Page 14
MD24 and MD30
Theory of Operation
3-6
#824820 - Revision A - November, 2013
OFF CONDITION
Closing drawer activates door sensing switch and
secondary interlock switch. (Monitor switch contacts
are opened.) When oven is plugged in, 120 volts A.C.
is supplied to power unit.
1. Display will show flashing "Enjoy Your Drawer Touch
Clear and Touch Clock". To set any program or set
clock, you must first touch STOP/CLEAR pad. The dis-
play will clear, and " : " will appear.
COOKING CONDITION
Program desired cooking time by touching NUMBER
pads. Program power level by touching POWER
LEVEL pad and then a Number pad. When the
START pad is touched, the following operations occur:
1. Contacts of relays are closed and components
connected to relays are turned on as follows.
RELAY CONNECTED
COMPONENTS
RY-1 Oven
lamp
RY-2 Power
transformer
RY-3 Stirrer
motor
RY-4 Fan
motor
2. 120 volts A.C. is supplied to primary winding of power
transformer and is converted to about 3.3 volts A.C.
output on the filament winding, and approximately
2370 volts A.C. on high voltage winding.
3. Filament winding voltage heats magnetron filament
and High Voltage winding sends voltage to voltage
doubler circuit.
4. Microwave energy produced by magnetron is chan-
nelled through waveguide into cavity feedbox, and
then into cavity where food is placed to be cooked.
5. Upon completion of cooking time, power transformer,
oven lamp, etc. are turned off, and generation of
microwave energy is stopped. Oven will revert to OFF
condition.
6. When drawer is opened during a cook cycle, monitor
switch, door sensing switch, secondary interlock
switch, relay (RY1) and primary interlock relay are
activated with following results. The circuits to the stir
fan motor, cooling fan motor, and high voltage compo-
nents are de-energized, oven lamp remains on, and
digital read-out displays time still remaining in cook
cycle when door was opened.
7. The monitor switch electrically monitors the operation
of secondary interlock switch and secondary interlock
relay and is mechanically associated with drawer so
that it will function in following sequence.
DESCRIPTION OF OPERATING SEQUENCE
8. When drawer opens from closed position, primary
interlock relay (RY2) and secondary interlock switch
open their contacts. Contacts of relay (RY1) remains
closed. Then monitor switch contacts close.
9. When the drawer is closed from open position, moni-
tor switch contacts open first. Then contacts of second-
ary interlock switch and door sensing switch close, and
contacts of the relay (RY1) open.
If secondary interlock switch and primary interlock relay
(RY2) fail with contacts closed when drawer is opened,
closing of monitor switch contacts will form a short circuit
through fuse, secondary interlock switch, relay (RY1)
and secondary interlock relay (RY2), causing fuse to
blow.
POWER LEVEL P-0 TO P-90 COOKING
When Variable Cooking Power is programmed, 120 volts
A.C. is supplied to power transformer intermittently
through contacts of relay (RY-2) which is operated by
power unit within a 32 second time base. Microwave
power operation is as follows:
VARI-MODE
ON TIME
OFF TIME
Power 10(P-HI)
32 sec.
0 sec.
(100% power)
Power 9(P-90)
30 sec.
2 sec.
(approx. 90% power)
Power 8(P-80)
26 sec.
6 sec.
(approx. 80% power)
Power 7(P-70)
24 sec.
8 sec.
(approx. 70% power)
Power 6(P-60)
22 sec.
10 sec.
(approx. 60% power)
Power 5(P-50)
18 sec.
14 sec.
(approx. 50% power)
Power 4(P-40)
16 sec.
16 sec.
(approx. 40% power)
Power 3(P-30)
12 sec.
20 sec.
(approx. 30% power)
Power 2(P-20)
8 sec.
24 sec.
(approx. 20% power)
Power 1(P-10)
6 sec.
26 sec.
(approx. 10% power)
Power 0(P-0)
0 sec.
32 sec.
(0% power)
Note: ON/OFF time ratio does not correspond with
percentage of microwave power, because approx. 2
seconds are needed for heating of magnetron fila-
ment.
