Power source circuit for microwave oven

A power source circuit for a microwave oven is provided with first and second door switches that are opened by opening the door of the microwave heating chamber and a third door switch having a movable contact that is converted to first and second fixed contacts in response to closing and opening the door respectively. The circuit insures safe cut off of the microwave magnetron even if the first door switch does not open upon opening the door and also in the event of grounding of the primary of a high voltage transformer.

BACKGROUND OF THE INVENTION 
Field of the Invention 
The present invention relates, in general, to a power source circuit for a 
microwave oven. In particular, the invention relates to a power source 
circuit incorporated on the primary side of a high voltage transformer. 
Description of the Related Art 
In general, in a power source circuit for a magnetron of a microwave oven, 
the primary side of a high voltage transformer is connected to the main 
power source. A high voltage rectifier circuit is provided on the 
secondary side, and the D.C. output of the high voltage rectifier is 
applied to the magnetron. In this case, usually the circuit on the primary 
side of the high voltage transformer is constructed so as to prevent the 
magnetron from oscillating while the door of the heating chamber is open. 
For example, FIG. 5 shows the conventional technique. In FIG. 5, normally 
when the door of the heating chamber is opened, the primary winding of 
high voltage transformer 22 is isolated from the main power source 21 by 
opening first and second door switches 18 and 19. Also, a third door 
switch 30 is closed when the door is opened. So even if, due to some 
malfunction such as welding of the contacts of door switch 18, the 
contacts of switch 18 should fail to open when the door is opened, a safe 
condition is ensured by the immediate melting of fuse 26, due to the 
formation of a short circuit of the main power source 21 through the 
closed switch 30 as shown by the broken line in FIG. 6. 
However, the above construction is still subject to the problem that if 
there should be some failure in the insulation of the high voltage 
transformer 22, a safe condition would not be guaranteed when the door is 
opened. Thus, if the bus on the side of the second door switch 19 of the 
pair of buses from the main power source 21 is live, (i.e., is the 
non-grounded side in the power transmission system), and if the primary 
winding of high voltage transformer 22 is grounded by failure of 
insulation etc. then when third door switch 30 is closed by opening the 
door while the timer switch 31 is closed, a path will be formed as shown 
by the broken line in FIG. 7. If a current flows through this path, there 
is a possibility that the oscillating condition of the magnetron 15 will 
be maintained even after the door is opened. In this case, if the house is 
equipped with a ground leakage circuit breaker, the power source will be 
cut off by operation of the ground leakage circuit breaker. But the 
existence of the possibility of such oscillation of the magnetron 15 is 
itself very undesirable from the safety point of view. 
To deal with this risk, it would be possible to provide two door switches 
18 and 32 which open with opening of the door on both buses of the main 
power source 21 as shown in FIG. 8. However, with such a construction, it 
is possible for fuse 26 to melt only after both two switches 18 and 19 
have welded, and thus this construction is still worse from the point of 
view of safety. 
SUMMARY OF THE INVENTION 
Accordingly, it is an object of the present invention to provide a power 
source circuit for a microwave oven wherein safety can be improved by 
reliably preventing the magnetron from oscillating when the door is open 
even if there is some failure of insulation of the high voltage 
transformer. 
In accordance with the present invention, there is provided a power source 
circuit for a microwave oven which includes a first and second bus, a high 
voltage transformer and first, second and third door switches. The first 
and second buses are connected to a power source. The high voltage 
transformer has a primary winding with first and second ends connected to 
the first and second buses respectively. The first door switch is 
connected between the first bus and the first end of the primary winding 
and is opened in response to opening of a door of the microwave oven. 
Likewise, the second door switch is connected between the second bus and 
the second end of the primary winding and is opened by opening of the 
microwave oven door. The third door switch has a movable contact and a 
first and second fixed contact. The movable contact makes electrical 
contact with the first fixed contact in response to closure of the door 
and contacts the second fixed contact in response to opening of the door. 
The movable contact and the first fixed contact are connected together and 
connected between the first switch and the first end of the primary 
winding when the door is closed. Further, the movable contact and the 
second fixed contact are connected together and connected between the 
first switch and the second bus when the door is opened.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The preferred embodiment of the present invention will now be described in 
more detail with reference to the accompanying drawings. Like reference 
numerals designate like or corresponding parts throughout the drawings. 
As shown by the overall external view of FIG. 4, an inner box 12 is 
accommodated in an outer box 11. A front face of the inner box 12 can be 
opened so that an interior of the inner box 12 can be used as a heating 
chamber 13. At the bottom of the heating chamber 13, a turntable 14 is 
provided. The food is placed on the turntable 14. The microwaves emitted 
from a magnetron 15 (FIG. 1), to be described, are guided into the heating 
chamber 13. The front face of the heating chamber 13 can be opened and 
closed by means of a door 16 that is pivoted to the outer box 11. Two 
engagement claws 17 for locking the door 16 in a closed condition are 
provided projecting from the side edge opposite to the pivoted side of the 
door 16. 
The electrical layout will now be described in reference to FIGS. 1 and 2. 
A total of three door switches 18, 19 and 20 are provided in outer box 11. 
