Abstract:
A safety circuit to disrupt power to a heating element of an appliance to be powered through a solid state switch (Tri-ac Q 1 ), typically a triac, from an AC power source (V 1 ) having a positive half cycle and a negative half cycle delivering power. A low resistance condition is sensed by detecting either the current through or voltage across the solid state switch during the positive half cycle and the negative half cycle of the AC power line, when the solid state AC switch is not actuated. A fault signal is generated to interrupt power to the heating element, preferably by a crowbar circuit opening a fuse, whenever the low resistance condition is detected.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    This application is based on U.S. provisional application Ser. No. 61/782,714 filed Mar. 14, 2013. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to safety circuits for AC operated appliances, for example though not limited to, heating pads and electric blankets, using solid state switching circuits to activate a load to assure the integrity of a solid state switching circuit controlling activation of the load. 
       BACKGROUND OF THE INVENTION 
       [0003]    Heating pads and electric blankets typically use heating elements powered by AC line voltage, where the temperature of the heating elements is controlled, and the safety continuously monitored to protect against over heating. Typically, power to the heating element of such devices is switched on by a solid state switch such as a triac. The integrity of the triac is a key factor in the safety of the product. Should a triac fail in the shorted condition, continuous heating can result in overheating, and the user could be exposed to suffering burns and the chance of a fire can occur. It is thus important to detect a shorted power switch condition and to disconnect the power before allowing an unsafe condition to develop. 
         [0004]    Using multiple circuits in a heating element of a heating pad or electric blanket provides better detection of overheating due to a larger portion of the heating wire being affected by a bunch condition. In this way, overheating can often be recognized sooner by the temperature control circuitry. 
         [0005]    The Weiss/Lin U.S. Pat. No. 5,420,397 discloses a safety circuit for a PTC heater wire that detects a break in the wire and quickly turns the power off before an arc can cause the highly flammable wire to catch fire. The circuit of this patent uses a triac to switch on and off the power by a time proportion relative to the heat setting. In one embodiment, two triacs are used in series to mitigate the effect of one of the triacs failing by becoming shorted inasmuch as the second triac would disrupt the power. In another embodiment, a second triac is used in a crowbar circuit to open the power fuse if power to the PTC wire is detected during the off state of the power control triac. 
         [0006]    Heating pads and electric blankets typically have higher wattage than is needed to stabilize at the desired temperature. The extra power is typically provided in order to quickly bring the surface of the heating pad or electric blanket up to the desired temperature. This is termed the preheat mode that drives the heater wire to a higher temperature for a short period of time. After the preheat mode, a controller measures the temperature and maintains the wire at a target temperature according to the setting selected by the user. In this case, the power to maintain the desired temperature may be as little as  20 % of the total available power. The solid state switch, typically a triac, can fail by becoming shorted in either the full wave or half wave condition. Even a failure in the half wave condition could provide  50 % of the power continuously and eventually result in overheating of the heating element. Attempts to detect a triac short in the positive half cycle only, such as in the Kohn/Levy patent application US2013/0015174 A1, leave vulnerable the situation where the triac may be shorted in the negative half cycle, and a runaway temperature could result. 
         [0007]    Appliances other than heating pads and electric blankets have heating elements powered from an AC line and use triacs to switch the power on and off to control temperature by connecting the heating element to AC power when the temperature is below a preset value, and disconnecting the heating element from the AC power when the required temperature is reached. Other types of AC operated appliances use electronic AC switches to operatively connect and disconnect power to the load. Failure resulting from a shorted electronic power switch (e.g., a triac) in such appliances also can lead to unsafe and uncontrollable temperature raise of the heating element, or other unsafe conditions. 
         [0008]    In microcontroller (MCU) based circuits, an MCU is used to measure temperature of the heater wire and provide a control signal for the triac. These MCU circuits are quite often powered from a non-isolated low voltage power supply connected to the power line, and providing just single polarity DC voltage, e.g. + 5 V. Having a single polarity power supply provides obstacles for direct detection of the opposite polarity or bipolar signals. 
