Abstract:
The present invention relates to a temperature compensated timer for a heating appliance that heats a food item during a heating cycle. The timer includes a coil having first and second terminals and presenting a resistance between the terminals that is a function of temperature changes of the coil. A timing circuit is coupled to the first and second terminals of the coil. The timing circuit operates to drive a current through the coil to maintain a heating cycle and to generate a delay signal a delay time after the heating cycle is initiated. The delay time has a value that is a function of the resistance of the coil. The delay signal is operable to remove the current from the coil to terminate the heating cycle.

Description:
TECHNICAL FIELD 
     The present invention relates generally to heating appliances such as toasters or toaster ovens, and more specifically to a temperature compensated timing circuit that controls the duration of a heating cycle of the appliance to compensate for the increasing internal temperature of the appliance on successive heating cycles. 
     BACKGROUND OF THE INVENTION 
     In heating appliances, such as toasters and toaster ovens, a food item is placed in a bread cavity of the appliance and toasted for a desired time, which is known as the heating cycle of the appliance. The duration of the heating cycle determines the extent to which the food item is cooked or toasted. For example, in a conventional toaster, the time required to toast successive food items to the same extent decreases for each successive heating cycle due to the bread cavity transmitting a certain amount of heat to the food item once the bread cavity has been warmed during previous heating cycles. In other words, the already warm bread cavity transmits a certain amount of heat to the food item in addition to the heat generated by the appliance during the heating cycle, resulting in less time being required to toast the food item. As a result, if the duration of the heating cycle is constant, food items placed in the bread cavity during subsequent heating cycles will be toasted more than those in previous heating cycles due to the additional heat transmitted from the bread cavity. To compensate for heat transmitted from the bread cavity to the food item during the second and subsequent heating cycles, conventional toasters include a timing circuit that compensates for this heat by reducing the duration of successive heating cycles. 
     FIG. 1 is a schematic of a conventional timer  10  that compensates for heating of the bread cavity by reducing the duration of successive heating cycles, as will now be described in more detail. In the timer  10 , an external force is applied to close a switch  12  thereby applying an AC voltage from a voltage generator  14  to an input node  16  of the timer  10 . An external lever (not shown) of the toaster containing the timer  10  is typically pushed down to apply the external force to close the switch  12  and to lower a food item into the bread cavity. The external lever is typically maintained in a down position by a mechanical latching mechanism (not shown) thereby maintaining the switch  12  closed. A coil  23  generates an electromagnetic force when energized to release the mechanical latching mechanism, thereby allowing the food item to be raised from the bread cavity and allowing the switch  12  to open, as indicated by the dotted line  25 , as will be described in more detail below. However, when the switch  12  is closed, the AC voltage from the voltage generator  14  on the node  16  is rectified by a diode  18 , and the magnitude of this rectified voltage is reduced by a voltage divider formed by series-connected resistors  20  and  22 . 
     A capacitor  26  is coupled to a node  24  defined between the resistors  20  and  22 , and filters the rectified voltage to provide approximately a DC supply voltage on the node  24 . As explained below, a timing circuit  28  receives the supply voltage on node  24  and generates a first trigger signal V t1 , on a node  29  a delayed time after the switch  12  is closed to apply the supply voltage to the circuit  28 . The timing circuit  28  includes a resistor  30  and a variable resistor  32  connected in parallel with a resistor  34  and a thermistor  36 . The thermistor  36  presents a resistance having a value that is a function of the temperature of the thermistor, as understood by those skilled in the art. The thermistor  36  has a negative temperature coefficient so that as the temperature of the thermistor increases, the value of the resistance presented by the thermistor decreases. Typically, the thermistor  36  is mounted near the bread cavity, of the toaster and thus presents a resistance having a value that is a function of the temperature within the bread cavity. The resistor  34  and thermistor  36  in parallel with the resistor  30  and the variable resistor  32  present an equivalent resistance R T  between the node  24  and a capacitor  38  coupled between the node  29  and ground. The capacitor  38  and equivalent resistance R T  together form an RC circuit with the voltage across the capacitor  38  having a value that varies as a function of time. The time dependence of the voltage across the capacitor  38  is determined by the values of the equivalent resistance R T  presented by the resistors  30 - 34  and thermistor  36  and the capacitor  38 , as well understood by those skilled in the art. In operation of the timing circuit  28 , the voltage on the node  24  is applied through the equivalent resistance R T  to charge the capacitor  38  and thereby develop first trigger signal V t1 . The rate at which the capacitor  38  charges and thus the rate at which the magnitude of the first threshold signal V t1  increases is a function of the resistance presented by resistors  30 - 34  and thermistor  36 , as previously described. A diode  52  and resistor  54  discharge the capacitor  38  when switch  12  is open. 
