Abstract:
A heating appliance such as an electric heating blanket having a control circuit which controls the application of power to the heating element of the blanket based on the condition of the heating element. The circuit senses the voltage at the end of the heating element in order to determine if the heating element has a short or an open circuit condition therein. Under normal conditions, the sensed voltage will be above a predetermined threshold value. If the sensed voltage falls below the threshold value, the control circuit shuts off power to the heating element. The control circuit will keep power off until the fault condition has been corrected and power has been removed and reapplied to the control circuit.

Description:
FIELD OF THE INVENTION 
     The present invention relates to heating devices, particularly circuits for controlling heating devices. 
     BACKGROUND INFORMATION 
     The use of positive thermal coefficient (PTC) elements in electric heating pads and blankets is well known. Typically, a PTC element comprises an electrically conductive PTC plastic material arranged between two conductors. If, however, one of the conductors in intimate contact with the PTC material breaks, arcing may occur. Since the heating wire used in heating pads and electric blankets is typically made very thin and flexible and is subjected to repeated flexing from use, conductor breaks in the heating wire are common. When a conductor break occurs, a line voltage can develop across the break causing an arc to jump across the break. Such an arc can raise the temperature of the PTC material to auto ignition, which can start a fire. If allowed to continue for an extended period of time (e.g., approximately 250 ms or more) such arcing will likely ignite a fire. 
     A safety circuit for preventing this condition from continuing and possibly causing a fire is described in U.S. Pat. No. 4,436,986 to Carlson. When the Carlson circuit detects a conductor break in the heating element, it generates a current surge that blows an input power fuse, thereby disabling the application of power to the heater. The fuse, however, must be sized to handle currents of two or three times the continuous current rating of the heater in order to accommodate the current inrush associated with the start-up characteristics of the PTC material. The Carlson circuit also relies on the fuse to deactivate the appliance in all possibilities of short circuits. 
     Typically, an adjustable bimetallic control switch is used to provide differing heat settings for PTC-based heating appliances. As current flows through the bimetallic element, the element heats up and bends due to the differential expansion of the metals incorporated in the element. The deflection causes the contacts to open and interrupt the current to the heater and the bimetallic element to cease bending. The bimetallic element then cools down until contact is again made and the cycle repeats. The deactivation of this type of electromechanical control is typically accomplished by blowing a fuse that is in series with the switch. 
     Modern electrical power controls use solid state switching devices such as silicon control rectifiers (SCR), power transistors, solid state relays and triacs. U.S. Pat. No. 4,315,141 to Mills describes a temperature overload circuit having a pair of solid state switches biased by a temperature sensitive capacitive element. In such control systems, a small signal controls the switching of larger load currents. 
     Logic integrated circuits or microprocessors can be used to control high-speed solid state power switching devices. Such processors are typically capable of operating at speeds many times the 50 or 60 Hz frequency of typical AC power sources. This capability makes it possible to control each AC cycle and perform switching as the AC power waveform crosses zero thereby lowering the noise generation associated with AC switching and improving efficiency. Microprocessors and logic ICs, however require programming, thereby adding a significant level of complexity, customization and thus cost. 
     U.S. Pat. No. 5,420,397 to Weiss et al. describes a microcontroller-based detection circuit for limiting arcing time by either disabling the microcontroller or switching off the power. An interruption in either the hot or neutral AC power conductors will signal the microcontroller and, after a short time period, the microcontroller enters a safety mode condition in which power to the PTC heater is turned off. In order to prevent repetitive arcing by continuously restarting the microcontroller, the safety mode is reset only by removing power and waiting a predetermined time interval. Repeated and prolonged arcing will cause the arc zone to heat up, such that the arc causes the PTC material to break down, creating a carbon conduction path contributing to the volatility of the fault. 
     Typically, electric blankets and heating pads can be disconnected from their control circuits to allow the electric blanket or heating pad to be washed. For safety purposes, if the control circuit is turned on before the heating element is connected or if the heating element is disconnected while power is applied, the control circuit should go into a safety mode and deactivate the application of power. 
     A problem with known control circuits is that a fault cannot be detected before full power is applied. This can be very dangerous, since as soon as full power is provided, arcing may occur, which could result in electrocution and/or fire. It is therefore desirable to provide the unit with some means for detecting a fault before full power is applied. 