         [0009]    In the case of a heating pad or other appliances having a heating element powered from an AC line, it has been found to be advantageous to use two circuits, one circuit is powered by the positive half cycle of the AC power line, typically 120 VAC, and the second circuit is powered by the negative half cycle of the AC power line, as described in United States patent application US20130134149. Heating elements typically used have positive temperature coefficient characteristics, for example when nickel is used, and the temperature is determined by the measured resistance for both circuits. The first circuit resistance is measured during the positive half cycle of the AC power line, and the second circuit resistance is measured during the negative half cycle. The requirement to measure the resistance in the negative half cycle for the second circuit has led to a requirement to determine the conduction of the triac both when power should be applied and when power should not be applied. Conductance of the triac when the power should not be applied is indicative of a triac short. Triacs can be shorted for either the positive or negative half cycles of the AC line, or even for both cycles. For single circuit and multiple circuit heating pads failure to detect the AC switch failure for either half cycle may lead to power during that cycle being applied to the heating element, and cause overheating. 
         [0010]    The same principles apply to other types of the electronic AC switches, e.g. MOSFET based AC switches, BJT based AC switches, thyristor based AC switches, triac equivalents, etc. 
         [0011]    With an appropriate circuit arrangement, the MCU can detect failures and shut the circuit down under certain abnormal conditions, such as overheating, a wire break or when the triac fails short. 
         [0012]    Referring to  FIG. 1 , illustrating a prior art circuit, MCU U 1  triggers the Triac Q 1  to energize the heating element HT 1  in response to the reading of a temperature sensor (not shown.) The MCU U 1  periodically stops the trigger pulses to evaluate the condition of the triac Q 1 . The Triac Detector circuit  10  comprising a transistor Q 2 , reverse voltage protection diode D 3 , current limiting resistor R 2  and a load resistor R 1 , detects voltage presence on the MT 2  terminal of the triac Q 1 , and outputs the low level to the MCU when voltage is present, and the high level when there is no voltage (triac conducting). The MCU compares the response of the triac detector circuit to the current status of the triac Q 1  control signal (trigger). If the control signal is present, and the triac is expected to conduct current, the MCU expects a high level form the detector. When there is no control signal, the MCU expects the low level from the detector during the positive half cycle of the power line voltage. Triac short failure is detected when the MCU receives the high level while the triac is not triggered. When such failure is detected, the MCU triggers the Crowbar circuit, comprising a Triac Qc and a current limiting resistor Rlim, which blows the Fuse F 1  and disconnects the Heater HT 1  from the power line. 
         [0013]    The prior art circuit of  FIG. 1 , however, is not responsive to a negative voltage at MT 2  terminal of the triac Q 1 . The triac, however, has both negative and positive switching facilities (for positive and negative half cycles of the power line) that can fail independently. The circuit of  FIG. 1  does not protect against the short failure of the negative half cycle control part of the triac Q 1 . Therefore, there is a need to provide detection of the triac failure for both the positive and negative half cycles of the power line. 
       OBJECT OF THE INVENTION 
       [0014]    It is, therefore, an object of the present invention to provide for an appliance utilizing a heating element powered from AC supply, a circuitry that detects a short in a solid state switch such as a triac in both the positive and negative half cycle of an AC power supply so that in this way an appliance utilizing a heating element powered by the positive half cycle and the negative half cycle of the AC power will be protected from overheating from a failure of the solid state switching element in either the positive or negative half of the AC cycle. 
       SUMMARY OF THE INVENTION 
       [0015]    According to the invention, a safety circuit is provided for use in disrupting power to a heating element of an appliance to be powered through a solid state switch, typically a triac, from an AC power source having a positive half cycle and a negative half cycle delivering power. A low resistance condition is sensed by detecting either the current through or absence of voltage across the solid state switch during the positive half cycle and the negative half cycle of the AC power line, when the solid state AC switch is not actuated. A fault signal is generated to interrupt power to the heating element, preferably by a crowbar circuit opening a fuse, whenever the low resistance condition is detected. 
         [0016]    In one embodiment, the solid state switch is selectively triggered to supply power to the heating element during only a predetermined number of cycles of the applied AC power source, providing a duty cycle limited average to the load. A circuit interrupter in series with the heating element and solid state switch disrupts power to the heating element when a larger current is established indicating the solid state switch is passing more than the predetermined number of cycles to the heating element. 