     A diac  40  receives the first trigger signal V t1  on a first terminal and has a second terminal coupled through series connected resistors  42  and  44  to ground. When the first trigger signal V t1  has a magnitude less than a predetermined breakdown voltage, the diac  40  presents a high impedance and no current flows through the diac. When the first trigger signal V t1  exceeds the breakdown voltage, the diac  40  turns ON and current flows from the node  29  through the diac  40  and series-connected resistors  42  and  44 . The resistors  42  and  44  operate as a voltage divider, with the voltage across the resistor  44  being applied as a second trigger signal V t2  to a silicon controlled rectifier (SCR)  46 , which is connected in series with the coil  23  and a resistor  50 . When the second trigger signal V t2  exceeds a second breakdown voltage, the SCR  46  turns ON causing current to flow from the node  24  through the resistor  50  and coil  23 , thereby energizing the coil. The resistor  50  reduces the magnitude of the voltage applied across the coil  23  when the SCR  46  is turned ON. As mentioned above, energizing the coil  23  releases a mechanical latching mechanism (not shown) to allow the switch  12  to open and the food article to be raised from the bread cavity. 
     The overall operation of the timer  10  during a heating cycle of a conventional appliance containing the timer will now be described in more detail. Initially, assume the switch  12  is open, isolating the voltage generator  14  from the node  16 . To initiate a heating cycle, an external force is applied to close the switch  12  thereby applying the voltage from the generator  14  to the input node  16 . When the voltage from the generator  14  is applied on the input node  16 , the diode  18  rectifies this voltage and the supply voltage on node  24  is developed, as previously described. In response to the voltage on the node  24 , the capacitor  38  begins charging at a rate determined by the value of the equivalent resistance R T  presented by resistors  30 - 34  and thermistor  36 . The variable resistor  32  is adjusted in relation to a “toast darkness” scale to control the duration of the heating cycle. As previously described, the thermistor  36  has a negative temperature coefficient so that as the temperature in the bread cavity increases the value of the resistance presented by the thermistor  36  decreases. Thus, as the temperature of the bread cavity increases, the equivalent resistance R T  presented by the resistors  30 - 34  and the thermistor  36  decreases, causing the capacitor  38  to charge at a faster rate. The voltage across the capacitor  38  corresponds to the first trigger signal V t1 , and as the capacitor  38  charges the magnitude of the first threshold voltage V t1  increases at a rate determined by the value of the equivalent resistance R T0 . Once the first trigger signal V t1  reaches the breakdown voltage of the diac  40 , the diac  40  turns ON causing current to flow through resistors  42  and  44 . In response to this current flow through the resistor  44 , the magnitude of the second trigger signal V t2  exceeds the breakdown voltage of the SCR  46 , turning ON the SCR so that current flows through the SCR to thereby energize the coil  23 . When the coil  23  is energized, the switch  12  opens, isolating the voltage generator  14  from the node  16  and thereby terminating the heating cycle of the appliance. 
     In a conventional toaster, when the coil  23  is energized causing the switch  12  to open a bread carriage within the toast cavity is typically released causing a portion of the toasted bread to extend beyond the top of the toaster so that it may be removed. It should also be noted that when the switch  12  opens causing the rectified voltage to be removed from the node  24 , the capacitor  38  may discharge through the diode  52  and resistor  54  to thereby remove charge from the capacitor  38  so that residual charge remaining on the capacitor  38  does not adversely affect the time of subsequent heating cycles. 
