GB2319910A

GB2319910A - Detecting faults in heating apparatus

Info

Publication number: GB2319910A
Application number: GB9725172A
Authority: GB
Inventors: Yoshio Okamura
Original assignee: Toshiba Corp
Current assignee: Toshiba Corp
Priority date: 1996-11-29
Filing date: 1997-11-27
Publication date: 1998-06-03
Anticipated expiration: 2017-11-27
Also published as: KR19980041900A; GB9725172D0; CN1146703C; GB2319910B; CN1184231A; KR100312917B1; JP3288234B2; GB9725477D0; JPH10160167A; TW349165B; MY123783A

Abstract

The door of a microwave oven is associated with at least two safety switches 24,25 connected in power supply lines 31,32 and operating in response to opening and closing of the door. A switch 29 is operated when the door is opened, and causes a microcomputer control circuit 35 to measure the resistance of a path A through the switches 24,25 to determine whether the switches are operating correctly. Visible and audible indications are provided for fault conditions such as welded switch contacts, broken wires, deterioration of primary transformer winding 34a, and poor condition of other oven components. The oven may also be disconnected from the power supply, or subsequent operation may be prevented.

Description

HEATING APPARATUS This invention relates to a heating apparatus provided with a plurality of safety switches which are connected to power supply lines so as to be operated in response to opening and closing of a door of a heating chamber.

FIG. 8 shows an electrical circuit arrangement of safety switches provided in a conventional microwave oven serving as the above-described type heating apparatus. Two safety switches 4 and 5 are connected to power supply lines 2 and 3 extending from a power supply plug 1 connected to a commercial power source respectively. A fuse 6 is connected to the power supply line 2 between the safety switch 4 and the power supply plug 1. A monitor switch 7 and a primary winding 8a of a transformer 8 are connected between the power supply lines 2 and 3 so as to be located farther away from the power source than the safety switches 4 and 5. The safety switches 4 and 5 are turned off when a door closing a heating chamber is opened whereas they are turned on when the door is closed. The monitor switch 7 is a shortcircuiting switch and is turned on when the door is opened, whereas it is turned off when the door is closed. The transformer 8 constitutes a power supply circuit for supplying electric power to a magnetron. In the abovedescribed arrangement, the monitor switch 7 is turned on in the case where the door is opened when the safety switches 4 and 5 fail to thereby remain in an ON state. Consequently, since a short circuit is formed such that an overcurrent flows, the fuse 6 melts to open a circuit. Thus, the microwave oven can be prevented from being used under the condition where the safety switches 4 and 5 are in failure.

In the above-described arrangement, however, the overcurrent flows even for a short period of time when the fuse 4 melts. This is undesirable from a view point of safety. Furthermore, upon flow of the overcurrent, a breaker of a power distribution box sometimes operates in response thereto, thereby cutting off the power.

Furthermore, no short circuit is formed when either safety switch 4 or 5 has failed. Consequently, the microwave oven is used under the condition where one of the safety switches is in failure. Additionally, a time difference is necessitated between a switching operation of each of the safety switches 4 and 5, and a switching operation of the monitor switch 7. More specifically, when the door is opened, the monitor switch 7 needs to be turned on after the safety switches 4 and 5 have been turned off. On the other hand, when the door is closed, the safety switches 4 and 5 need to be turned on after the monitor switch 7 has been turned off. This arrangement requires a dedicated lever member for providing the above-mentioned time difference.

Consequently, the construction of the microwave oven is complicated, and accordingly, the manufacturing cost thereof is increased.

FIG. 9 shows, by way of example, the above-described safety switches 4 and 5, and the monitor switch 7 provided in the main body of the microwave oven. As shown, one safety switch 4 is operated by an operating member 10 mounted on the door 9, whereas the other safety switch 5 is operated by another operating member 11 mounted on the door 9 and a connecting lever 13. The monitor switch 7 is operated by the operating member 11 and the lever 13 for providing the time difference.

Therefore, an object of the present invention is to provide a heating apparatus in which an overcurrent can be prevented from flowing when one or both of the safety switches have failed, the failure of either one of the safety switches can be detected, and a simplification of the construction and a reduction in the manufacturing cost can be achieved.

The present invention provides a heating apparatus comprising a heating chamber having an opening, a door for closing and opening the opening of the heating chamber, at least two safety switches connected to power supply lines to be capable of cutting off both poles thereof respectively, each safety switch including a plurality of contacts and performing a switching operation in response to opening and closing of the door, switch detecting means for detecting a connected state of the contacts of each safety switch, door detecting means for detecting opening and closing of the door, and control means for controlling the switch detecting means so that the switch detecting means performs a detecting operation while the door is open.

