Abstract:
In a toaster, a toasting carriage is latched during toasting by an electromagnet actuated by a timer control circuit, and released when toasting time has elapsed. One of two rectifying circuits is always live and supplies a DC current to the timer control circuit and another, separate rectifying circuit supplies a DC current to energise the electromagnet when the toasting cycle is initiated. A first switching module switches a current flow through the heating element, while a second switching module isolates the always live rectifying circuit from the heating element. Switches of the first and second switching modules are disposed in an array and are closed by movement of the toasting carriage to the toasting position. More responsive and stable electromagnetic latching contributes to reducing risks associated with a fire that could occur during use of the toaster.

Description:
TECHNICAL FIELD 
     The present invention relates to an electrically heated toaster with a toasting carriage moveable between an extended ejection position and a toasting position in which it is latched during toasting by an electromagnet actuated by a timer circuit. In particular, it relates to toasters of this type having features to avoid burning of items being toasted. 
     BACKGROUND OF THE INVENTION 
     So-called “pop-up” toasters incorporate a toasting chamber with a toasting carriage which can be lowered against a biasing spring for a toasting cycle. A handle is provided for moving the carriage to the toasting position. At the same time as being lowered the toasting carriage makes switch contacts for energising toaster elements and to begin the toasting cycle. The coil of the electromagnet is supplied with current during the toasting operation in order to hold the toasting carriage in a lowered position. At the end of the toasting operation, the supply of current to the electromagnet is stopped, and the toasting carriage rises to “pop-up” the toasted item under the action of the spring. 
     A prior art toaster of this type has a power circuit that supplies the heating elements with AC power and is operated by a power switch actuated by movement of the toast carriage to the toasting position. Separate from the power circuit, a control circuit to adjust toasting time includes an integrated circuit (IC). Adjustment is accomplished by varying a time constant by means of a manually variable resistor and dependent upon other mode buttons selected by the user (e.g. a “reheat mode” for reduced heating time or a “bagel mode” for extended heating). To improve the reliability of the electromagnet holding action in toasters of this type the applicant designed a toaster described in its Chinese Patent No. 200920003318.X with two DC power supply circuits fed from the mains power. The first DC supply circuit includes a first rectifier connected by a tapping at an intermediate position along the heating element, and includes switches operable for supplying current to operate the electromagnet, and the electromagnet is thus only operable when the heating elements are switched on. The second DC supply circuit supplies current to the control circuit. This second DC supply includes a second rectifier connected to the AC power terminals, such that the control circuit is always live (i.e. has electric potential to electrical ground or neutral) whenever the toaster is plugged into the mains. In the presence of a latching signal from the controller, power supply to the latching electromagnet is maintained. To provide this control signal but isolate the live second DC supply circuit from the heating elements, an optical coupler was used. Although this prior art toaster operates satisfactorily, there is an ongoing need for performance improvements in toasters of this type. 
     Products containing sugars and fats can, in some situations, catch fire when being toasted. With toasters of the above-described type where the elements are controlled by a timer, independently of the heat load, there is a risk that a fire could be intensified if the heating elements continue to operate while an item to be toasted is burning. There is therefore a need for an improved timer-controlled toaster with anti-burn features which will mitigate the likelihood of a dangerous fire occurring. It is an object of the present invention to overcome or substantially ameliorate the above-mentioned disadvantages or more generally to provide an improved electric toaster. 
     DISCLOSURE OF THE INVENTION 
     According to one aspect of the present invention there is provided an electric toaster including: 
     a pair of AC power terminals; 
     a heating element; 
     a toasting carriage moveable from an ejection position against resilient means to a toasting position; 
     a first switching module electrically connected between the AC power terminals and the heating element for switching the current to pass through the heating element; 
     an electromagnet energised for latching the toasting carriage in the toasting position; 
     a first rectifying circuit operable for supplying DC current to energise the electromagnet, the first rectifying circuit being electrically connected to a branch from the heating element; 
     a timer control circuit for controlling operation of the electromagnet and release the toasting carriage; 
     a second rectifying circuit connected by a branch between the first switching module and the AC power terminals for supplying DC current to the control circuit, the second rectifying circuit; 
     a second switching module electrically connected between the heating element and the first rectifying circuit for isolating the first rectifying circuit from the heating element; 
     a third switching module controlled by the timer control circuit and electrically connected between the electromagnet and the first rectifying circuit, and 
     wherein switches of the first and second switching modules are disposed in an array near the toasting position and are closed by movement of the toasting carriage to the toasting position. 