     SUMMARY OF THE INVENTION 
     The present invention provides a circuit for protecting users against failures of PTC wire heating elements in electrical heating appliances. In an exemplary embodiment, a sub-circuit located in the heating device (e.g., blanket, pad) senses the voltage at the end of the PTC wire and provides a heating element status signal to a control circuit. The control circuit mixes the status signal with a sample of the power supplied to the PTC wire to provide a signal representative of the condition of the PTC wire. The circuit of the present invention preferably detects when the temperature control cycles power to the PTC wire to prevent false tripping. If the signal indicates the presence of a fault, power is removed from the PTC heating wire. Power is kept off until the circuit is reset. 
     In a further exemplary embodiment, the sensing circuit uses phase in-coding to allow more than one sense circuit to use the same signal line, thereby reducing the number of conductors between the heating device and the control circuit. The heating device thus can be coupled to the circuit with a small number of conductors (e.g., 3 wires for a single heating element and 4 wires for two heating elements). 
     In an exemplary embodiment, the circuit of the present invention can be reset by removing power from the circuit (e.g., unplugging the appliance from a wall outlet). If power is reapplied while the fault is still present, the circuit will not reset. 
     The circuit of the present invention monitors each cycle of the AC power applied to the heating elements, thereby providing a fast response time for detecting intermittent failures before they develop into dangerous conditions. 
     Moreover, the common failure mode of the circuit components will cause a trip condition, thus deactivating the heater. Improper coupling of the heater to the circuit will also preferably cause the circuit to trip. 
     The circuit of the present invention can be implemented with a low parts count, using conventional components, thereby providing high reliability and low-cost. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is a block diagram of a first exemplary embodiment of a circuit in accordance with the present invention. 
     FIG. 2 is a schematic diagram of the exemplary circuit of FIG.  1 . 
     FIG. 3 is a block diagram of a second exemplary embodiment of a circuit in accordance with the present invention. 
     FIG. 4 is a schematic diagram of the exemplary circuit of FIG.  3 . 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 is a block diagram of an exemplary heating appliance  100  in accordance with the present invention. The appliance  100  comprises a circuit  110  and a heating blanket module  170 . The module  170  comprises a heating wire element  175  comprising, for example, PTC wire, or the like. Power, such as 120 VAC is applied to the circuit  110 , which controls the application of power to the heating blanket module  170 . While the exemplary embodiment described comprises a heating blanket, the present invention is applicable to a wide variety of heating appliances. 
     Generally, the circuit  110  can sense both breaks and shorts of the heating wire element  175  by sensing the voltage at the end of the heating wire element and adding in a sampling of the power applied to the heating wire element, if the sensed voltage is above a predetermined threshold, then the circuit  110  maintains power to the blanket wire  175 . If the sensed voltage is less than a predetermined trip voltage, the circuit  110  disconnects power from the heating wire  175 . Either a break or a short in the heating wire  175  will cause the voltage at the end of the wire to go to below the trip point. Preferably, after such a loss of voltage the control circuit  110  can only be reset if power is removed from the circuit and reapplied (e.g., power is removed by disconnecting the AC line cord). 
     The safety circuit  110  contains a power latch block  115 , a power supply block  120 , a filter network block  125 , a mixer/level shifter block  130  and a temperature controller  150 . 
     The heating blanket module  170  comprises a single heating element  175 . A dual heating element embodiment is described below in connection with FIGS. 3 and 4. The heating blanket module  170  comprises a voltage sensing sub-circuit  177  that generates a BLANKET OK signal that is provided to the control circuit  110 . The BLANKET OK signal indicates the condition of the heating element  175  as determined from the voltage that is sensed at the end of the heating element  175 . 
     The temperature controller  150  is arranged between the power control circuitry  110  and the heating element  175  and serves to control the application of power to the heating element in accordance with the temperature of the heating element and a desired temperature set by the user. The temperature controller  150  may be any suitable duty cycle regulating device (e.g., solid state or mechanical) as is familiar in the art. 
     FIG. 2 shows a schematic diagram of an exemplary embodiment of a circuit in accordance with the present invention. AC line power (e.g., 120 volts AC) is applied across LN and LH (i.e., LH is the “hot” side of the line and LN is the “neutral” side.) The power supply block  120  comprises capacitors C 1  and C 2 , resistors R 1  and R 10  and diodes D 1  and D 2 , arranged as shown in FIG.  2 . This sub-circuit reduces, rectifies and filters the AC line voltage applied to the circuit at LN and LH. 