The invention also provides a heating apparatus comprising a heating chamber having an opening, a door for closing and opening the opening of the heating chamber, at least two safety switches connected to power supply lines to be capable of cutting off both poles thereof respectively, each safety switch including a common contact and a door opening contact and performing a switching operation in response to opening and closing of the door, switch detecting means, connected to the door opening contacts connected to the common contacts of the respective safety switches when the door has been opened, for detecting a conductive state between the door opening contacts, door detecting means for detecting opening and closing of the door, and control means for controlling the switch detecting means so that the switch detecting means performs d detecting operation while the door is open.

In each heating apparatus, the control means preferably controls the switch detecting means so that the switch detecting means performs the detecting operation for a predetermined period of time starting when the door is opened. When the switch detecting means detects an abnormal condition of at least one of the safety switches, the control means preferably informs of the abnormal condition, prohibits a heating operation, or informs of the abnormal condition and prohibits the heating operation.

The switch detecting means preferably detects a resistance between the door opening contacts of the safety switches and further detects a first abnormal condition in which at least one of the safety switches has failed1 when the detected resistance value is an infinity. In this constitution, the switch detecting means detects a second abnormal condition when the detected resistance value is larger than a set value. Furthermore, the switch detecting means detects a third abnormal condition when the detected resistance value is smaller than a set value.

The heating apparatus preferably further comprises a plurality of loads provided between the power supply lines and a plurality of relays for turning on and off the respective loads. In this apparatus, the control means sequentially turns on the relays and controls the switch detecting means so that the switch detecting means performs the detecting operation to detect failure in each load. The heating apparatus further comprises an operation panel including at least one operation key. In this constitution, when the operation key is operated in a mode differing from a usual mode thereof while the door is open, the control means sequentially turns on the relays and controls the switch detecting means so that the switch detecting means performs the detecting operation to detect failure in each load.

The invention will be described, merely by way of example, with reference to the accompanying drawings, in which: FIG. 1 is an electrical circuit diagram for explaining the principle of a detecting operation performed in a microwave oven of one embodiment in accordance with the present invention; FIG. 2 is a schematic longitudinal side section of the microwave oven; FIG. 3 is an electrical circuit diagram showing part of a control circuit and safety switches; FIG. 4 is a view similar to FIG. 1, showing the case where one of the safety switches has failed; FIG. 5 is a flowchart showing a routine for detecting failure of the safety switches; FIG. 6 is an electrical circuit diagram showing a microwave oven with a heater; FIG. 7 is a flowchart showing a routine for detecting failure of the safety switches; FIG. 8 is a view similar to FIG. 1, showing a prior art; and FIG. 9 is a view similar to FIG. 2, showing the prior art.

One embodiment of the present invention will be described with reference to FIGS. 1 to 7. The invention is applied to a microwave oven in the embodiment. First, the principle of detection of failure in safety switches will be described with reference to FIGS. 1 to 5. Ref erring to FIG. 2, a part of the interior of an equipment chamber of the microwave oven and safety switches provided in the part are schematically shown. A main body 21 of the microwave oven defines therein a heating chamber (not shown) having a front opening. A door 22 is provided for closing and opening the opening of the heating chamber. The main body 21 further defines therein the equipment chamber 23 adjacent to the heating chamber. Two safety switches 24 and 25 are provided in the equipment chamber 23. For example, each of the safety switches 24 and 25 comprises a microswitch. The upper safety switch 24 is actuated by an operating member 26 mounted on an upper right-hand end of the door 22 to thereby perform a switching operation. The lower safety switch 25 is actuated via a connecting lever 28 by an operating member 27 mounted on a lower right-hand end of the door 22 to thereby perform a switching operation.

Each of the safety switches 24 and 25 includes two contacts a and b and a common contact c, as shown in FIG. 1.

Each safety switch is closed between the contacts c and a when the door 22 is closed. Each safety switch is closed between the contacts b and c when the door is opened. The contact b of each safety switch constitutes a door opening state contact in the invention. A door switch 29 is provided in the vicinity of the lower safety switch 25 to be disposed side by side, as shown in FIG. 1. The door switch 29 also comprises a microswitch. The door switch 29 is actuated via the connecting lever 28 by the operating member 27 to thereby perform a switching operation. Namely, the door switch 29 is operated in synchronism with the lower safety switch 25. The door switch 29 is turned on when the door 22 is closed, whereas it is turned off when the door 22 is opened. Consequently, opening and closing of the door 22 can be detected on the basis of turn-on and turn-off of the door switch 29. Thus, the door switch 29 constitutes door detecting means in the invention. The door switch 29 may be turned off when the door 22 is closed, whereas it may be turned on when the door 22 is opened.

