Note: Descriptions are shown in the official language in which they were submitted.
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The present invention relates to a door interlock
arrangement and more particularly to a microwave oven
having a door position sensing arrang~ment for pxeventing
the operation of the oven when the oven door is not
properly closed,
Sa~ety requires that microwave ovens be supplied with
means for preventing the generation o~ microwave energy
when access to the cavit~ is possible, In applicable
5tQ~tQ~
~ regulations it is ~tcs that a microwave oven cavity shall
have a minimum of two operative interlocks including a
concealed safety interlock which must not be operable by
any part of the body or by the use of a rod 3 millimeters
or greater in diameter and with a useful length of ten
centimeters,
The prior art, such as U,S, Patent Mo,3,5239170
dated August 4, 1970 - Boehm discloses a microwave oven
wherein a pair o~ switches are mounted on the door frame9
the door being provided wlth a pair of plungers for operat-
ing the switches respectively when the door is in its closed
position~ Such an arrangement is typical in that the door
or at least a portion thereof serves to actuate a plunger
associated with a switch ~orming a portion of an interlock
system for the microwave oven, U~S, Patent 3,329,795
dated July 4, 1967 - Long discloses an arrangement wherein
a pair of switches are opera~le in response to pressure
~rom the door such that when the door is in its closed
position both the switches are positioned to close the power
circuit to microwave generator. Long also di~cl~ses an
arrangement wherein a conductive material is applied to the
frangible glass material of the door, the conductor being
coupled with the oven operational circuitry such that,
should the oven door glass crack, the conductor located on
- 1 - ~
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the surface of the glass will break to open the power
circuit to the microwave generator. U S. Patent 3,624,334
dated November 30, 1971 - Chapell discloses a microwave
oven control circuit having first and second interlock
switches operative when the door is open to apply power
to the filament only of the magnetron and when the door
is closed to apply power to the magnetron plate such that
if either switch malfunctions when the door is open, the
other operative switch interrupts the power to the magnetron
plate.
In all the above-mentioned examples of the prior art,
the door of the oven is an active member only to the ex-
tent that the closing of it applies pressure to a self-
contained switch ~or the activation thereof. No electrical
current flows through any portion or part of the door to
be coupled into the operational circuitry of the oven
through switches formed of mating contacts on the oven
door and the door frame respectively so as to provide a
safety interlock wherein the door is a truly active member
thereof. U S Patent 3,091,675 dated May 28, 1963 - Sholtes
assigned to the same assignee as the present invention dis-
closes an appliance having an interloc~ switch assembly
mounted within a hollow door and arranged to prevent
operation of the appliance when the door is open. However,
the switch is an integral assembly,both contacts being
located within the switch which is, in turn, located within
the door
It is desirable to provide a safety interlock for a
microwave oven wherein the door itself, or at least a portion
3a thereof, forms a portion of an electrical circuit such that
the opening of the door breaks the electrical circuit to
remove power from the operating components of the oven, so
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as to prevent generation of microwave energy. By the
present invention thexe is provided such a safety in-
terlock in the form of a door position sensing arrangement
for a microwave oven which is rather simple of construction,
highly relia~le and of fairly low cost and which meets the
requirements hereinbefore described.
In accordance with the present invention, there is
provided in an electronic oven having an oven liner front
frame defining an opening into an oven cavity and a cloor
o A arranged for gaining access into and for closing the cavity~ ~n~
à door position sensing arrangement. Included are a pair
o~ contacts, including a first contact secured to the liner
front frame and a second contact secured to the door in
mating relationship with the irst contact when the door
i~ in the closed position, the second contact being
electrically insulated from the first contact when the door
is open and electrically engaged with the first contact
when the door is closed, the contacts being coupled
electrically into an oven c:ircuit, such that, when the door
is open with the contacts disengaged, electrical power will
be removed from operational components of the oven to
e~ect shutdown thereof.
In the accompanying drawings:
FIGURE 1 is perspective view of a table-top microw~ve
oven shown with the ovsn door in the open position~ the
oven incorporating the pref~rred embodiment of the present
invention;
FIGURE 2 is a schematic representation of circuitry
incorporating the preferred embodiment of the present in-
30vention; and
FIGURE 3 is a portion of the schematic repr~sentation
of FIGURE 2 in some~hat modified form
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Referring to Figure 1 of the accompanying drawing,
there is shown a table top electronic oven 10 of the
microwave type wherein the present invention is incorporated.
Microwave oven 10 is of standard construction having an
oven cooking cavity 12 framed by a box-like oven liner 14
having a front frame 16 defining an opening into cavity 12.
A front opening access door 18 is hingedly secured along
its lower edge by appropriate means (not shown) to front
frame 16 for movement between an open position as shown
and a closed position as shown and a closed position
(not shown) in mating engagement with front rame 16.
handle is provided (not shown) suitably secured to the
door 18 for aiding in opening and closing the door.
A control panel 20 is shown positioned beside cooking
cavity 12 in a frontal position to occupy substantially
the same vertical plane as the oven door 18 when in its
vertical, closed position. The electrical controls of the
oven are mounted on this control panel, and are suitably
coupled to appropriate circuitry for providing proper
operation of the microwave oven.