         [0017]    In another embodiment, the safety circuits detecting either the current through or absence of voltage across the solid state switch during the positive half cycle and the negative half cycle of the AC power line, when the solid state AC switch is not actuated, provide a logical fault related signal to an MCU or other logical circuit, which actuates a circuit interrupter, e.g. a crowbar circuit, to disrupt power to the heating element. 
         [0018]    In yet another embodiment, the safety circuits are used with an analog input of the MCU, providing simplified means to detect either the current through or absence of voltage across the solid state switch during the positive half cycle and the negative half cycle of the AC power line, when the solid state AC switch is not actuated. When the fault condition is detected, the MCU actuates a circuit interrupter, e.g. a crowbar circuit, to disrupt power to the heating element. This embodiment is further adapted to a heating appliance with dual heating elements each formed of a positive temperature coefficient flexible wire and individually powered through a solid state switch by a respective half cycle of the AC power source, where the provided bi-polar current detection means are further utilized for independent temperature control of each of the positive half cycle operated and the negative half cycle operated heating circuit. 
         [0019]    These and other objects, features and advantages of the present invention are provided by a circuitry providing a safety circuit that detects a short in either the positive or negative half cycle of the solid state switching element to initiate a crowbar circuit to increase current to the fuse through which AC power is supplied to the appliance to open the fuse. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0020]      FIG. 1  is a circuit diagram of a prior art circuit for the AC switch failure detection, limited to the positive half cycle; 
           [0021]      FIG. 2  is a circuit diagram for the AC switch failure detection for both AC half cycles by sensing a voltage drop across the solid state switching device; 
           [0022]      FIG. 3  is a circuit diagram for the AC switch failure detection during both AC half cycles by sensing current through the solid state switching device; 
           [0023]      FIG. 4  is a circuit diagram of an alternate circuit for the AC switch failure detection during both AC half cycles; 
           [0024]      FIG. 5  is a circuit diagram for an alternate circuit for the AC switch failure detection during both AC half cycles; 
           [0025]      FIG. 6  is a circuit diagram for an alternate circuit controlling power to a heating element from an AC power source during both AC cycles; 
           [0026]      FIG. 7  is a circuit diagram for an alternate circuit for the AC switch failure detection during both AC half cycles by using analog inputs; 
           [0027]      FIG. 8  illustrates the relation of the input analog signals to the logical signals in the MCU of the circuitry of  FIG. 7 ; 
           [0028]      FIG. 9  is a circuit diagram for an alternate circuit for the AC switch failure detection during both AC half cycles by using analog inputs; 
           [0029]      FIG. 10  illustrates the signals of  FIG. 9 ; 
           [0030]      FIGS. 11   a  and  11   b  illustrate circuit diagrams for an alternate circuit controlling power to a heating element from an AC power source during both AC cycles by analog inputs; 
           [0031]      FIG. 12  is a circuit diagram for a preferred circuit for the AC switch failure detection during both AC half cycles in a dual circuit heating pad controller; 
           [0032]      FIG. 13  illustrates the signals of the circuitry of  FIG. 12 ; 
           [0033]      FIG. 14  is a circuit diagram for a duty cycle enabled protection responsive to the AC switch failure at either half cycle; 
           [0034]      FIG. 15  illustrates the signals of the circuitry of  FIG. 14 ; 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0035]    The triac detector circuitry  12  shown in  FIG. 2  detects a short during either half cycle of AC power controlled by a triac to a heating device. A PNP transistor Q 3  provides a required negative voltage detection capability. This bi-polar voltage detector operates as follows. When the triac Q 1  is conducting, the voltage drop across the triac is below 1 Vrms. This voltage is too small to turn the switch Q 2  on. Voltage divider R 3  R 5  keeps Vbe of Q 3  at about 0.15V, which keeps the switch Q 3  off as well. The detector output is high. When positive line voltage is present, Q 2  turns on, and the detector output is low. Diode D 2  protects the base of Q 3  from reverse voltage. When negative line voltage is present, Q 3  turns on, and turns on the switch Q 2 , and the detector output is low. 