Referring to FIG. 1, power supply lines 31 and 32 extend from a power supply plug 30 which is to be connected to a commercial power supply. A fuse 33 and the safety switch 24 are connected to the power supply line 31. The safety switch 25 is connected to the other power supply line 32. A primary winding 34a of a transformer 34 is connected between the power supply lines 31 and 32. More specifically, the contact a of the safety switch 24 is connected to one end of the fuse 33. The common contact c of the safety switch 24 is connected to one end of the primary winding 34a. The contact a of the other safety switch 25 is connected to the power supply plug 30. The common contact c of the safety switch 25 i8 connected to the other end of the primary winding 34a. Consequently, the safety switches 24 and 25 are capable of cutting off both poles of the respective power supply lines 31 and 32. The contact b of the safety switch 24 is connected to a first terminal 36 of a control circuit 35. The contact b of the other safety switch 25 is connected to a second terminal 37 of the control circuit 35. The door switch 29 has two contacts c and a connected to third and fourth terminals 38 and 39 of the control circuit 35 respectively.

The transformer 34 constitutes a power supply circuit for supplying a driving power supply to a magnetron (not shown). To a secondary winding 34b of the transformer 34 are connected a main relay and various electrical components composing the power supply circuit for the drive of magnetron. The control circuit 35 has functions of switch detecting means and control means in the invention. The control circuit 35 is composed of a printed circuit board 40, a microcomputer 41 and various electrical components, as shown in FIG. 3. The microcomputer 41 has a function of controlling an overall operation of the microwave oven. For this purpose, the microcomputer 41 is incorporated with control programs. A constant voltage (DC 5V, for example) delivered from a DC power supply circuit 42 is supplied to the control circuit 35, as shown in FIG. 1. The DC power supply circuit 42 is connected to a section of the power supply line 31 between the fuse 33 and the contact a of the safety switch 24 and to a section of the power supply line 32 between the power supply plug 30 and the contact a of the safety switch 25.

Referring to FIG. 3, an arrangement for detecting an abnormal condition or failure of the safety switches 24 and 25 by the microcomputer 41 will be described. The contact b of the safety switch 24 is connected via a resistance 43 to an input terminal 41a of the microcomputer 41. The contact b of the safety switch 24 is further connected via a noise reducing capacitor 45 to a grounding conductor provided on the printed circuit board 40 to thereby be grounded. The contact b of the other safety switch 25 is connected to another grounding conductor provided on the printed circuit board 40 to be grounded. The microcomputer 41 delivers a constant DC voltage (DC 5V, for example) from an output terminal 41b thereof and converts an analog voltage signal supplied to the input terminal 41a thereof to a corresponding digital voltage signal. The microcomputer 41 further processes the digital voltage signal, for example, to determine a voltage value.

Each of the safety switches 24 and 25 is closed between the contacts c and b when the door 22 of the microwave oven is open. Accordingly, a current path A is formed through the output terminal 41b of the microcomputer 41, a resistance 44, the contacts c and b of the safety switch 24, the primary winding 34a of the transformer 34, and the contacts c and b of the safety switch 25 to be grounded.

When the current path A is formed, the DC voltage delivered from the output terminal 41b of the microcomputer 41 is divided by the resistance 44 and a resistance of the current path A. A divided voltage signal is supplied to the input terminal 41a of the microcomputer 41. Consequently, the microcomputer 41 is capable of detecting the magnitude of the resistance of the current path A on the basis of the voltage signal supplied to the input terminal 41a thereof.

The case where either safety switch has failed due to, for example, contact welding will be described. As shown in FIG. 4, when the safety switch 24 has failed and the door 22 of the microwave oven is opened with the contacts c and a being kept closed, the current path is cut off between the resistance 44 connected to the output terminal 41b of the microcomputer 41 and the ground. Under this condition, the DC voltage delivered from the output terminal 41b of the microcomputer 41 is divided by the resistance 44 and an infinity resistance. Accordingly, when supplied with the divided voltage signal, the microcomputer 41 determines that the resistance of the current path A is infinity and accordingly, that at least one of the safety switches 24 and 25 has failed or a wire composing the current path A has been broken.

