Specifications

Ver SG 9/200 48 Sharp Electronics Corporation
PROTECTION CIRCUITS
Several protection circuits are used in microwave ovens. Besides the sensor shut down
circuits mentioned previously, monitor switches and temperature fuses are also used.
The monitor switch circuit is designed to prevent the oven from energizing any circuit of
the oven while the door is open. It is also designed to immediately interrupt all circuits if
the door opens while the oven is in operating mode. The circuit contains a combination
of two switches and a relay on the control assembly. In the models that have mechanical
timers, a three switch combination is used.
The mechanical timer ovens have the three switches mounted on the door latch assembly.
The primary switch monitors the hot side of the power line while the secondary switch
monitors the neutral side. These switches are both normally open switches and will close
when the door latch hook energizes them. The monitor switch is a normally closed
switch and will open when the door latch energizes it. When the door is closed, the
switches must be so aligned that the monitor switch is energized first. Now that the
monitor switch is open the primary and secondary switches can close allowing current to
pass to the remaining circuits of the oven. If the switches are out of alignment or a
mechanical problem exists with the door, the monitor switch will remain in the normally
close position. When the primary and secondary switches close, a dead short will be
placed across the line via the three switches. This, of course, will result in the monitor
fuse opening. If the monitor fuse is open due to a failure in the door switches, it is
recommended that all three switches be replaced. This is due to the amount of current
that will pass through the switch. Even though the amount of time that the current passes
through the switch contact can be measured in microseconds, it is enough time to do
permanent damage to the switch contacts.
Ovens that use the solid state controls have two switches and a relay for the protect
circuit. While the monitor and secondary switch function the same as in mechanical
timer models, the primary switch has been replaced by the primary interlock relay. The
interlock relay has the same function as the primary switch. This relay is activated by the
door sensing switch and again controlled by the door latch assembly. If a monitor fuse is
open due to a door problem, replace the monitor and secondary switch. It is not
necessary to replace the control assembly or primary relay on the control. This due to the
construction of the relay. It is capable of carrying the heavy current load where as the
switch is not.
It is easy to see the importance of ordering the correct switch. The monitor switch is
normally closed and the interlock switches are normally open.