Combination thermal and mechanical safety switch for microwave heating oven

In a microwave heating oven, a safety switch 301 is disposed in an external position relative to a heating chamber 100. A displacement rod 302, which is made of polyprophylene or the like low dielectric constant plastic substance, is disposed through and across the heating chamber 100. Displacement rod 302 serves as a thermal switch, so that at an abnormally high temperature displacement rod 302 melts down, thereby opening the safety switch 301 and disengaging the microwave generator.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention generally relates to heating ovens; for example, 
electric or microwave heating ovens. The invention particularly relates to 
microwave ovens having mechanical safety switches linked to respond to the 
open-close action of their front doors, which mechanical switches also 
serve as thermally responsive safety switches. Thus, the switches both 
guard against operation of the microwave when the door is open, and 
against operation when the microwave is overheated. 
2. Description of the Related Art 
In the conventional microwave heating oven, a box which constitutes a 
heating chamber, is further enclosed in an external case. Because of such 
construction, it is difficult to make such a heating oven at a low cost, 
or to make such a heating oven easy in manufacture, or to make such a 
heating oven compact. In order to solve such problems, a heating oven in 
which the box of the heating chamber serving as the external case has been 
tried. For example, a heating oven wherein the control box containing 
electric parts is disposed at the side of the heating chamber, is shown in 
patent application Sho No. 59-105036, or a heating oven wherein the 
control box disposed below the heating chamber, is shown in patent 
application Sho No. 59-119319. 
FIG. 1 shows one of such conventional microwave heating ovens. In FIG. 1, 
the control box 2 containing electric parts is disposed below the heating 
chamber 1. The front door 3 is disposed at the front opening 1a of such 
heating chamber 1. The front door 3 has a hook 3a for locking the front 
door 3. And the safety switch 7 is disposed at the position involving such 
hook 3a on the side wall of the heating chamber. 
FIG. 2 shows a control circuit of such a conventional microwave heating 
oven. In FIG. 2, the control circuit comprises, a magnetron 31, a high 
voltage transformer 32, a high voltage capacitor 33, a diode 34, a cooling 
fan motor 4, a timer control motor 5, and so on. In the safety standard, 
for example UL standard, at least two safety switches 7 and 8, responsive 
to the open-close action of the front door 3 must be included in the 
design of the microwave. Further, a short switch 10 also must be disposed 
on such electric circuit, to melt down the fuse 9 in case switches 7 and 8 
become stuck closed when front door 3 is open. Furthermore, the 
conventional microwave heating oven has a thermal switch 11 which is, for 
example, a fuse switch or a bimetal switch, so as to break off the 
electric circuit for making microwave 30 and the cooling fan motor 4 in 
case of abnormal high temperature or fire trouble of food. In practical 
design of the microwave heating oven, the position of the thermal switch 
is determined after designing of a passage of the exhaust gas from the 
cooked food by trial and error of the experiment. 
As improvement toward simplicity of the configuration of the heating oven 
proceeds, however, the safety switch responding to the open-close action 
of the front door must be disposed at the position to be showered by the 
microwave, which is shown in FIG. 1. So that, such a heating oven has the 
following problems. First, shower of microwave destroys the safety switch, 
second at receiving such microwave shower, wires of control circuit 
radiate electric noise which a badly affect the television set or the 
radio. 
Further, in a microwave heating oven such as shown in patent application 
Sho No. 59-119319, the walls of the heating chamber serves as the external 
case, and so the heat is conveyed toward external part easily. Therefore, 
there is a strong demand for the quick responding thermal switch which can 
break down the electric circuit in a short time. 
SUMMARY OF THE INVENTION 
The purpose of the present invention is to provide a microwave heating oven 
having safety swiches, at least one of which serves dually as a thermal 
switch. 