     If the external force is again applied to close the switch  12  and initiate another heating cycle, the timer  10  operates in the same manner as previously described to energize the coil  23  a delay time after the switch  12  is closed. During this subsequent heating cycle, however, the bread cavity may still be warm from the previous cycle and thus the thermistor  36  presents a smaller resistance than during the prior heating cycle. As a result, the resistance R T  presented by the resistors  30 - 34  and thermistor  36  is smaller than during the previous heating cycle, causing the capacitor  38  to charge more quickly and thereby reducing the delay time of the timer  10 . More specifically, the signal V t1  more quickly exceeds the breakdown voltage of the diac  40 , causing the diac to turn ON faster. As previously described, when the diac  40  turns ON, the signal V t2  is generated to trigger the SCR  46 , energize to the coil  23 , and terminate the heating cycle. Because the SCR  46  turns ON faster, the duration of the heating cycle is reduced accordingly. As previously described, this is desirable because toast placed in the bread cavity during the subsequent heating cycle will be toasted by a certain amount due to residual heat transmitted to the bread from the heated bread cavity. Thus, the delay time of the current heating cycle is decreased to toast the bread during the second heating cycle by the same amount as that during the first heating cycle. 
     Another conventional timer used in controlling the duration of heating cycles in a toaster includes a digital timer, such as an MC4541, coupled to a temperature sensitive capacitor. The capacitor functions as a temperature sensor, presenting a capacitance having a value that is a function of temperature. In operation, a coil is energized at the start of a heating cycle. The coil generates an electromagnetic force that is applied to hold the bread carriage within the cooking cavity during the heating cycle. During the heating cycle, the digital timer generates an oscillating signal having a frequency that is a function of the value of the capacitor. The frequency of the oscillating signal determines when the digital timer activates a transistor coupled to the coil to thereby de-energize the coil and terminate the heating cycle. 
     In the conventional timer  10 , several factors make it difficult to maintain a consistent level of toasting during successive heating cycles. First, the precise location of the thermistor within the bread cavity is critical. The thermistor  36  must be positioned so that the resistance presented by the thermistor  36  varies as a function of the temperature in the bread cavity to properly adjust the delay time of the timer  10  and maintain consistent toasting among heating cycles. The position of the thermistor, however, may not be consistent from one toaster to the next, causing unwanted variations in the delay time of the timer  10 . Another factor that adversely affects the levels of toasting is the inherent nonlinearity of the thermistor  36 , which causes the delay time to be adjusted by amounts that do not properly compensate for increased temperatures in the bread cavity. The tolerance of the thermistor  36  is typically relatively large for less expensive thermistors, and such variations in the value of the resistance presented by the thermistor  36  among timing circuits  28  results in variations in the delay times among the timing circuits  28 . An additional problem with the timer  10  may arise if the coil  23  fails “open.” In this situation, when the SCR  46  turns ON, coil  23  is not energized so the switch  12  remains closed causing power to be continually applied to the toaster. This may result in a potentially dangerous situation as the toaster becomes increasingly hot. The prior art circuit including the digital timer and capacitor as described above does not present this same problem since the associated coil is energized at the start of a heating cycle and a failed open coil would prevent a heating cycle from being initiated. 
     There is a need for a timer to reliably control and adjust the duration of heating cycles in a toaster in order to maintain consistent levels of toasting of food items among successive heating cycles. 
     SUMMARY OF THE INVENTION 
     According to one aspect of the present invention, a temperature compensated timing circuit includes a coil having first and second terminals that presents a resistance between the terminals that is a function of temperature changes of the coil due primarily to current flowing, through the coil when energized and/or heat transfer to the coil from heating elements. A power circuit generates a first voltage, and a switching circuit is coupled between the coil and the power circuit. The switching circuit operates in response to an external condition to apply the first voltage across the first and second terminals to energize the coil, and thereafter operates to isolate the power circuit from the coil responsive to the coil being deenergized. A timing circuit is coupled to the first and second terminals of the coil. The timing, circuit operates in a first mode when the coil is energized to time a delay time having a value that is a function of the resistance of the coil. The timing circuit operates on a second mode upon expiration of the delay time to deenergize the coil and thereby cause the switching circuit to isolate the power circuit from the coil. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic of a conventional timing circuit for a heating appliance. 
