2.1. Variable power cooking
control
High Voltage Inverter Power Supply (U) controls output power
by the signal from Digital Programmer Circuit (DPC). Power
relay always stay on, but PWM (Pulse Width Modulation) signal
controls microwave output power.
NOTE:
The ON/OFF time ratio does not correspond with the
percentage of microwave power since approximately 2
seconds are required for heating of magnetron
filament.
Variable Power Cooking
POWER
SETTING
OUTPUT
POWER(%)
APPROX.
MANUAL MICROWAVE DUTY
ON(SEC)
OFF(SEC)
HIGH
100%
22
0
DEFROST
25%
15
7
MEDIUM
60%
22
0
LOW
45%
22
0
SIMMER
20%
14
8
WARM
10%
7
15
2.2. Inverter power supply circuit
The Inverter Power Supply circuit powered from the line
voltage, 230-240V 50Hz AC input supplies 4,000V DC to the
magnetron tube, and functions in place of the H.V. transformer,
the H.V. capacitor and H.V. diode.
1. The AC input voltage 230-240V 50Hz is rectified to DC
voltage immediately.
2. DC voltage will be supplied to the switching devices called
IGBT. These devices are switched ON-OFF by the 20 to 40
kHz PWM (pulse width modulation) signal from the
microcomputer in the DPC.
3. This drives the High voltage transformer to increase voltage
up to 2,000V AC.
4. Then the half-wave doubler voltage rectifier circuit,
consisting of the H.V. diodes and capacitors, generates the
necessary 4,000V DC needed for the magnetron.
5. Output power of the magnetron tube is always monitored by
the signal output from the current transformer built into the
inverter circuit.
6. This signal is fed back to the microcomputer in the DPC to
determine operating conditions and output necessary to
control PWM signal to the Inverter Power Supply for control
of the output power.
2.3. Turbo defrost, Auto cook
When the Auto Control feature is selected and the Start pad is
tapped:
1. The digital programer circuit determines the power level and
cooking time to complete cooking and indicates the
operating state in the display window.
2. When cooking time in the display window has elapsed, the
oven turns off automatically by a control signal from the
digital programmer circuit.
2.4. Thermistor
The thermistor which fixed on magnetron detects magnetron
temperature and make power down when the temperature
becomes abnormal high. A normal thermistor´s resistance
values for temperature ranges between 10-30 degree C is from
35K
Ω
to 110K
Ω
.
2.5. Convection Cooking Control
The digital programmer circuit controls the ON-OFF time of the
heater in order to control oven cavity temperature.
1. After selecting desired oven cavity temperature of
Convection (the range of selected oven temp is 100°C-
220°C) and pressing [Start] pad, a high level out of the
digital programmer circuit and is applied to coil of power
relay (RY4).
2. When the contacts of power relay RY4 close, power source
voltage is applied to the heater and the heater turns on.
3. When the oven temperature reaches the set temperature,
the digital programmer circuit senses the temperature
through oven temp sensor and stops supplying high level
signal to the coil of power relay RY4 and the heater turns
off.
4. After the heater turns off, the oven temperature will
continue increasing a while and then decrease as shown in
Figure.
When the oven temperature drops below the set
temperature, the digital programmer circuit senses the
signal and starts supplying high level signal to the coil of
power relay again.
2.6. Grill cooking
Grill cooking is accomplished by upper heaters only. One grill
cooking cycle is 33 seconds.
1. During grill cooking, the digital programmer circuit controls
power relay RY4´s ON-OFF time. In all three grill cooking
categories, RY4´s ON-OFF time are shown in Figure.
GRILL CATEGORY
GRILL (RY4)
ON (sec.)
OFF (sec.)
1
33
0
2
24
9
3
18
15
2 DESCRIPTION OF OPERATING SEQUENCE
6
NN-CT585S / NN-CT565M / NN-CT555W