A microwave heating oven in accordance with the present invention 
comprises, 
a heating chamber for containing and heating a heating object therein, 
a microwave generator for generating microwave for heating the heating 
object, 
a front door for covering a front opening part of the heating chamber, 
a safety switch which is disposed behind a microwave-shield and is for 
switching off the operation of the microwave generator, responding to 
opening action of the front door, 
physical displacement means for transmitting the opening action of the 
front door to the safety switch, disposed across and through the heating 
chamber. 
As a result of the above-mentioned configuration, the safety switches are 
safely shielded from the microwave and hence are not broken by the 
radiation of the microwave, and on the other hand the switch is quickly 
opened in the event of abnormally high temperatures of fire trouble.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
A preferred embodiment of a microwave heating oven in accordance with the 
present invention is shown in FIG. 3, FIG. 4, FIG. 5, FIG. 6, FIG. 7 and 
FIG. 8. 
As shown in FIG. 3, a box made of a microwaveshielding substance, such as 
metal, and constituting a heating chamber 100 of such embodiment is 
designed to serve also as an external case, so as to make the size of the 
microwave heating oven compact and reduce cost. Panels 101, 102, 103, 104, 
106, 107 and so on, which are for example stainless steel, are made in box 
shape by known bending and calking and the like. A control box 200 in 
which a magnetron 201, a high voltage transformer 202, a high voltage 
capacitor 203, a cooling fan motor 204, and so on are contained, is 
disposed below the heating chamber 100. At a front opening window of the 
heating chamber 100, a front door 110 is connected rotatably to the panel 
107 by a known mechanism (which is not shown in the drawings). And a front 
part of the control box 200 is covered by a control panel 120. A timer 207 
for controlling the cooking time, (namely radiation time of microwave,) 
and a knob of the timer 208 are disposed at the front face of the control 
panel 120. 
As shown in FIG. 6 which is a perspective view of the heating oven in 
accordance with FIG. 3, the front door 110 has a projection part 110b and 
a hook part 110c on its inner face. The projection part 110b turns on a 
safety switch 206 disposed on the control box 200 when the front door 110 
is closed, and turns off the safety switch 206 when the front door 110 is 
opened. And the hook part 110c is caught to be locked and released to be 
unlocked by a known mechanism in the control box 200, responding to the 
close action and open action of the door knob 110d of the front door 110 
(the lock mechanism is not shown in the drawings), respectively. 
The control panel 120 has openings 120a and 120b, respectively for 
receiving the projection part 110b and the hook part 110c of the front 
door 110. And the lock mechanism serves as another safety switch. These 
mechanism and the safety switches are shown in detail in FIG. 8 and 
explained later. 
A rear cover 105 is fixed on the rear part of the heating chamber 100, and 
the space 300, which is enclosed by the panel 102 of the heating chamber 
100 and the rear cover 105, constitutes the exhaust passage of the heating 
oven to exhaust the vapor or the like from the food 130 of heating object 
by the absorption of the cooling fan 204a. On the inner face of the rear 
cover 105, a safety switch 301 which turns off or on responding to the 
open-close action of the front door 110, is disposed. A displacement rod 
302, which transmits the open-close action of the front door 110 to the 
safety switch 301, is disposed through the upper part of the heating 
chamber 100. And also, it is supported at both end parts in a direction 
shown by arrow marks A and B in FIG. 3 by bushings 304 and at inbetween 
parts by guide holders 305. The guide holders 305 are held by the panels 
102 and 103 of the heating chamber 100. The displacement rod 302 is always 
charged by a spring of safety switch 301 (which is not shown in figures) 
so as to be pushed out of the panel 107. When the front door 110 is 
opened, the displacement rod 302 slides in the direction arrowed A by the 
force of the spring of the safety switch 301 and is pushed out of the 
panel 107, so that the right end 302b of the displacement rod 302 is 
separated from the safety switch 301, and the safety switch 301 is opened. 