The microcomputer 41 is further capable of detecting an ON-state and OFF-state of the door switch 29. For example, one end of the door switch 29 is grounded and the constant DC voltage is supplied via a resistance to the other end of the door switch 29. A node of the other end of the door switch 29 and the resistance is connected to an input terminal of the microcomputer 41. Based on the level of the voltage signal supplied to the input terminal, the microcomputer 41 determines in which the door switch 29 is, the ON-state or OFF-state. When the door switch 29 is in the ON-state, the microcomputer 41 determines that the door 22 of the microwave oven is closed. When the door switch 29 is in the OFF-state, the microcomputer 41 determines that the door 22 is opened.

The operation of the microwave oven or particularly, a control manner for determining failure of the safety switches 24 and 25 etc. will be described with reference to FIG. 5. FIG. 5 shows a part of the control program stored in the microcomputer 41 and more specifically, control contents for determining failure of the safety switches 24 and 25 etc. First, the microcomputer 41 detects the state of the door switch 29 to thereby determine whether the door 22 has been opened, at step S1. When the door 22 is opened, the microcomputer 41 determines in the affirmative (YES at step S1), thereby executing a detecting operation for detecting failure of the safety switches 24 and 25 etc. for a predetermined period of time (for several seconds, for example). More specifically, the microcomputer 41 advances to step S2 to deliver the constant DC voltage from the output terminal 41b thereof and to detect the resistance of the current path A formed through the contacts c and b, the primary winding 34a of the transformer 34, and the contacts c and b of the safety switch 25 on the basis of the voltage signal supplied to the input terminal 41a thereof. The microcomputer 41 further determines at step S3 whether the detected resistance value R is an infinity. When the detected resistance value R is the infinity, the microcomputer 41 determines that at least one of the safety switches 24 and 25 has failed or a wire composing the current path A has been broken (YES at step S3). The microcomputer 41 then informs of a first abnormal condition at step S4. In informing of the first abnormal condition, an indication, "EEEE" is displayed on a display mounted in an operation panel or more specifically, a 7-segment type display for clock display, and a buzzer (not shown) is actuated. The information of first abnormal condition indicates that at least one of the safety switches has failed. The microcomputer 41 then advances to step S5 to stop the subsequent overall operation of the microwave oven, namely, to prohibit the heating operation.

On the other hand, when the detected resistance value R is not the infinity, the microcomputer 41 determines in the negative at step S3 (NO at step S3), advancing to step S6 to determine whether the resistance value R is larger than a predetermined upper limit resistance value Rmax. The resistance value of the current path A substantially depends upon a resistance value of the primary winding 34a of the transformer 34 when the two safety switches 24 and 25 are normal. Thus, the resistance value of the current path A is within a predetermined range although it varies from product to product. With progress of wear of electrical components such as the primary winding 34a of the transformer 34, the resistance value R of the current path A exceeds the upper limit resistance value Rmax On the other hand, with progress of deterioration.of insulation in the electrical components such as the primary winding 34a of the transformer 34, the resistance value R of the current path A reduces below a lower limit resistance value Rmin Accordingly, when the primary winding 34a of the transformer 34 is worn, the microcomputer 41 determines in the affirmative at step S6, advancing to step S7 to inform of a second abnormal condition. In informing of the second abnormal condition, an indication, "sO1" is displayed on the display of the operation panel, and the buzzer is actuated.

The information of the second abnormal condition indicates that the primary winding 34a of the transformer 34 is worn.

Since the second abnormal condition does not interfere with the heating operation, the microcomputer 41 completes a control routine for determining failure of the safety switches 24 and 25 etc. (checking operation), returning to a main routine to be on standby for operation of various keys of the operation panel.

When the resistance value R is equal to or smaller than the upper limit resistance value RmaXt the microcomputer 41 determines in the negative at step S6, advancing to step S8 to determine whether the resistance value R is smaller than the lower limit resistance value Ruin. The resistance value R becomes smaller than the lower limit resistance value Rmin with progress in the deterioration of insulation in the primary winding 34a. In this case, the microcomputer 41 determines in the affirmative at step S8, advancing to step S9 to inform of a third abnormal condition. In informing of the third abnormal condition, an indication, "E02" is displayed on the display of the operation panel, and the buzzer is actuated. The information of third abnormal condition indicates that the insulation of the primary winding 34a of the transformer 34 is deteriorated. Since the third abnormal condition does not interfere with the heating operation as in the second abnormal condition, the microcomputer 41 completes a control routine for determining failure of the safety switches 24 and 25 etc. (checking operation), returning to a main routine to be on standby for operation of various keys of the operation panel. Although the heating operation is allowed in the cases of the second and third abnormal conditions as described above, the subsequent overall operation of the microwave oven may be prohibited, instead.

