Abstrict
A microwave oven includes an electric heater disposed in the cooking
chamber for cooking by radiant heat. A temperature sensor senses
a temperature of the heater, and a door sensor senses an open state
of the oven door. If the oven door is open, and the heater temperature
is at or above a reference temperature (i.e., a temperature which
could burn a user), an alarm, such as an audio and/or visual alarm,
is activated to warn the user.
Claims
What is claimed is:
1. A microwave oven comprising:
a main body forming a cooking chamber having a door;
a microwave generator for supplying microwaves to the cooking chamber;
an electric heater located at an upper portion of the cooking chamber
for
cooking by direct heat;
a first temperature sensor for sensing an overall internal temperature
of the cooking chamber;
a second temperature sensor disposed adjacent the heater for sensing
a temperature of the heater;
a door sensor for sensing an open state of the door; and
an alarm for emitting a warning signal warning that the heater
is hot, said alarm operably connected to the second temperature
sensor and the door sensor for emitting the warning signal only
in response to the simultaneous occurrence of: the second temperature
of the heater being at or above a reference temperature, and the
door being open.
2. The microwave oven according to claim 1 wherein the alarm emits
an audio signal.
3. The microwave oven according to claim 1 wherein the second temperature
sensor comprises a bimetallic switch including a laminate of metals
having different thermal expansion coefficients, the bimetallic
switch closing in response to the heater temperature being at or
above the reference temperature.
4. The microwave oven according to claim 3 wherein the door sensor
is a switch which closes when the door is open, the door sensor
switch and bimetallic switch being connected in series with the
alarm.
5. The microwave oven according to claim 1 wherein the second temperature
sensor comprises a thermistor having a resistance varying according
to changes in temperature; the thermistor disposed in a circuit
to which a voltage is applied, for converting the voltage in accordance
with the thermistor resistance; a control unit connected to the
circuit for detecting the converted voltage and comparing the converted
voltage with a reference voltage; the control unit also connected
to the door sensor and the alarm to activate the alarm in response
to the door being open and the comparison between the converted
voltage and reference voltage indicating that the heater temperature
is at or above the reference temperature.
Description BACKGROUND OF THE INVENTION
1) Field of the Invention
The present invention generally relates to a microwave oven having
an electric heater for cooking by radiant heat.
2) Description of the Prior Art
A microwave oven is a cooking appliance which cooks food with high
speed frequency energy of 2,450 MHz. This energizes food molecules
to move at high speeds to produce frictional heat. Such a microwave
oven also has a heater in its cooking chamber to provide a grilling
function for cooking food using direct heat.
FIG. 1 is a front view of a conventional microwave oven. As shown
in FIG. 1, the conventional microwave oven includes a cooking chamber
10 in which food is cooked by high frequency energy; a door 20 for
opening and closing the cooking chamber 10; a rotary motor (not
illustrated); a rotary stand 11 connected with the rotary motor;
and a tray 12 adapted to be positioned on the rotary stand 11, and
on which the food is placed.
The conventional microwave oven also includes a heater 13 for performing
the grill function; a thermistor 14 which senses the temperature
of the cooking chamber 10; a control panel having a display 30 showing
the operation information of the microwave oven and a function selection
part 40; a magnetron antenna (not illustrated) mounted on the sidewall
of the cooking chamber's upper portion for emitting high frequency
energy to the cooking chamber 10; and a magnetron (not illustrated)
installed in a space at the rear of the display 30 for applying
the high frequency energy to the magnetron antenna.
The heater 13 is positioned on the rear upper portion of the cooking
chamber 10 and is able to be rotated by a predetermined angle. The
heater 13 is held in place by a supporting member each provided
on a side wall of the cooking chamber 10. Thus, when a user cooks
with the heater 13, a lamp of the cooking chamber 10 (not illustrated)
is switched on to illuminate the cooking chamber 10, and the tray
11 then rotates. Heat is produced when electricity is applied to
the heater 13. When the programmed cooking time is complete, the
electricity applied to the heater 13 is shut off, and the heater
13 stops producing heat. When the cooking operation is complete,
the microwave signals the user.
The heater 13 remains hot for a predetermined period of time even
after the cooking time has finished, since its temperature diminishes
slowly. Since the conventional microwave oven cannot measure the
temperature of the heater 13, the user may not know that the temperature
of the heater 13 is higher than the microwave oven's internal temperature.
Thus, when the user tries to cook food with the conventional microwave
oven before the heater 13 cools, he may touch the hot heater 13
as he either puts the food being cooked in the cooking chamber 10
or takes it out of the cooking chamber 10. If his or her hand touches
the heater 13, he or she may get burnt, and instantaneously drop
the food that he or she is holding.
SUMMARY OF THE INVENTION
The present invention involves a safety system for a microwave
oven that can obviate disadvantages of the conventional technique.
It is an objective of the present invention to provide a safety
system for a microwave oven with a heater which warns a user if
the surface temperature of the oven's heater is over a predetermined
point to prevent the chance of a burn injury to the user.