     The output of the power supply sub-circuit  120  is coupled to the coil of a relay K 1 . The relay K 1  comprises normally open contacts arranged in series with a conductor that provides power to the heating element  175  of the blanket module  170 , via the temperature controller  150 . When the contacts of the relay K 1  are closed, power is applied to the heating element  175  via the temperature controller  150 . The normally open contacts of the relay K 1  close when the relay&#39;s coil is energized. Under normal operation, when AC line power is applied across LH and LN, the relay K 1  is activated to supply power to the heating element  175  of the blanket module. 
     A silicon controlled rectifier (SCR) SCR 2  is coupled across the coil of relay K 1 . When SCR 2  is triggered on (as described below), it shorts the coil of relay K 1 . With SCR 2  on, the current through the relay coil is diverted through SCR 2 , the coil is deactivated and the normally open contacts of the relay K 1  are opened, thereby removing power from the heating element  175 . The resistor R 1  is coupled in series with the relay coil in order to limit the current through SCR 2  when the relay coil is shorted out by SCR 2 . 
     Once triggered, SCR 2  will stay on as long as current flows through it. Since the current that is diverted through SCR 2  is derived directly from the AC line power, SCR 2  will remain on—and thus relay K 1  will remain deactivated—until the AC line power is removed from the circuit  110  (e.g., the appliance is unplugged from the AC power outlet). A safety latching mechanism is thus provided. 
     A further switching device, SCR 3 , provides a trigger signal for turning on SCR 2  and thus deactivating the relay K 1 . A capacitor C 3  is coupled between the cathode and gate of SCR 2  to prevent noise spikes from triggering SCR 2 . Resistors R 4  and R 5  serve to adjust the sensitivity of the triggering of SCR 2 . A network comprising a resistor R 16 , a diode D 3  and a capacitor C 4 , arranged as shown, causes SCR 2  to fire a predetermined time interval after SCR 3  turns off. In the exemplary embodiment shown, the predetermined time interval is preferably approximately 2-6 ms so sensing can still occur in a half cycle of the 60 Hz AC line voltage (or 8 ms). The noise filtering and delay network thus provided prevents false triggering of SCR 2  by removing high frequency noise and adding a delay to the trigger signal. This filtering provides substantial immunity against noise. Furthermore, the primary power control device, relay K 1 , is not sensitive to AC line surges or spikes. 
     Voltage sensing at the end of the PTC wire element  175  is accomplished with a voltage sensing sub-circuit  177 , as mentioned above. In the exemplary embodiment of FIG. 2, the voltage sensing sub-circuit  177  comprises a resistor voltage divider R 14 , R 15  and a transistor Q 1 . If the voltage across the end of the PTC wire element  175  is greater than a minimum set point voltage, the switch Q 1  will be turned on. This generates the BLANKET OK signal which is provided to the control circuit. When Q 1  is on, SCR 3  is turned on through resistors R 13  and R 9 , which in turn, keeps SCR 2  from firing. Until SCR 2  is triggered, the relay K 1  stays energized. If the voltage at the end of the blanket is lost either from a short or an open circuit, switch Q 1 , will turn off. This, in turn, drops out the trigger switch SCR 3  and allows SCR 2  to fire, deactivating the relay K 1  and turning off power to the heating element  175  in the blanket. 
     The control circuit  110  mixes the BLANKET OK signal with a sample of the power applied to the blanket and is level shifted to provide a signal to the trigger switch SCR 3 . Mixing in a sample of the power applied to the PTC element prevents tripping when the temperature controller cycles power to the PTC wire. More specifically, when the temperature controller  150  cycles off power to the heating element  175 , there will be no voltage at the end of the heating element. To prevent this from triggering SCR 2  and thus deactivating the relay K 1 , a resistor R 8  will hold SCR 3  on, thereby preventing the deactivation of the relay K 1 . When the temperature controller  150  re-cycles power to the heating element  175 , the resistor R 8  is shorted out and the transistor Q 1  is once again allowed to control the firing of SCR 3  in accordance with the voltage sensed at the end of the heating element  175 . 
     The voltage sensing sub-circuit  177  is preferably located in the blanket  170 . With such an arrangement, only three conductors are required to couple the blanket  170  to the control circuit  110 , namely: two wires for applying power to the blanket (including a common or ground) and a wire for the voltage sense signal (BLANKET OK). Preferably, improperly connecting the blanket  170  to the control circuit  110  will cause a tripping of the circuit, thereby preventing the application of power to the improperly connected blanket. 
     The voltage sensing sub-circuit  177  advantageously operates with low current, thereby reducing the portion of power dissipated in the control circuitry and improving the overall efficiency of the heating appliance. 