According to the above-described microwave oven, the detecting operation is executed upon opening of the door 22 in the case where at least one of the two safety switches 24 and 25 has failed. In the detecting operation, the conductive state between the contacts b of the respective safety switches 24 and 25 or more specifically, the resistance value of the current path A is detected. Based on the results of detection1 the microcomputer 41 determines that at least one of the switches 24 and 25 has failed (the first abnormal condition), when the detected resistance value is the infinity. Since this arrangement eliminates the monitor switch 7 (see FIGS. 8 and 9) required in the prior art, an overcurrent is prevented even when one or both of the safety switches 24 and 25 fail. This can improve the safety. Furthermore, since the monitor switch 7 and the lever 13 dedicated thereto in the prior art are not required, the number of parts can be reduced and accordingly, the construction can be simplified.

Consequently, the manufacturing cost of the microwave oven can be reduced.

In the above-described microwave oven, the door switch 29 is provided as the door detecting means for detecting opening and closing of the door 22, for example. The door switch 29 may be operated in synchronism with the safety switch 25. In this case, the lever 13 dedicated to the monitor switch can completely be eliminated. Furthermore, since the door switch is usually independent of from the monitor switch in the conventional microwave ovens, the door switch does not increase the manufacturing cost of the microwave oven. Additionally, the door switch is provided in the prior art in order that a microcomputer may recognize opening and closing of the door.

Furthermore, in the above-described constitution, the microcomputer 41 executes the detecting operation to detect the conductive state between the contacts b of the safety switches 24 and 25 or the resistance value of the current path A for the predetermined period of time (several seconds, for example) starting at the time the door is opened. Thus, the detecting operation is reliably completed in several seconds when the user opens the door. That is, when the user opens the door, the failure of one or both of the safety switches 24 and 25 is automatically detected in several seconds and informed to the user. Consequently, the user can quickly find the failure of the safety switches 24 and 25.

When determining that at least one of the safety switches 24 and 25 has failed (the first abnormal condition), the microcomputer 41 prohibits the subsequent overall operation of the microwave oven. Consequently, the heating operation can reliably be prevented from being executed with one of the safety switches remaining in failure. In this case, since the user cannot operate the microwave oven, he or she is obliged to have the microwave oven repaired. Consequently, the safety can be improved.

Furthermore, when the detected resistance value R is larger than the upper limit resistance value Rmaxi # the microcomputer 41 determines that the microwave oven is in the second abnormal condition, for example, that the primary winding 34a of the transformer 34 is worn, and informs of the second abnormal condition. Consequently, the user can reliably recognize that the microwave oven is in the second abnormal condition. Furthermore, when the detected resistance value R is smaller than the lower limit resistance value Ruin, the microcomputer 41 determines that the microwave oven is in the third abnormal condition, for example, that the insulation of the primary winding 34a is deteriorated, and informs 9f the third abnormal condition.

Consequently, the user cfln reliably recognize that the microwave oven is in the third abnormal condition.

The above-described coirstitution of the microwave oven is directed to the explanation of the principle of detecting an abnormal condition of the safety switches. In the actual microwave oven, various loas and relays for turning on and off the respective loads ate added to the above-described constitution. A more specific constitution of a microwave oven with a heater will now be described with reference to FIGS. 6 and 7. In FIG. 6,;the identical parts are labeled by the same reference symbols as in FIG. 1.

Referring to FIG. 6, a'main relay 46 is connected to a portion of the power supply line 32 nearer to the transformer 34 than the safety switch 25. A parallel circuit of a chamber lamp 47 and an electric motor 48 for driving a turntable (not shown is connected between a portion of the power supply line 31 nearer to the transformer 34 than the safety switch 24 and a portion of the power supply line 32 nearer to the transformer 34 than the main relay 46. A serial circuit of a grill heater 49 and a grill relay 50 for energizing and deenergizing the heater 49 is connected in parallel with the chamber lamp 47 and the motor 48 between the power supply lines 31 and 32.

A parallel circuit of an oven heater 51 and an electric motor 52 of a fan (not shown) for producing hot air and a serial circuit of the heater 51 and an oven relay 53 for energizing and deenergizing the motor 52 are connected between the power supply lines 31 and 32. A serial circuit of an electric motor 54 of a blowing fan (not shown) for supplying air to the heating chamber and a fan motor relay 55 for energizing and deenergizing the motor 54 is connected between the power supply lines 31 and 32.