And when the front door 110 is closed, the inner face 110a of the front 
door 110 contacts with the left end 302a of the displacement rod 302, and 
so, the displacement rod 302 is withdrawed in the direction arrowed B, and 
the safety switch 301 is closed. Accordingly, the safety switch 301 is 
responding to the open-close action of the front door 110. 
On the bottom of the heating chamber 100, a magnetron 201 which generates 
the microwave, and a stirrer blade 209 which radiates the microwave 
equally on the food 130, are disposed. The food 130 is partitioned from 
the magnetron 201 and the stirrer blade 209 by a partition 140 made of 
ceramics or the like insulating substance. 
The safety swich 301 which is mentioned above, also serves as a thermal 
switch. The thermal switch is for sensing the abnormal high temperature or 
heat in the heating chamber 100 caused by the fire trouble of the food or 
the like, and for cutting off the electric circuit of microwave generating 
circuit 250 and the cooling fan motor 204, so that the opening of the 
thermal switch 301 prevents propagation of heat to the outside of the 
heating oven in an early stage. 
The constitution and operation of the thermal switch is as follows. 
The displacement rod 302 is made of low dielectric loss materials such as 
polypropylene, so that it does not ignite at normal heating temperatures 
of the microwave, but it easily changes its' shape at abnormally high 
temperature caused by the fire trouble of the food or the like. 
Accordingly, in case of the fire trouble of the food which is shown in 
FIG. 4, the center part of the displacement rod 302 changes its shape, and 
does not withstand the force of the spring of the safety switch 301 (which 
is not shown in figures). So, the spring returns to normalcy and the right 
end 302b is pushed in the direction designated by arrow mark A. Then, the 
safety switch 301 is turned off and cuts off the electric circuit of 
making microwave 250 and the cooling fan motor 204. 
FIG. 5 is a cross-sectional plan view of the heating oven in accordance 
with FIG. 3. 
On the center of the heating chamber 100, the food 130 of heating object is 
disposed. The displacement rod 302, the guide holder 305 and the safety 
switch 301 are disposed on the right end part of the heating chamber 100. 
And also these switching mechanisms is disposed on the upper part of the 
heating chamber 100, so the heating chamber 100 is hardly restricted by 
these switching mechanisms. When the microwave radiated the food 130 in 
cooking, the vapor or the like are produced. Such vapor is drafted through 
the punched holes 102a of the panel 102 in FIG. 5 and exhausted out of the 
heating oven by the cooling fan 204a in FIG. 3. The exhaust passage is 
shown by the arrow marks. 
FIG. 7 shows an example of the electric circuit of this embodiment of the 
microwave heating oven. In the electric circuit the above-mentioned at 
least two safety switches 206 and 301 to comply with the safety standard 
are provided. The safety switches 206 and 301 are responsive to the 
open-close action of the front door 110. Further, a short-circuiting 
switch 280 is also provided in such electric circuit, to turn on in case 
of failure of the safety switches 206 and 301, so as to melt down a fuse 
260. Incidentally, 270 designates a main switch. As shown in FIG. 8, the 
short-circuiting switch 280 is provided to connect two power lines at the 
part beyond other safety switch 206, 301 and a fuse 260 in paralel with 
loads in this apparatus, and serves as lock mechanism of the front door 
110. 
In such electric circuit, a timer motor 207a of a timer 207 which controls 
the heating time, the cooling fan motor 204 and an electric circuit for 
making microwave are connected in paralel. Further, the microwave 
generating circuit 250 comprises the magnetron 201, the high voltage 
transformer 202, the high voltage capacitor 203, the diode 210 and so on. 
As mentioned above, the present invention uses a thermal switch 301 as one 
of safety switches, and the safety switches are disposed at a position 
thermally isolated from the heating chamber, so that the safety switches 
of the present invention have high reliability. The electric circuit of 
the present invention has simplicity, and the microwave heating oven can 
be made without losing the compactness and the low cost, in comparison 
with the conventional compact type microwave heating oven.