         [0036]    The detector of  FIG. 2  detects triac conduction based on the voltage drop across the triac. 
         [0037]    Another method of detection of the triac conduction is based on sensing current passing through the triac. The circuit of  FIG. 3  shows such a detector using a current transformer (CT). This circuit outputs positive voltage to the MCU when the triac Q 1  is conducting on either the positive or negative half cycle of the AC power line voltage. The voltage is proportional to the magnitude of current passing through the triac. The CT turn ratio is selected so that the nominal current through the triac creates voltage on the output of the detector corresponding to the high logic level. Resistor R 3  limits input current of the MCU at current spikes. 
         [0038]    An AC optocoupler (OC) enabled circuit of  FIG. 4  provides bi-polar detection of the triac Q 1  current based on a voltage drop across the current sense resistor R 8 . The output of the circuit is high when the triac is off, and goes low when any polarity current is detected. The values on the schematic correspond to the heater current of about 0.5 Amp. 
         [0039]    A simple and inexpensive bi-polar current detector is presented on  FIG. 5 . This current detector comprises two NPN switches Q 2  and Q 3  that share the load resistor R 1 . The output of the circuit is high, when the triac is off. Positive current through the current sense resistor R 6  turns the switch Q 2  on, and the output goes low. The output goes low, when negative current through R 6  turns Q 3  on. Resistors R 4  and R 5  limit the base currents of the corresponding transistors. R 6  value on the schematic corresponds to the heater current of about 0.5 Amp. The current sense resistor of the circuit of  FIG. 5  dissipates about the half of the heat of the current sense resistor of  FIG. 4 . 
         [0040]    An inductor or a capacitor may be used in place of current sense resistor of  FIGS. 4  and  FIG. 5  to reduce heat generated by the current sense component. To further reduce heat, and make the circuit work on a wide range of currents, the circuit of  FIG. 6  uses a pair of inversely paralleled diodes D 1  and D 2  in place of a current sense resistor. 
         [0041]    The circuits discussed up to this point are designed to work with digital inputs of an MCU or any logic driven protection circuit. Having an analog input available opens up other opportunities for bi-polar voltage and current detection. An on-board or external A-to-D converter (ADC) or a pair of on-board or external analog comparators may be used as an analog input of the MCU or any other logic driven protection circuit. 
         [0042]    With the analog input, the voltage detector circuit of  FIG. 2  can be transformed into a simple arrangement of  FIG. 7 . Voltage divider R 1  R 3  provides a bias for the analog input of the MCU. Resistor R 2  together with the bias network forms a voltage divider that scales down the power voltage. A voltage comparator or an ADC of the MCU detects the status of the triac Q 1  as shown on  FIG. 8 . 
         [0043]      FIG. 9  depicts a bi-polar current detector version for an MCU with an analog input. This circuit features a voltage divider R 1  R 2  that sets the bias at 1 V, and a DC blocking capacitor C 3 . A voltage comparator or an ADC of the MCU detects the status of the triac Q 1  as shown on  FIG. 10 . 
         [0044]    The two circuits shown on  FIG. 11  A and  FIG. 11  B demonstrate yet another version of the bi-polar current detector working in conjunction with an analog input of the MCU. Both circuits feature a small value current sense resistor and a bias circuit that shifts a zero at the MCU input up by about  500 mV. The value of the current sense resistor is selected for a heater current of about  0 . 5  Amp. The circuits work similar to the detector of  FIG. 9 . 
         [0045]    Bi-polar current detection circuits of  FIGS. 11  a and  11  b can be used for temperature control of the heating pad with a heating element using a positive temperature coefficient (PTC) wire that changes its resistance with temperature. The bi-polar detection capability enables independent temperature control of each circuit of the dual circuit heating pad, where one circuit is powered by a positive half cycle of the power line, and another circuit by the negative half cycle. An example of such controller is shown on  FIG. 12 . 
