the control circuit ( FIG. 9 ), or it can be located on a separate printed circuit board within the blanket  16 . Locating the protection circuit ( FIG. 8 ) within the blanket  16  has the added advantages of only requiring a two-wire connecting cord and protection against tampering in the form of connecting the blanket directly to AC power. A safety TRIAC Q 2  is triggered if significant leakage current reaches the guard wire from the heating wire because of a breakdown in the safety wire anywhere along the safety wire. Using diodes D 4 -D 7  to “switch ends” during each half cycle ensures that there is always sufficient potential difference between the heating wire and the guard wire at all points along the length of the safety wire. Without diodes D 4 -D 7 , the closer the breakdown locus to L 1  end ( FIG. 9 ), the less chance of detection, and a breakdown at the L end would not be detectable. For example, using D 4 -D 7 , consider the following breakdown locations: 
         [0026]    1. Breakdown at H 1  end of heater: Maximum voltage potential will occur when L 1  (line) is most negative with respect to L 2  (neutral). In this case, nearly the full peak line voltage appears on the guard, the guard being positive with respect to L 1 . A 1  of the triac (Q 2 ) is connected to the fused side of the line (L 1 ), so if its gate is electrically connected to the guard via D 7 , this will turn on Q 2  as soon as the trigger conditions of the specific triac are met. This will open the fuse and permanently disconnect the heater from the power source. 
         [0027]    2. Breakdown at H 2  end of heater: Maximum voltage potential will occur when L 1  is most positive with respect to L 2 . Nearly the full line voltage appears on the guard via D 6 , the guard in this case being negative with respect to L 1  . Again, the Q 2  will turn on once trigger conditions are met and open the fuse. 
         [0028]    3. Worst case, midpoint of heater: During negative half-cycles, the guard wire will be made positive with respect to L 1  by an applied potential of approximately ½ the peak line voltage. During positive half-cycles, the guard will be negative by ½ the peak line voltage. 
         [0029]    While one or more preferred embodiments of the invention have been described above, it should be understood that any and all equivalent realizations of the present invention are included within the scope and spirit thereof. The embodiments depicted are presented by way of example and are not intended as limitations upon the present invention. Thus, those of ordinary skill in this art should understand that the present invention is not limited to these embodiments since modifications can be made. Therefore, it is contemplated that any and all such embodiments are included in the present invention as may fall within the scope and spirit of the invention.