     Preferably the AC power terminals are electrically connected to respective phase and neutral conductors joined by the heating element, the switches of the first switching module comprise a first switch and a second switch connected to the phase and neutral conductors respectively such that with any one of the first and second switches open the current supply to the heating element is cut; the first rectifying circuit comprises two legs and the switches of the second switching module comprise a third switch and a fourth switch, each electrically connected in one of the two legs, such that with both the third and fourth switches open the first rectifying circuit is isolated from the heating element. 
     Preferably the switches of the first switching module further comprise a fifth switch serially connected with the first switch to the phase conductor. 
     Preferably a thermal overload switch is electrically connected in series between the first and fifth switches. Preferably the thermal overload switch includes a bimetallic switching element, and does not require manual resetting after operation, and can be tripped multiple times. 
     Preferably an input signal line, electrically connected between the first and fifth switches, sends a signal indicative of closing of the first switch to the timer control circuit. 
     Preferably a latching block to which a ferrous part is fixed for cooperating with the electromagnet, the latching block being moveable between a latched position for latching the toasting carriage in the toasting position, and a released position, wherein in the toasting position the latching block abuts the first and second switches and the carriage, or a fixture on the carriage, abuts the third, fourth and fifth switches to thereby close the first, second, third, fourth and fifth switches. 
     In another aspect the invention provides an electric toaster comprising: 
     a pair of AC power terminals; 
     a heating element; 
     phase and neutral conductors electrically connecting the heating element to the AC power terminals; 
     a toasting carriage moveable from an ejection position against resilient means to a toasting position; 
     a first switch and a second switch connected to the phase and neutral conductors respectively such that with any one of the first and second switches open the current supply to the heating element is cut; 
     a hook disposed on the toasting carriage; 
     a moveably mounted latching block cooperating with the hook for latching the toasting carriage in the toasting position, the latching block being moveable between a latched position in engagement with the hook and a released position disengaged from the hook; 
     a ferrous part fixed to the latching block; 
     an electromagnet cooperating with the ferrous part to hold the latching block in its latched position thereby latching the toasting carriage in the toasting position; 
     a first rectifying circuit operable for supplying DC current to energise the electromagnet, the first rectifying circuit being electrically connected to a branch from the heating element and comprising a supply and return leg; 
     a timer control circuit for controlling operation of the electromagnet and release the toasting carriage; 
     a second rectifying circuit connected by a branch between the first switching module and the AC power terminals for supplying DC current to the control circuit; 
     a third switch and a fourth switch, each electrically connected in one of the supply and return legs of the first rectifying circuit, such that with both the third and fourth switches open the first rectifying circuit is isolated from the heating element. a switching device controlled by the timer control circuit and electrically connected between the electromagnet and the first rectifying circuit, and 
     wherein the first, second, third and fourth switches are each single-pole single-throw normally-open switches disposed in an array, and when the toasting carriage is lowered the carriage, or a fixture on the carriage, abuts the third and fourth fifth switches to thereby close the third, and fourth switches; and the carriage, or a fixture on the carriage, abuts the latching block which in turn abuts the first and second switches and to thereby close the first and second switches to initiate a toasting cycle. 
     This invention provides an electromagnetically-latched timer-released toaster which is effective and efficient in operational use, which provides a high degree of safety and reliability, with faster response time and improved stability of the electromagnetic latching, all of which contribute to greatly reducing risks associated with a fire occurring during use. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Preferred forms of the present invention will now be described by way of example with reference to the accompanying drawings, wherein: 
         FIG. 1  is a block diagram of the circuit of the toaster of the invention; 
         FIG. 2  is a schematic of the electrical circuit of the toaster of the invention; 
         FIG. 3  is a perspective view of a toasting carriage of the toaster of the invention; 
         FIG. 4  is a perspective view of a latching block of the toaster of the invention; 
         FIG. 5  is a perspective view of a switch array of the toaster of the invention; 
         FIG. 6  is a perspective view showing the toasting carriage, latching block and switch array assembled onto the toaster of the invention, and 
         FIG. 7  is a schematic of the latching mechanism of the toaster of the present invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to  FIGS. 1-4  of the drawings, the toaster  100  includes a bridge rectifying circuit  10  connected to AC power terminals  11  for supplying a low voltage DC current to a timer control circuit  12 . A first switching module  13  is electrically connected between the AC power terminals  11  and a heating element  14  for switching the current to pass through the heating element. A second switching module  15  is electrically connected between the heating element  14  and a rectifying circuit  16  for supplying DC current to energise an electromagnet  17 . 