A range relay 56 is connected to a portion of the power supply line 31 which is further connected to one end of the primary winding 34a of the transformer 34. The control circuit 35 or the microcomputer 41 thereof on/off-controls the main relay 46, the grill relay 50, the oven relay 53, the fan motor relay 55 and the range relay 56. A motor relay for energizing and deenergizing the motor 48 may be connected in series to the motor 48, and the control circuit 35 may on/off-control the motor relay. Consequently, the motor 48 can be controlled to be energized and deenergized independent of the chamber lamp 47.

The control circuit 35 controls a display 57 provided in the operation panel and receives switch signals from various operation keys 58 on the operation panel. The display 57 comprises a display section for displaying the current time and a cooking time, and a display section for displaying a cooking menu. The keys 58 include a menu selecting key, a time setting key, a start key, a cancel key, etc.

The operation of the microwave oven or particularly, a control manner for determining failure of the safety switches 24 and 25 etc. or the detection executed upon opening of the door will be described with reference to FIG. 7. FIG. 7 shows a part of the control program stored in the microcomputer 41 of the control circuit 35 and more specifically, control contents for determining failure of the safety switches 24 and 25 etc.

First, the microcomputer 41 detects the state of the door switch 29 to thereby determine whether the door 22 has been opened, at step Slow. When the door 22 is opened, the microcomputer 41 determines in the affirmative (YES at step S107), thereby executing a detecting operation (steps 5102 to S113) for detecting failure of the safety switches 24 and 25 etc. for a predetermined period of time (for one or two seconds, for example). More specifically, the mi

More specifically, a resistance between the contacts b of the respective safety switches 24 and 25 is detected. When the detected resistance is the infinity, the microcomputer 41 determines that at least one of the safety switches 24 and 25 has failed. The microcomputer 41 stores the results of determination.

The microcomputer 41 then advances to step S104 to check whether the chamber lamp 47 and the motor 48 are worn or deteriorated in the insulation on the basis of the detected resistance value. This checking process is carried out in the same manner as the above-described steps S6 and S8 in FIG. 5. That is, when the detected resistance value R is larger than an upper limit resistance value Rmax11 the microcomputer 41 determines that the microwave oven is in the second abnormal condition or that the electrical components or the chamber lamp 47 and motor 48 are worn.

When the resistance value R is smaller than a lower limit resistance value R inlt the microcomputer 41 determines that the microwave oven is in the third abnormal condition or that the electrical components are deteriorated in the insulation. The microcomputer 41 stores the results of determination. Subsequently, the microcomputer 41 turns off the main relay 46 and then advances to step S105 to turn on the main relay 46 and the grill relay 50. The microcomputer 41 further advances to step S106 to check whether the grill heater 49 is worn or deteriorated in the insulation. This checking process is also carried out in the same manner as the above-described steps S6 and S8. More specifically, the microcomputer 41 detects the resistance between the contacts b of the respective safety switches 24 and 25. When the detected resistance value R is larger than an upper limit resistance value Rmax2I the microcomputer 41 determines that the microwave oven is in the second abnormal condition or that the electrical component or the heater 49 is worn.

When the resistance value R is smaller than a lower limit resistance value Rmin2' the microcomputer 41 determines that the microwave oven is in the third abnormal condition or that the heater 49 is deteriorated in the insulation. The microcomputer 41 stores the results of determination.

When turning off the main relay 46 and the grill relay 50, the microcomputer 41 advances to step S107 to turn on the main relay 46 and the oven relay 53. The microcomputer 41 further advances to step S108 to check whether the oven heater 51 and the motor 52 are worn or deteriorated in the insulation. This checking process is also carried out in the same manner as the above-described steps S6 and 58.

More specifically, the microcomputer 41 detects the resistance between the contacts b of tbe respective safety switches 24 and 25. When the detected resistance value R is larger than an upper limit resistance value RzaX3, the microcomputer 41 determines that the microwave oven is in the second abnormal condition or that the electrical components or the heater 51 and the motor 52 are worn. When the resistance value R is smaller than a lower limit resistance value Ruin31 the microcomputer 41 determines that the microwave oven is in the third abnormal condition or that the electrical components or the heater 51 and the motor 52 are deteriorated in the insulation. The microcomputer 41 stores the results of determination.

When turning off the main relay 46 and the oven relay 53, the microcomputer 41 advances to step S109 to turn on the main relay 46 and the range relay 56. The microcomputer 41 further advances to step S110 to check whether the primary winding 34a of the transformer 34 is worn or deteriorated in the insulation. This checking process is also carried out in the same manner as the above-described steps S6 and 58. More specifically, the microcomputer 41 detects the resistance between the contacts b of the respective safety switches 24 and 25. When the detected resistance value R is larger than the upper limit resistance value RaX, the microcomputer 41 determines that the microwave oven is in the second abnormal condition or that the electrical component or the transformer 34 is worn.