These switches are actuated in a manner linked to the opening and closing 
of the door 16. The first and second door switches 18 and 19 are of the 
single-throw type having a pair of movable contacts 18a and 19a and a pair 
of fixed contacts 18b and 19b. The construction is such that when the door 
16 of the heating chamber 13 is closed, the two engagement claws 17 of the 
door 16 press on actuators (not shown), closing contact pairs 18a, 18b and 
19a, 19b. But when the door 16 is opened, the engagement claws 17 separate 
from the actuators with the result that these contact pairs are opened. In 
contrast, third door switch 20 is of double-throw type having a movable 
contact 20a and first and second fixed contacts 20b and 20c. The 
connection condition of the contacts of the third door switch 20 is 
changed over by an actuator (not shown) being pressed by the door 16. That 
is, when the door 16 of the heating chamber 13 is closed, the door 16 
presses on the actuator, bringing movable contact 20a into contact with 
first fixed contact 20b. When the door 16 is opened, the actuator moves 
movable contact 20a into the condition contacting the second fixed contact 
20c. 
On a high voltage transformer 22 for supplying power to a magnetron 15 from 
main power source 21, there are wound a primary winding 23, a secondary 
winding 24 for high voltage use, and a tertiary winding 25 for the heater 
of the magnetron 15. A timer motor 27 is connected in parallel with 
primary winding 23 of the high voltage transformer 22. A timer switch 28 
that is opened and closed by the timer motor 27 is connected to one lead 
wire 23a of the primary winding 23 of the high voltage transformer 22. 
Between one of the buses 21a of the main power source 21 and the timer 
switch 28, there are arranged, connected in series, a fuse 26, the first 
door switch 18 and the movable contact 20a and first fixed contact 20b of 
third door switch 20. Also, between the other bus 21b of main power source 
21 the other lead wire 23b of the primary winding 23 of the high voltage 
transformer 22, the second door switch 19 is connected in series. 
Furthermore, second fixed contact 20c of the third door switch 20 is 
connected to the other bus 21b of main power source 21. The secondary 
winding 24 of the high voltage transformer 22 is connected between the 
plate and cathode of the magnetron 15 through a high voltage rectifier 
circuit 29, while the tertiary winding 25 is connected to the heater of 
the magnetron 15. 
In operation, when the food that is to be heated is placed inside heating 
chamber 13 and the door 16 is closed, as shown in FIG. 2, the first and 
second door switches 18 and 19 are closed. The movable contact 20a of the 
third door switch 20 is put in contact with first fixed contact 20b. 
Thereupon, when the desired time is set by turning timer switch 28 ON, a 
current is passed to the timer motor 27 and the primary winding 23 of the 
high voltage transformer 22, thereby causing magnetron 15 to oscillate and 
the food in the heating chamber 13 to be irradiated with microwaves. When 
the set time has elapsed, the timer switch 28 is opened by the rotation of 
the timer motor 27, causing the magnetron 15 to stop oscillating and 
cooking to be terminated. 
Assume now that the door 16 is opened when the timer switch 28 is closed 
before the set time has elapsed. Under ordinary conditions, when the first 
door switch 18 and the second door switch 19 are functioning normally, the 
two switches 18 and 19 are opened by the opening of the door 16 of the 
heating chamber 13. The primary winding 23 of the high voltage transformer 
22 is isolated from the main power source 21, cutting off the supply of 
power to the magnetron 15. Thus, the situation of microwaves being 
discharged to the outside from the heating chamber 13 whose door 16 has 
been opened can be reliably prevented. 
Consider now the exceptional case where, even though the door 16 is opened, 
the contacts 18a, 18b of the first door switch 18 function abnormally and 
do not open, due for example to contact welding between contacts 18a and 
18b. However, in this embodiment of the invention, on opening the door 16, 
the connection condition of movable contact 20a and second fixed contact 
20c of the third door switch 20 is always changed over. In this case, as 
shown in FIG. 3(a), the primary winding 23 of the high voltage transformer 
22 is isolated from the main power source 21 and a short circuit of the 
main power source 21 is formed through a fuse 26, the first door switch 18 
and the third door switch 20. Thus, the oscillation of the magnetron 15 
will still be stopped, so the situation of microwaves being discharged 
from the heating chamber 13 to the outside can be reliably prevented. The 
abnormal condition is made obvious by melting of the fuse 26. 
Furthermore, considering the case where, of the pair of buses 21a, 21b of 
the main power source 21, the bus 21b on the side of second door switch 19 
is live and some failure of insulation etc. of the high voltage 
transformer 22 has allowed the other lead wire 23b of the primary winding 
23 to become grounded. Conventionally, this would have resulted in a 
condition in which power could be supplied to the magnetron 15 (see FIG. 
7). However, in the present embodiment, in such a case, opening of the 
door 16 changes over the movable contact 20a of the third door switch 20 
from the condition contacting the first fixed contact 20bto a condition 
contacting the second fixed contact 20c. Thus, the primary winding 23 of 
the high voltage transformer 22 is isolated from the main power source 21 
(see FIG. 3(b)). As a result, the magnetron 15 will still be prevented 
from oscillating and a situation in which the microwaves are discharged to 
the outside from the heating chamber 13 can be reliably prevented. 
As described above, with this embodiment, even when the contacts fail to 
open with opening of the door 16 due to malfunction of a door switch, the 
oscillation of the magnetron 15 can be prevented by isolating primary 
winding 23 of the high voltage transformer 22 from the main power source 
21 by melting of the fuse 26 etc. Also, even if insulation breakdown of 
the high voltage transformer 22 should occur, the situation of the 
magnetron 15 oscillating can be reliably prevented in accordance with the 
polarity of the main power source. Consequently, in such a case also, user 
safety can be fully guaranteed without waiting for the domestic ground 
leakage circuit breaker to operate. 
Numerous other modifications and variations of the present invention are 
possible in light of the above teachings. It is therefore to be understood 
that, within the scope of the appended claims, the present invention can 
be practiced in a manner other than as specifically described herein.