     The toaster  100  includes a toasting carriage  20  for supporting an item to be toasted (not shown). The toasting carriage  20  is mounted to slide along an upright rail  21  fixed at one end to the chassis  22  of the toaster and at the other end to a base  26  of the toaster. The carriage  20  is moveable from a raised ejection position (shown in  FIG. 2 ) against a return spring  23  to a lowered toasting position (not shown). The carriage  20  is held in the toasting position, by cooperation with the electromagnet  17 . 
     In particular, the carriage  20  includes a pivotally mounted hook  24  spring-biased away from the carriage to engage with a latching block  25  mounted to the base  26 . At one end the latching block  25  is mounted to pivot about a transverse axis  27 , and at an opposing end the latching block  25  has an abutment surface  28 . A ferrous part  29  is fixed to the latching block  25  for cooperating with the electromagnet  17 . The latching block  25  is moveable between the latched position shown, for latching the toasting carriage in the toasting position, and a released position (not shown) in which the abutment surface  28  is pivoted upwardly about axis  27 . When the carriage  25  is lowered to the toasting position, the hook  24  engages the latching block  25 , pushing it to its latched position where it is then held by the electromagnet  17 , before the hook  24  is pivoted by engagement with the inclined face  30  and then retained in a recess (not shown) in the latching block  25 , thereby holding the carriage in the toasting position. 
     The toasting cycle is initiated by moving the toasting carriage  20  to the toasting position, and the resulting operation of the switching modules  13  and  15 . As the rectifying circuit  11  and timer control circuit  12  receive current whenever the terminals  11  are connected to mains power, and as soon as a signal indicative of actuation of the switching module  13  is received along input signal line  31 , the timer control circuit  12  is able to transmit a latching signal along output line  32  to the switching module  18 , which is then operated to activate the electromagnet  17 . As the timer control circuit  12 , which preferably comprises a microprocessor  56 , is always live there is no delay while it is configured and before it is able to process the input and provide the latching signal. Having a bridge rectifying circuit  16  for powering the electromagnet  17 , which is separate from the rectifying circuit  10  powering the timer control circuit  12 , improves reliability and responsiveness of the carriage latching. The switching module  15  serves to isolate the heating element  14  from a fault current passing from the always live timer control circuit  12 , so that a user cannot unexpectedly find the element  14  to be live. 
     Referring to  FIG. 6 , the AC terminals  11  comprise terminals L and N connected to phase and neutral conductors  36  and  37  respectively. The phase and neutral conductors  36 ,  37  are joined by the heating elements  38 - 40 . In a two-slice toaster the power is split between elements  38  and  39  in the outer sides of the two toasting chambers  41 ,  42 , while the larger element  40  spans the inners sides of both toasting chambers  41 ,  42  In use terminals L and N are connected to the electrical mains. The switching module  13  includes three single-pole, single-throw normally-open switches, designated SW 1 , SW 2  and SW 5  that are mechanically operated in a manner described in detail below with reference to  FIGS. 2-5 . The switching module  13  further includes a thermal overload switch  35 , such as a bimetallic thermostatic switch mounted to the chassis of the toaster and electrically connected to the phase conductor  36 . The thermal overload switch  35  is set to open at a temperature above that resulting from normal operation, to respond to an event such as an item being toasted catching fire. The first switch SW 1  and second switch SW 2  are connected to the phase and neutral conductors  36 ,  37  respectively. The second switch SW 2  controls power to the heating elements, should the polarity of the power supply be reversed. The fifth switch SW 5  is also redundant, and is provided as an additional safeguard, such that with any one of the first, second and fifth switches SW 1 , SW 2 , SW 5  open the current supply to the heating elements is cut to avoid burning. 
     The upper current carrying line through the heating element  38 ,  39  includes a voltage divider connected to a first leg  43  of the first rectifying circuit  16 . The other side of the first rectifying circuit  16  is connected with the neutral conductor  37 . The first rectifying circuit  16  includes a smoothing capacitor  45  across the output of the first bridge rectifier  46 , and resistors  47 ,  48 . 
     The switching module or switching transistor  18  is electrically connected in series with the resistor  48  and the electromagnet  20  between the positive and negative lines of the first rectifying circuit, while the emitter of the switching transistor  18  is connected to an output line  32  from the timer control circuit  12 . In this manner the timer control circuit  12  actuates the switching transistor  18  for supplying current to the electromagnet coil  20 . 
     In the phase and neutral lines between the switching module  13  and the AC terminals L, N are respective nodes  49 ,  50  from which upper and lower branches extend to a second diode rectifier  51  of the second rectifying circuit  10 . 