     In a preferred embodiment, in order to reapply power to the heating blanket  170 , the fault condition must be corrected (e.g., by repairing or replacing the heating blanket) and power must be removed and reapplied to the control circuit  110 . If the fault condition is still present when power is reapplied to the circuit  110 , the circuit will not reset and thus will not reapply power to the blanket  170 . 
     A further advantageous feature of the exemplary embodiment shown is that the common failure mode of the components used will cause a trip condition. As such if the circuit  110  fails due to component failure, power will be removed from the blanket  170 . The circuit  110  preferably must be operating normally in order to apply power to the blanket  170 . 
     FIG. 3 shows a block diagram of an exemplary embodiment of a dual-element heating appliance  300 , in accordance with the present invention. The exemplary appliance  300  comprises a control circuit  310  and a heating blanket module  370  having a first PTC wire heating element  375  and a second PTC wire heating element  376 . The heating blanket module  370  comprises a first voltage sensing sub-circuit  377 , for sensing the voltage at the end of the first PTC heating element  375 , and a second voltage sensing sub-circuit  378 , for sensing the voltage at the end of the second PTC heating element  376 . The voltage sensing sub-circuits  377 ,  378  generate a combined BLANKET OK signal, as described more fully below, which indicates the condition of the heating elements  375 ,  376 . 
     The control circuit  310  is similar in function to the single-element control circuit  110  of FIG.  1 . The circuit  310  includes two temperature controllers  350 ,  351 , one for each heating element  375 ,  376 . The circuit  310  also includes two mixer/level shifters  330 ,  331 , one for each heating element, a power latch  315 , a power supply  320  and a filter network  325 . 
     FIG. 4 shows a schematic diagram of an exemplary dual heating element appliance, such as that of FIG.  3 . The voltage sensing sub-circuit  377  for sensing the voltage at the end of the PTC wire element  375  comprises a resistor voltage divider (R 1 , R 2 ) and a transistor Q 1 . The voltage sensing sub-circuit  378  for sensing the voltage at the end of the PTC wire element  376  comprises a resistor voltage divider (R 3 , R 5 ) and an SCR SCR 1 . The voltage sensing sub-circuit  378  is active during the positive half cycle of the AC power whereas the voltage sensing sub-circuit  377  is active during the negative half cycle. As such, depending on which half cycle the AC power is in, SCR 1  or Q 1  can provide a trigger signal to fire SCR 2 . Both signals can thus share a common line (BLANKET OK) from the blanket module  370 . 
     As with the embodiment of FIGS. 1 and 2, if the voltage sensed at the end of the heating element  375  is greater than a threshold value, the transistor Q 1  will be on, thereby holding off the trigger signal for SCR 2  and allowing the relay K 1  to provide power to the heating elements. Similarly, if the voltage sensed at the end of the heating element  376  is greater than a threshold value, SCRl will be on, thereby holding off the trigger signal for SCR 2  and allowing the relay K 1  to provide power to the heating elements. Q 1  will only conduct during the positive half cycle, receiving its gating signal from SCR 1 . SCR 3  will only conduct during the negative half cycle, receiving its drive signal from Q 1 . 
     The signals generated by the sub-circuits  377 ,  378  are separated in the control circuit  310  and mixed with samples from the PTC input power. The mixed signals are then level shifted by a transistor Q 2  (for the first heating element) and an SCR SCR 3  (for the second heating element). Mixing of a sample of the input power to the PTC wire heating elements prevents tripping when each of the temperature controllers  350 ,  351  cycles power to the respective PTC wire heating element  375 ,  376 . The two signals from the level shifters are combined to produce the trigger signal for SCR 2  which causes SCR 2  to short the relay coil. The combined signal passes through the filter network to prevent false tripping. If the BLANKET OK signal is active for both half cycles, SCR 2  stays off and the power control relay, K 1 , stays energized. A fault condition in either PTC wire will cause the BLANKET OK signal to go inactive for that phase, thereby activating SCR 2 , dropping out K 1  and removing power to the entire blanket module  370 . 
     Resistors R 12  and R 6  are included to prevent false triggering of SCR 2  when the temperature controllers  350 ,  351  cycle off power to the heating elements. 
     Any failures in the system, from the blanket-heating element through the voltage sensing sub-circuits, to the switch devices, to the triggering device and finally the relay would cause power to the blanket to be shut down. 
     As with the single-element embodiment described above, the dual-element embodiment has a low wire count between the heating device (i.e., the blanket  370 ) and the control circuit  310 . Namely, there is one common wire, two power wires (one for each heating element) and a wire for the voltage sense signal (BLANKET OK).