In order to obtain the above-mentioned objective of the present
invention, there is provided a microwave oven which comprises a
main body forming a cooking chamber having a door. A microwave generator
supplies microwaves to the cooking chamber. An electric heater is
located at an upper portion of the cooking chamber for cooking by
direct heat. A first temperature sensor is provided for sensing
an overall internal temperature of the cooking chamber. A second
temperature sensor is disposed adjacent to the heater for sensing
a temperature of the heater. A door sensor is provided for sensing
an open state of the door. An alarm is operably connected to the
second temperature sensor and the door sensor for emitting a warning
signal in response to the sensed temperature being at or above a
reference temperature, and the door being open.
Preferably, the alarm emits an audio signal.
The second temperature sensor preferably comprises a bimetallic
switch including a laminate of metals having different thermal expansion
coefficients. The bimetallic switch closing in response to the heater
temperature being at or above the reference temperature. Preferably
the door sensor is a switch which closes when the door is open,
and which is connected in series with the alarm and the bimetallic
switch.
The second temperature sensor may comprise a thermistor having
a resistance varying according to changes in temperature. The thermistor
is disposed in a circuit to which a voltage is applied, for converting
the voltage in accordance with the thermistor resistance. A control
unit is connected to the circuit for detecting the converted voltage
and comparing the converted voltage with a reference voltage. The
control unit also is connected to both the door sensor and the alarm
to activate the alarm in response to: (a) the door being open and
(b) the comparison between the converted voltage and reference voltage
indicating that the heater temperature being at or above the reference
temperature.
BRIEF DESCRIPTION OF THE DRAWINGS
The objects and advantages of the invention will become apparent
from the following detailed description of preferred embodiments
thereof in connection with the accompanying drawings in which like
numerals designate like elements, and in which:
FIG. 1 is a front view of a microwave oven in accordance with a
conventional art;
FIG. 2 is a front view of a microwave oven in accordance with the
present invention;
FIG. 3 is a circuit diagram of a driver circuit for a safety system
of the microwave oven in accordance with a first preferred embodiment
of the present invention;
FIG. 4 depicts the control sequence of the driver circuit of FIG.
3; and
FIG. 5 is a circuit diagram of the microwave oven in accordance
with a second preferred embodiment of the present invention.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION
Preferred embodiments of the present invention will be described
in detail with reference to FIGS. 2-5. The same items as in FIG.
1 are given the same reference numerals and their description will
not be repeated.
FIG. 2 is a front view of a microwave oven having a heater temperature
sensor for measuring a surface temperature of a heater in accordance
with the present invention. FIG. 3 is a circuit diagram of a driver
circuit for a safety system of the microwave oven in accordance
with a first preferred embodiment of the present invention. FIG.
4 depicts the control sequence of the driver circuit of FIG. 3.
Referring first to FIG. 2, a microwave oven of the present invention
includes a heater temperature sensor 15 for checking the temperature
of a heater 13, and this heater temperature sensor 15 is disposed
at a predetermined point of the cooking chamber's ceiling, so as
to be close to the heater 13.
A function selection part 40 and display 30 are provided in the
front panel of the microwave oven to allow a user to manipulate
and confirm with ease, and the function selection part 40 has a
cooking selection switch, a grill mode selection switch, and a power
switch.
The display 30 shows a user information programmed through the
function selection part 40, and includes a visual alarm 50-1 which
gives a user a warning if the temperature of the heater 13 is high
when he or she opens a door 20 of the microwave oven. An audio alarm
such as a buzzer is provided to the outside of the display 30.
FIG. 3 is a circuit diagram of the inventive safety system, including
the heater temperature sensor 15 for sensing the temperature of
the heater 13, and visual and audio alarms 50-1 and 50-2.
As shown in FIG. 3, the inventive safety system includes a control
unit 90 to which the heater temperature sensor 15 is coupled in
series with a resistor R therebetween, and an alarm device 50 that
is operated by an output signal of the control unit 90. The control
unit 90 controls the alarm device 50 according to a value of voltage
converted by the heater temperature sensor 15 after sensing the
temperature of the heater 13. That is, the resistor 15 is selected
so that when a temperature greater than a reference temperature
is sensed, the alarm sounds. The reference temperature is selected
to be a value which will cause a user to be burned upon touching
the heater. Thus, for example a reference temperature of 50.degree.
C. can be selected.
As shown in FIG. 3, the alarm device 50 consists of the visual
alarm 50-1 and the audio alarm 50-2.
The heater temperature sensor 15 is preferably a thermistor, i.e.,
a temperature-sensitive resistor whose resistance increases as the
temperature increases.
Thus, the value of resistor R and the material of the thermistor
15 are chosen so that if the temperature of the heater 13 is higher
than the reference temperature, 50.degree. C., the control unit
90 sounds the alarm device 50. If the temperature of the heater
13 drops to 50.degree. C. or less, the control unit 90 deactivates
the alarm device 50.
For example, with the heater temperature above 50.degree. C., if
a constant voltage of 5 V is applied across the thermistor 15, the
control unit 90 will detect a value of 2.5 V or less. Accordingly,
a voltage of 2.5 V is used as a reference voltage. If it is determined
that the sensed voltage is lower than the reference voltage, it
is determined that the temperature of the heater 13 is higher than
the reference temperature, 50.degree. C., and a driving signal is
sent to the alarm device 50.