In accordance with the present invention, there is
provided a door position sensing arrangement comprising,
in the preferred embodiment, two pairs of contacts 22 and
24 including first contacts 26 and 28 respectively and
second contacts 30 and 32 respectively. First contacts
26 and 28 are mechanically secured to front frame 16 and
normally electrically insulated therefrom, the contacts
being depxessed below or within the front rame. In the
preferred embodiment they are located on the portion of
the frame opposite the door hinge member and are arranged
on that side at opposite ends thereof such as near the
corners of the door thereby to be separated by a distance
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greater than ten centimeters; that is9 the distance between
contacts 26 and 28 is greater than t~n centimeters, Second
contacts 30 and 32 are mechanically secured, in projecting
fashion, to the door 18 so as to be in mating engagement
relationship with first contacts 26 and 28 respectively
when the door 18 is in its closed posit:ion, Second contacts
30 and 32 are normally electrically insulated ~rom the door
while being coupled serially electrical:ly with each other
through insulated conductor 33 located within door 18 as
shown in Figure 2.
Referring now to Figure 2 there is shown7 by schematic
representation, a typical circuit for a microwave oven
which incorporates the sensing arrangement of the present
invention, Means are provided for coupling to a source
o~ electrical power and include lines Ll and L2, and
coupled thereacross, an oven load 34, A switch 36 is coupled
serially with load 34 such that when opened, power will be
removed to effect shutdown o the oven, Switch 36 is of
the normally-open type and is responsive to operation of
a relay coil 38 to become closed upon the actuation of the
coil 38, A step down trans~ormer 40 has its primary wi.nd-
ing 42 coupled across lines Ll and L2 and a low voltage
secondary winding 44 (for example 12 volts) coupled in
circuit for actuating relay solenoid 38 to produce an
acceptably safe, low voltage system, As can be seen,
contact pairs 22 and 24 are coupled serially electrically
with relay solenoid 38 across transformer secondary 44.
When door 18 is closed properly such that contacts 26 and
30 and likewise contacts 28 and 32 are in mating engage-
ment respectively and power is being delivered to lines Ll
and L2, relay solenoid 38 will be actuated so as to close
switch 36 thereby delivering electrical power to load 34,
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However, should an attempt be made to open the door
without first turning the oven off, the simple opening of
the door will breaX contacts 26 and 30 and~or contacts
28 and 32 to effect shutdown of the oven by removal of
power from load 34. It can be envisioned tha-t even the
slightest prying of the door corners will cause the break-
ing of the contacts closest thereto so as to incapacitate
the microwave oven. And, should for some reason the
contact pairs 22 and/or 24 become weldecl together, the
oven door 18 could not be opened. Applying physical force
to break the frozen contacts, ~hich would allow the opening
of the door, would also then effect the opening of the
circuit again effecting shutdown of the microwave oven
Such as arrangement provides monitoring oE the corners
of the door system, the most easily distorted portion of
the door, so as to provide a positive method o~ preventing
operation of the microwave oven with a distorted door
assembly. Such an arrangement may also serve to checX for
cleanliness of the sealing surfaces through contact re-
sistance and to prevent operation of an oven with bably
soiled contacts Such an arrangement is also capable of
limiting the microwave leakage to a minute amount be~ore
the interlock operates~ It is believed also that such an
interlock meets specific federal requirements for concealed
interlocks
It has also been envisioned that such an arrangement
might be used in a microwave oven wherein an electrically
conductive gasket, such as for example of a metal mesh,
is used to make door seal. With such an arrangement, the
gasket material would replace the two contacts of the door
and their joining conductor 33 and would therefore locate
against the contact points on the oven frame Since the
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contact points mounted on the front frame are sliyhtly
recessed, that is below the surface of the frame~ the
metal gasket would pull away from the contacts before it
disengaged the front frame With such an arrangement, the
generation of microwave energy would be stopped before the
mîcrowave closure between the door and the frame would be
broken.
There is shown in Figure 3, a somewhat modified ~orm
of the schematic representation of Figure 2 with correspond-
ing components numbered accordingly. Figure 2 may be broken
at terminals 46 and 48 such that Figure 3 may be placed
into the circuit of Figure 2. It will be noted that in
Figure 2, contact pairs 22 and 24 and more specifically
contacts 30 and 32 are coupled serially electrica:Lly
through the door 18 via conductor 33. In the arrangement
of Figure 3, while the contact pairs 22 and 24 are coupled
serially, this is effected through the coupling of contacts
26 and 28 serially electrically through the liner front
frame via conductor 33. With such an arrangement then,
contacts 30 and 32 are coupled by suitable insulated con-
ductors~, such as flexible wires 50 and 52 to terminals
46 and ~8 respectively through the door hinge member With
such an arrangement, electrical current still 10ws through
a portion of the door to be coupled into the operational
circuitry of the oven through what are essentially switches
formed of mating contacts on the oven door and the door
frame respectively so as to provide a safety interlock
wherein the door is a truly active member thereof.
0~ course~ it will be obvious to those skilled in the
art that the circuitry arrangement of either Figure 2 or
Figure 3 will work effectively using only one pair of
contacts. Referring to Figure 2, using only contacts 26
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and 30 to monitor the position of the door, contact 30 will
be coupled in circuit through a suitable insulated conductor,
such as a flexible wire, and may for example be passed by
suitable means through the hinge member and thence to couple
with relay solenoid 38
It should be apparent to those skilled in the art
that the embodiment described heretofore is considered to
be the presently preferred form o this invention. In
accordance with the Patent Statutes, changes may be made
in the disclosed apparatus and the manner in which it is
used without actually departing from the true spirit and
scope of this invention.