     FIG. 2 is an isometric view of a toaster including a timer according to one embodiment of the present invention. 
     FIG. 3 is an isometric view of one embodiment of a switching circuit connected to the timer of FIG.  2 . 
     FIG. 4 is a schematic of one embodiment of the timer of FIG.  2 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 2 is an isometric view of a toaster  200  including a timer  202  according to one embodiment of the present invention. The toaster  200  includes a shell  204  formed from two side panels  206 ,  210  and two end panels  208 ,  212  that may be integrally formed with each other. An outer bread guard  216  is positioned inside the side panel  206  and an identical outer bread guard  218  is positioned inside the side panel  210 , as shown. Two inner bread guards  220  and  222  are also positioned between the outer guards  216  and  218 . Each of the bread guards  216 - 222  includes a horizontal member  219  and vertical members  221 . A first bread cavity  215  is defined between the bread guards  216  and  220 , and a second bread cavity  217  is defined between the bread guards  218  and  222 . The bread guards  216 - 222  function to protect bread placed between the bread guards from heating elements (not shown in FIG. 2) positioned inside the side panels  206  and  210  as well as between the center bread guards  220  and  222 . 
     A first bread carriage (not shown) is contained within the first bread cavity  215  and functions to support a piece of bread as it is lowered into and raised from the bread cavity  215 . A second bread carriage (not shown) is similarly positioned within the second bread cavity  217  to support another piece of bread in the bread cavity  217 . Each of the bread carriages includes a lever portion extending through slots  224  and  226 , respectively, in the side panel  208 . The lever portions are pushed down to lower the respective pieces of bread on the bread carriages into the bread cavities  215 ,  217 . As the bread carriages are pushed down, the outer bread guards  216 ,  218  move toward the center of the corresponding bread cavity  215 ,  217 , as shown for the bread guard  218 . In this way, the bread guards  216 - 222  position the bread in approximately the centers of the bread cavities  215  and  217  so that the bread placed on the bread carriages is not positioned too close to the heating elements. The toaster  200  further includes an edge panel  228  positioned at the bottom of the side panel  208 . The timer  202  and a switching circuit  234  are shown mounted on the edge panel  228 . 
     FIG. 3 illustrates the switching circuit  234  in more detail. The switching circuit  234  includes a contact lever  236  and a contact assembly  238 , which includes first and second electrically conductive resilient blades  239 ,  240  that are selectively coupled to respective contacts  241  and  242 , as explained below. The contact lever  236  rotates about an axis  246  in a counter-clockwise direction in response to a force F applied as shown. When the contact lever  236  rotates it forces the blades  239 ,  240  against the contacts  241 ,  242 , respectively, to apply power to the toaster  200 . A portion of one of the bread carriages (not shown) applies the force F on the contact lever  236 . The timer  202  is also coupled to the switch assembly  238  to receive power through the switch assembly  238  during a heating cycle. As explained further below, the timer  202  drives an electromagnetic coil  23  that generates a magnetic field in an armature  243 . A keeper  245  mounted at the end makes contact with the armature  243 . The magnetic field maintains the keeper  245  in contact with the armature  243  during, the heating cycle so that the switch assembly  234  continues to apply power to the toaster  200 . At the end of the heating cycle as determined by the timer  202 , the timer  202  removes power from the electromagnet coil, thereby releasing the keeper  245 . The contact lever  236  is then allowed to rotate in a clockwise direction to allow the blades  239 ,  240  to separate from the contacts  241 ,  242 , respectively. Electrical power is then removed from the toaster  200 . 
     FIG. 4 is a schematic of the timer  202  of FIGS. 2 and 3 according to one embodiment of the present invention. The timer  202  includes the switching circuit  234 , which is shown schematically as including a switch  310  and an electromagnet coil  312 . In operation, the coil  312 , when energized, maintains the switch  310  closed. The coil  312  has a resistance that is a function of temperature, and the timer  202  adjusts the duration of the heating cycles of the toaster  200  containing the timer  202  in response to the value of the resistance and thus the temperature of the coil  312 , as will now be explained in more detail. Thus, the coil  312  is used as both a temperature sensor and an actuator for terminating the heating cycle. 