If the temperature of the heater 13 decreases below the reference
temperature, the temperature sensed by the thermistor 15 also drops.
Thus, the resistance of the thermistor 15 decreases, and if the
control unit 90 detects a voltage higher than the reference voltage
(2.5 V), it interprets that the temperature of the heater 13 is
lower than the reference temperature, 50.degree. C., and does not
operate the alarm device 50.
The following description concerns the mutual operation of the
above components of the microwave oven.
Referring to FIG. 4, depicting the control sequence of the driver
circuit of FIG. 3, if a user selects to cook using the heater 13,
the control unit 90 controls the power applied across the heater
13 so that the heater 13 produces heat. The display 30 then shows
the user the programmed information on the function selection part
40. When the programmed cooking time is over, the power applied
to the heater 13 is shut off so that the heater 13 does not emit
the heat, informing the user of completion of the cooking. Cooked
food is then removed from the cooking chamber 10. In addition, even
if the programmed cooking time is not over yet, the user may open
the door 20 in order to inspect the cooking process or to bring
the food out of the cooking chamber 10.
If the door 20 is opened, the control unit 90 senses it (Step 100).
Once the control unit 90 senses (Step 100) that the door 20 is being
opened, it detects (Step 200) the temperature of the heater 13 at
predetermined intervals using the thermistor 15. If the control
unit 90 does not sense the door 20 being opened, it returns to the
initial stage. The control unit 90 then determines (Step 300) if
the temperature of the heater 13 that it measured at Step 200 is
higher than the preset reference temperature (50.degree. C.). As
described above, referring to FIG. 3, the control unit 90 checks
whether a voltage of 2.5 V or less is detected when the voltage
of 5 V is applied to the thermistor 15.
If the control unit 90 determines (Step 300) that the temperature
of the heater 13 is higher than the preset reference temperature
(50.degree. C.), i.e., if it detects a voltage of 2.5 V or less,
the alarm device 50 is activated (Step 400). More specifically,
the lamp of the display 30 (i.e., the visual alarm device 50-1)
is switched on, and the audio alarm device 50-1, buzzer, is turned
on. Thus before a user puts his or her hand in the cooking chamber
10 in order to either take the food out of the cooking chamber 10
or bring it in the cooking chamber 10, the alarm device 50 lets
the user know that the heater 13 is still hot, thus preventing him
or her from getting a burn. On the contrary, if the control unit
90 determines that the temperature of the heater 13 is lower than
the preset reference temperature (50.degree. C.) at Step 300, i.e.,
if it detects a voltage over 2.5 V, the alarm device 50 is turned
off (Step 500). Operation then returns to the initial stage.
If the control unit 90 determines that the temperature of the heater
13 is higher than the preset reference temperature (50.degree. C.)
at Step 300, and then returns to the initial stage with the alarm
device turned on at Step 400, it continuously checks (Step 200)
the temperature of the heater 13 at predetermined time intervals.
The control unit 90 also compares (Step 300) the detected temperature
with the preset reference temperature. if the control unit 90 determines
that the detected temperature is lower than the preset reference
temperature, it controls the power applied to the alarm device 50,
thus stopping its operation (Step 500). In case that the control
unit 90 determines that the detected temperature is higher than
or equal to the preset reference temperature, it keeps operating
the alarm device 50 (Step 400). As the control unit 90 senses that
the door 20 is closed by the user, it deactivates the alarm device
50.
Another preferred embodiment of the present invention is now described
referring to FIG. 5.
A driver circuit for the safety system includes a bimetal sensor
15' serving to open and close its contact at a given point of temperature,
a door switch 70 sensing the opening and closing of the door 20,
and an alarm device 50. The sensor 15' is made of two laminated
metals having different coefficients of thermal expansion.
Once the temperature of the heater 13 within the cooking chamber
10 increases to a predetermined point and over, the bimetal sensor
15', closes its contact to turn on the driver circuit for the safety
system. If the temperature of the heater 13 goes below the predetermined
point, the bimetal sensor 15' opens its contact to turn off the
driver circuit. The door switch 70 is operated by a push button,
and opening and closing the door 20 turns the driver circuit on
and off.
Thus, when the temperature of the heater 13 increases to a predetermined
point and the door 20 is opened, the driver circuit is automatically
closed to drive the alarm device 50. If the user closes the door
20 when the temperature of the heater 13 is higher than the predetermined
point, the switch 70' opens to deactivate the alarm device 50.
In accordance with the present invention, as described above, the
extra heater temperature sensor is provided for a microwave oven
for measuring the temperature of the heater only. When the door
of the microwave oven is opened, and the measured heater temperature
is high enough to burn the user, the alarm device sounds a warning.
In such a manner, the present invention prevents the user from being
burnt by the hot heater.
Although the present invention has been described in connection
with preferred embodiments thereof, it will be appreciated by those
skilled in the art that additions, modifications, substitutions
and deletions not specifically described may be made without departing
from the spirit and scope and invention as defined in the appended
claims. |