     The second rectifying circuit  10  includes smoothing capacitors  52 ,  53  across the output of the second rectifier  51 , as well as a resistors  54  and Zener diode  55  which provide a voltage regulator, supplying DC voltage Vcc for powering the timer control circuit  12 . 
     The timer control circuit  12  includes a microprocessor or integrated circuit  56  of a known type, the operation of which will not be described in detail. The base of the transistor  18  is connected to an output pin of the IC  56 . The principal controls and sensors in the control circuit  12  include the variable resistor  57  used as the primary means for controlling the degree of toasting by the timing of the toasting cycle. Five mode switches  58 - 62  are provided for further controlling the cycle (the modes including “Frozen”, “Reheat”, “Bagel” etc), each having an associated LED  63 - 67  indicating the selected status of each mode. A “cancel” switch  68  allows each mode to be de-selected. A negative temperature coefficient thermistor  69  is used to monitor temperature. 
     Connected to the first switching module  13  at a node  70  in the phase line between the switch SW 1  and the thermal overload switch  35  is an activation-sensing circuit  71  which provides a DC signal to the integrated circuit  56  indicating that the switch SW 1  has been closed. The activation-sensing circuit  71  includes diode  72  and voltage divider  73 . Line  74  connects the output side of the activation-sensing circuit  71  to a pin of the IC  56 . 
     As shown in  FIGS. 2-5 , the switches SW 1 , SW 2 , SW 3 , SW 4 , SW 5  of the first and second switching modules  13 ,  15  are disposed in an array upon a circuit board  77 , that is generally upright and mounted to the base  26  adjacent the toasting position of the carriage  20 . The second and fifth switches SW 2 , SW 5  are disposed adjacent one another, and opposite the abutment surface  28  of the latching block  25 . As the carriage  20  is lowered a finger  78  extending downwardly from the carriage first abuts the first switch SW 1 , before a transversely projecting arm  79  abuts both the third and fourth switches SW 3 , SW 4  to close them simultaneously. 
       FIG. 6  schematically illustrates the toasting carriage  20  of the toaster and the electromagnetic latching mechanism  75  for holding the toasting carriage  20  down in its toasting position (not shown) against the spring  23  that is urging it to its extended position. The throws of the second and fifth switches SW 2 , SW 5  are shown schematically abutting abutment surface  28  of the latching block  25  by which they are closed, simultaneously. 
     In operation, when the terminals L, N are connected to the mains supply, current is supplied through the second rectifying circuit  10  for supplying current to the timer control circuit  12 . To toast a slice of bread or the like, the user places the item in one of the toasting chambers  41 ,  42  to sit upon arms (not shown) of the toasting carriage  20  and lowers the toasting carriage  20  toward the toasting position. 
     When the finger  78  first abuts and closes the first switch SW 1  the input signal line, electrically connected between the first switch SW 1  and thermal overload switch  35 , sends a signal to the activation-sensing circuit  71  which in turn provides a signal indicative of closing of the first switch to the timer control circuit  12 . The timer control circuit  12  then starts a timer and transmits a latching signal along output line  32 . 
     The hook  24  and latching block  25  initially abut, pivoting the latching block  25  downwardly against its release spring  81 , thereby closing the second and fifth switches SW 2 , SW 5  and substantially simultaneously the third and fourth switches SW 3 , SW 4  are closed by abutment with the carriage. Power is thus provided to the heating elements  38 - 40  and to the rectifying circuit  16  for powering the electromagnet  20 . With a current flowing between the collector and emitter of the switching transistor  18  and through the electromagnet  20  it attracts the ferrous block  29 , the magnetic circuit holding the ferromagnetic block  56  and electromagnet coil  20  together. With the hook  24  and latching block  25  mutually engaged the toasting carriage  20  is thereby latched in the toasting position. In this manner the current drawn through the electromagnet  20  is generally independent of the current drawn by the control circuit and strong, reliable latching action is achieved. At the end of the toasting time the controller  56  stops the latching signal along output line  32  to the switching module  18 , thereby opening the circuit powering the electromagnet  20  which then allows the latching block  25  to pivot to its released position (shown in  FIG. 7 ) and thus allows switches SW 2  and SW 5  to open. In this manner switches SW 2  and SW 5  cut power to the elements  38 - 40 , independently of the position of the toasting carriage  20 , so burning is prevented in the event that the toasting carriage  20  becomes stuck in the toasting position. 
     Aspects of the present invention have been described by way of example only and it should be appreciated that modifications and additions may be made thereto without departing from the scope thereof.