     An AC voltage generator  300  is coupled through the switch  310  to an input node  301 . When the switch  310  is open, the AC voltage generator  300  is isolated from the input node  301 , which corresponds to the open position of the switch circuit  234  shown in FIG. 3 where the contact lever  236  is raised. When the switch  310  is closed, the voltage generator  300  applies an AC voltage on the input node  301 . A diode  302  rectifies the AC voltage on the input node  301 , and this rectified voltage is applied through a resistor  304  to a capacitor  308 . The capacitor  308  filters the rectified voltage to develop a substantially DC voltage on a node  305 , and this DC voltage is applied through a resistor  306  to a node  314  coupled to one terminal of the coil  312 . The coil  312  is connected between the node  314  and ground, and is thus energized when the switch  310  is closed. 
     The voltage on the node  314  corresponds to the voltage across the coil  312 , and this voltage is applied to an RC delay circuit  318  formed by resistors  320 ,  322 , a variable resistor  324 , and a capacitor  328 . The resistor  320  and a resistor  326  function as a voltage divider to develop a reduced voltage on a node  327 , and this reduced voltage is applied through the resistor  322  and variable resistor  324  to charge the capacitor  328 . The values of the resistors  320 ,  322 , the variable resistor  324 , and the capacitor  328  are selected to provide the desired delay time of the delay circuit  318 . The precise value of the variable resistor  324  may be adjusted with reference to a “toast darkness” scale (not shown) to thereby adjust the delay time. The rate at which the capacitor  328  changes is also a function of the voltage on the node  327 , as will be described in more detail below. The voltage across the capacitor  328  corresponds to a first trigger signal V t1  that is applied to a base of an NPN transistor  330  having its collector coupled to the node  314 . The NPN transistor  330  acts as an emitter-follower to couple the trigger signal V t1 , less one diode-drop, to a gate of an SCR  334  as a second trigger signal V t2 . However, as is well known in the art, the base of the transistor  330  presents a high impedance to avoid shunting excess charging current from the capacitor. As the capacitor  328  charges, the magnitude of the first trigger signal V t1  increases accordingly. When the second trigger signal V t2  exceeds the trigger voltage of the SCR  334 , the SCR turns ON, coupling the node  314  to approximately ground to thereby deenergize the coil  312 . In other words, when the SCR  334  turns ON, current flows through the SCR  334  and not through the coil  312  to thereby deenergize the coil. The timer  202  further includes a diode  336  and resistor  338  that discharge the capacitor  328  when the SCR  334  turns ON and drives the node  314  to approximately ground. 
     The operation of the timer  202  during a heating cycle of the toaster  200  (FIG. 2) will now be described in more detail. To initiate a heating cycle, the external force F is applied to close the switch  310 , thereby causing a D.C. voltage to be generated at the node  305 . The voltage on the node  305  is applied through the resistor  306  to energize the coil  312 , causing the coil  312  to maintain the switch  310  closed even after the external force F is removed. At this point, the SCR  334  is turned OFF. The resistor  306  and the coil  312  form a voltage divider at the node  314 , and this voltage on the node  314  is applied to the RC delay circuit  318 . 
     Once the coil  312  is energized, the capacitor  328  begins charging to time the delay time of the timer  202 . Once the value of the signal V t1  coupled to the emitter of the transistor  330  as the second trigger signal V t2  exceeds the breakdown voltage of the SCR  334 , the SCR turns ON. The current flowing through the coil  312  is then shunted through the SCR  334  to deenergize the coil. When the coil  312  is deenergized, the coil no longer generates an electromagnetic force to keep the switch  310  closed, and thus the switch  310  opens, thereby terminating the heating cycle of the toaster  200 . 
     If the external force F is again applied a short time after the first heating cycle, the switch  310  is again closed to repeat the above-described operation during a second heating cycle. During the second heating cycle, the timer  202  operates in the same manner as previously described. However, during the second heating cycle and any subsequent heating cycles, however, the delay time of the timer  202  varies as a function of the resistance of the coil  312 , as will now be explained in more detail. 