When the resistance value R is smaller than the lower limit resistance value Rmin, the microcomputer 41 determines that the microwave oven is in the third abnormal condition or that the transformer 34 is deteriorated in the insulation.

The microcomputer 41 stores the results of determination.

When turning off the main relay 46 and the range relay 56 subsequently, the microcomputer 41 advances to step S111 to turn on the main relay 46 and the fan motor relay 55.

The microcomputer 41 further advances to step S112 to check whether the fan motor 54 is worn or deteriorated in the insulation. This checking process is also carried out in the same manner as the above-described steps S6 and S8.

More specifically, the microcomputer 41 detects the resistance between the contacts b of the respective safety switches 24 and 25. When the detected resistance value R is larger than an upper limit resistance value Rmax4, the microcomputer 41 determines that the microwave oven is in the second abnormal condition or that the electrical component or the fan motor 54 is worn. When the resistance value R is smaller than a lower limit resistance value in4' the microcomputer 41 determines that the microwave oven is in the third abnormal condition or that the fan motor 54 is deteriorated in the insulation. The microcomputer 41 stores the results of determination.

Successively, the microcomputer 41 turns off the main relay 46 and the fan motor relay 55 and then advances to step S113. When the stored results of determination include one or more types of abnormal condition, these types of abnormal condition are displayed on the display 57. When at least one of the safety switches 24 and 25 has failed, the microcomputer 41 prohibits overall subsequent operation of the microwave oven or the heating operation. When the abnormal condition other than the failure of one or both of the safety switches 24 and 25 is detected, the heating operation can normally be executed. Accordingly, the microcomputer 41 completes the control routine for determining failure of the safety switches 24 and 25 etc.

(checking operation), returning to the main routine to be on standby. Alternatively, the overall subsequent operation of the microwave oven may be prohibited when the abnormal condition other than the failure of one or both of the safety switches 24 and 25 is detected.

According to the foregoing embodiment, the same effect can be achieved as that obtained from the constitution for description of the principle of detecting operation.

Particularly in the embodiment, the plurality of relays are sequentially turned on so that the abnormal condition or failure of the plurality of electrical components serving as the plurality of loads can readily be determined. In the embodiment, furthermore, the checking of the safety switches 24 and 25 and the plurality of electrical components with respect to failure is completed in one or two seconds. it takes about three seconds for the magnetron to start oscillating upon energization. Accordingly, even if both safety switches 24 and 25 fail and are closed between the contacts c and a, the checking can reliably be completed before oscillation of the magnetron. Consequently, the safety in the operation of the microwave oven can sufficiently be ensured.

In the embodiment, the failure of the safety switches 24 and 25, and the failure of the electrical components are detected when the door 22 is opened. Rowever, only the failure of the safety switches 24 and 25 may be detected when the door 22 is opened. The failure of the electrical components may be detected when one of the operation keys is operated in a mode differing from a usual mode thereof while the door is open. In this constitution, since almost no driving sound of the relay is produced when the door 22 is opened, the driving sound gives little offense to the ear.

Purthermore, the above-described manner of starting detection of the failure of the electrical components by the specific operation of the operation key can be utilized as a method of diagnosis when service personnel repairs the microwave oven.

The failure of the safety switches 24 and 25 is detected and successively, the failures of the electrical components are detected in the foregoing embodiment.

However, the detecting operation may be stopped without detection of failure of the electrical components when the failure of the safety switches 24 and 25 has been detected.

Furthermore, when the failure of the safety switches 24 and 25 has been detected, the microcomputer 41 informs of the detected failure or displays the indication on the display 7 and actuates the buzzer, and prohibits the heating operation. However, the microcomputer 41 may only inform of the detected failure. In this case, since the heating operation can be executed by the microwave oven, the usability thereof can be improved. On the contrary, the heating operation may be prohibited without the informing operation upon detection of the failure of the safety switches 24 and 25. In this case, since the heating operation cannot be executed, the failure of the microwave oven can clearly be recognized, so that the user promptly has the microwave oven repaired.

In the embodiment, the failure of the safety switches 24 and 25 etc. is detected when the door 22 is opened.

However, the failure detecting operation may be carried out on condition that a predetermined one of the operation keys of the operation panel is operated while the door 22 is open. Furthermore, although the two safety switches 24 and 25 are provided on the power supply lines 31 and 32 in the embodiment, three or more safety switches may be provided.