         [0046]    The circuit of  FIG. 12  uses an MCU (not shown) that employs an ADC to measure a voltage drop across the current sense resistor R 6  during the positive and negative half cycles of the AC power line (V PTC     —     POS  and V PTC     —     NEG ) to a dual circuit heating pad. The MCU also measures the power line voltage scaled down by a voltage divider R 9  R 10  . To increase resolution, in this example the line voltage V LINE  is measured as amplitude of the positive half cycle, taking advantage of the symmetrical nature of the AC power line. The MCU uses the measurements to evaluate heating wire temperature for the positive and negative half cycle operated parts of the heating element independently, as shown of  FIG. 13 . The resistance of the positive and negative halves of the heating element is measured as ratios of the power line voltage measurement to the amplitude of the corresponding half cycle measurement of the voltage drop across the current sense resistor. The resulting values that directly correspond to the temperatures of the positive and negative halves of the heating element are compared to a preset level, and if the preset level is reached, the heating element part of the corresponding polarity is turned off for the duration of the immediately following heating period. 
         [0047]    The V BASE     —     POS  and V BASE     —     NEG  voltage measurements are taken by the MCU when the triac is off to evaluate the proper operation of the positive and negative halves of the triac, and also to remove the power supply ripple effect on the temperature measurements. If either V BASE     —     POS  and V BASE     —     NEG  voltage departs from a known DC offset for more than allowed preset value, the MCU detects a triac Q 2  failure, and activates the crowbar circuit Q 1  R 2  to blow the fuse F 2  and disconnect the heating element from the power. This arrangement provides protection not only from a short in any half of the triac, but also in case of excessive leakage current through the triac. 
         [0048]    The same circuit arrangement can be used with a single circuit heating element comprising PTC wire. In this case the negative half cycle resistance reading may be used further to improve accuracy of the measurement and to assess the condition of the negative part of the AC switch (triac). 
         [0049]    The same circuit arrangement can be used with a multiple circuit heating element comprising PTC wire, where the heating element circuits are sequentially connected (one at a time) to the control circuit for resistance measurements by an additional switching circuit. In this case the control circuit may provide condition evaluation for all AC switches in the heating element operation circuitry. 
         [0050]    For the non-PTC types of the AC powered heating elements, or when other techniques are used to control the heating element temperature, the circuits of  FIG. 2-FIG .  8 ,  FIG. 9 , and  FIG. 11  A, and  FIG. 11  B provide positive detection of the AC switch failure for both positive and negative halves of the switching structure. 
         [0051]    When no crowbar circuit is available, and/or no logic driven protection circuitry is used, a Duty Cycle Protection method provides adequate protection from a short circuit failure of either part of the AC switching element. 
         [0052]    Duty Cycle Protection is a passive technique that provides adequate electronic AC switch failure protection without use of extra active components or sophisticated algorithms. The hardware arrangement is given on  FIG. 14 . The principle of operation is illustrated by diagrams of  FIG. 15 . Under normal working conditions the triac Q 1  is triggered every third cycle of the power line voltage. The heating element HT 1  is constructed to provide adequate power at a 33.3% duty cycle. The Fuse F 1  is sized to allow continuous operation at an average current corresponding to the 33.3% duty cycle, and blow, when the current is exceeded by more than 15%. If either positive or negative half of the triac fails due to a short, the average current doubles as the duty cycle goes 66.7%, and the fuse F 1  blows. If both halves of the triac fail due to a short, the average current triples, and the fuse F 1  blows even faster. 
         [0053]    The bi-polar temperature control and/or AC switch protection mechanisms described above can be used on any AC powered devices, where a complete or partial failure of the AC switch may lead to a dangerous situation or undesired performance. Examples of such devices include but are not limited to motor controls, light controls, microwave ovens, conventional ovens, etc. 
         [0054]    The AC switch may comprise a triac, a triac equivalent, a MOSFET AC switch, a thyristor based AC switch, a solid state relay, or any other electronic circuit capable of controlling AC power in response to the control signal. 
         [0055]    It should be noted that in the above described circuits, an inductor or a capacitor may be used in place of the resistor to measure current. As one skilled in the art would know, differing sources of impedance would suffice. 
         [0056]    It is also understood that other arrangements of the voltage and current based AC switch failure detection circuits may be employed by those skilled in art to provide detection of the positive and negative halves of the AC switch without departing from the scope and spirit of the present invention. 
         [0057]    The embodiments herein described are provided for the purpose of illustration and not limitation of the present invention.