     As explained above, the coil  312  is energized during the entire duration of a heating cycle. As the switch  310  is closed to activate successive heating cycles, the temperature of the coil  312  increases due primarily to self heating of the coil  312  caused by the current flowing through the coil. However, some heating of the coil may occur because of heat transferred to the coil from heating elements (not shown) in the toaster. Since the resistance of the coil  312  is a function of the temperature and the resistance of the coil is utilized to adjust the delay time of the timer  202 , the delay time of the timer  202  is a function of the temperature of the coil. More specifically, during the second heating cycle, the coil  312  has a larger resistance than during the first heating cycle due to the temperature of the coil  312  being higher. As a result, the voltage on the node  314  is greater than during the first heating cycle. The increased voltage on the node  314  causes the capacitor  328  to be charged at a faster rate. As a result, the second trigger signal V t2  reaches the breakdown voltage of the SCR  334  more quickly to terminate the second heating cycle earlier than the first heating cycle was terminated. 
     If the voltage across the capacitor  328  is assumed to be approximately linear in the voltage range of interest, the change in the voltage on the node  321  has a proportional affect on the time required for the voltage across the capacitor  328  to reach a specific value. For example, assume that the voltage on the node  321  equals 2.39 volts and the duration of a heating cycle equals t 1  when the coil  312  is at room temperature. Furthermore, assume that the voltage on the node  321  equals 2.99 volts when the coil  312  has a temperature of 100° C. In this example, the duration of the heating cycle when the coil  312  equals 100° C. is approximately equal to 2.39/2.99 t 1 . In other words, the duration of the heating cycle t 2  when the coil  312  has a temperature of 100° C. is only 80% (2.39/2.99) of the duration of the heating cycle t 1  when the coil  312  is at approximately room temperature. 
     In one embodiment, the coil  312  is a copper coil. The resistance of copper exhibits a linear positive temperature coefficient, as will be understood by those skilled in the art. Moreover, the temperature coefficient for copper is extremely consistent so that the temperature coefficient does not vary greatly from one coil to another. As a result, the use of the coil  312  enables the duration of successive heating cycles to be more precisely adjusted since the temperature coefficient among coils  312  are consistent. In contrast, the thermistor  36  (FIG. 1) of the conventional timer  10  has a non-linear temperature coefficient that may vary greatly from one thermistor to another. The coil  312  may alternately be an aluminum coil. Aluminum presents a higher resistance than copper so a smaller coil may be utilized to develop the voltage applied to the timing circuit  318 . In the timer  202 , the position of the coil  312  is not critical as was the position of the thermistor  36  in the conventional timer  10  since most of the heating of the coil  312  results from current flowing through the coil  312 . In the embodiment of FIG. 4, the timing circuit  318  is formed from analog timing circuitry, but one skilled in the art will realize the timing circuit may also be formed from a microprocessor, microcontroller, or other digital device. 
     The SCR  334  may also be arranged to receive heat from one of the power resistors  304  and  306 . In one embodiment of the timer  202 , the SCR  334  is physically positioned on the toaster  200  to receive heat from the bread cavities  215 ,  217 . As understood by those skilled in the art, the magnitude of the second trigger signal V t2  required to trigger the SCR  334  decreases as the temperature of the SCR increases. Thus, the magnitude of the signal V t2  required to trigger the SCR  334  decreases as the temperature increases within the bread cavities  215 ,  217 . As a result, the variation in the breakdown voltage of the SCR  334  as a function of temperature may also be utilized to adjust the duration of successive heating cycles of the timer  202 . Furthermore, the base-to-emitter voltage of the transistor  330  typically decreases with increasing temperature as it is heating during subsequent heating cycles. As a result, the second trigger signal V t2  has a magnitude that is closer to the magnitude of the first trigger signal V t1  to further decrease the duration of subsequent heating cycles of the timer  202 . In this way, the duration of successive heating cycles of the timer  202  is adjusted in response to variations in the resistance of the coil  312  along with variations in the breakdown voltage of the SCR  334  and variations in the base-emitter voltage of the transistor  330 . 
     It is to be understood that although various embodiments of the present invention have been set forth in the foregoing description, the above disclosure is illustrative only, and changes may be made in detail while remaining within the broad principles of the invention. Accordingly, the invention is to be limited only by the appended claims.