In this case, two or more safety switches are preferably provided on one power supply line.

In the foregoing embodiment, the microcomputer 41 detects the conductive state between the contacts b of the two safety switches 24 and 25, namely, the resistance of the current path A for the purpose of detecting failure of the safety switches. The connected state of the contacts of each safety switch may be detected by the microcomputer 41, instead. More specifically, the safety switches 24 and 25 are grounded at one ends thereof, and the constant DC voltage is supplied via resistances to the other ends of the safety switches. Nodes of the other ends of the safety switches and the resistances are connected to two input terminals of the microcomputer 41 respectively. In this constitution, the microcomputer 41 determines whether the safety switches 24 and 25 are turned on or off, based on the levels of the voltage signals supplied to the respective input terminals thereof, thereby detecting the connected state of the contacts. When the door 22 is opened or when the microcomputer 41 detects opening of the door 22 on the basis of a switch signal from the door switch 29, the microcomputer 41 detects the connected state of the contacts of the respective safety switches 24 and 25 in the abovedescribed manner, thereby determining whether the safety switches have failed. Also, when three or more safety switches are provided, the connected state of contacts of the safety switches 24 and 25 may be detected by the microcomputer in substantially the same manner as described above.

The foregoing description and drawings are merely illustrative of the principles of the present invention and are not to be construed in a limiting sense. Various changes and modifications will become apparent to those of ordinary skill in the art. All such changes and modifications are seen to fall within the scope of the invention as defined by the appended claims.

Claims

WE CLAIM:

1. A heating apparatus comprising: a heating chamber having an opening; a door for closing and opening the opening of the heating chamber; at least two safety switches connected to power supply lines to be capable of cutting off both poles thereof respectively, each safety switch including a plurality of contacts and performing a switching operation in response to opening and closing of the door; switch detecting means for detecting a connected state of the contacts of each safety switch; door detecting means for detecting opening and closing of the door; and control means for controlling the switch detecting means so that the switch detecting means performs a detecting operation while the door is open.

2. A heating apparatus comprising: a heating chamber having an opening; a door for closing and opening the opening of the heating chamber; at least two safety switches connected to power supply lines to be capable of cutting off both poles thereof respectively, each safety switch including a common contact and a door opening contact and performing a switching operation in response to opening and closing of the door; switch detecting means, connected to the door opening contacts connected to the common contacts of the respective safety switches when the door has been opened, for detecting a conductive state between the door opening contacts; door detecting means for detecting opening and closing of the door; and control means for controlling the switch detecting means so that the switch detecting means performs a detecting operation while the door is open.

3. A heating apparatus according to claim 1 or 2, wherein the control means controls the switch detecting means so that the switch detecting means performs the detecting operation for a predetermined period of time starting when the door is opened.

4. A heating apparatus according to claim 1 or 2, wherein, when the switch detecting means detects an abnormal condition of at least one of the safety switches, the control means informs of the abnormal condition.

5. A heating apparatus according to claim 1 or 2, wherein, when the switch detecting means detects an abnormal condition of at least one of the safety switches, the control means prohibits a heating operation.

6. A heating apparatus according to claim t or 2, wherein, when the switch detecting means detects an abnormal condition of at least one of the safety switches, the control means informs of the abnormal condition and prohibits a heating operation.

7. A heating apparatus according to claim 2, wherein the switch detecting means detects a resistance between the door opening contacts of the safety switches and further detects a first abnormal condition in which at least one of the safety switches has failed, when the detected resistance value is an infinity.

8. A heating apparatus according to claim 7, wherein the switch detecting means detects a second abnormal condition when the detected resistance value is larger than a set value.

9. A heating apparatus according to claim 7, wherein the switch detecting means detects a third abnormal condition when the detected resistance value is smaller than a set value.

10. A heating apparatus according to any one of claims 2 to 9, further comprising a plurality of loads provided between the power supply lines and a plurality of relays for turning on and off the respective loads, and wherein the control means sequentially turns on the relays and controls the switch detecting means so that the switch detecting means performs the detecting operation to detect failure in each load.

11. A heating apparatus according to claim 10, further comprising an operation panel including at least one operation key, and wherein, when the operation key is operated in a mode differing from a usual mode thereof while the door is open, the control means sequentially turns on the relays and controls the switch detecting means so that the switch detecting means performs the detecting operation to detect failure in each load.

12. A heating apparatus substantially as hereinbefore described with reference to Figures 1 to 7 of the accompanying drawings.