Panasonic Microwave Ovens with Inverters Quick start guide

Technical Guide
Microwave Ovens with Inverters
Panasonic Services Company
National Training

Prepared by:
César Perdomo
Panasonic Services Company
National Training Department
Secaucus, NJ
Warning
This service information is designed for experienced repair technicians only and is not designed for use by
the general public. It does not contain warnings or cautions to advise non-technical individuals of potential
dangers in attempting to service a product. Products powered by electricity should be serviced or repaired
only by experienced professional technicians. Any attempt to service or repair the product or products
dealt with in this service information by anyone else could result in serious injury or death.

Objective
The objective of this course is to provide the student with information about the
latest technology used in Panasonic’s new line of microwave ovens.
Upon successful completion of this course, the student will learn the differences
between the high voltage circuit used in most microwave ovens, and the Inverter
technology used in this line of Panasonic microwave ovens.
The student will also have a better understanding of the new features and the
function of the major components of the microwave oven.

Table Of Contents
Microwave ...........................................................................................................1
What are Microwaves?......................................................................................1
How do microwaves cook?................................................................................1
Inverter Technology............................................................................................2
Difference Between MWO without Inverter and MWO with Inverter..................2
Major components..............................................................................................4
Magnetron.........................................................................................................4
How to diagnose for an open filament or a shorted magnetron.........................4
Oven cavity .......................................................................................................5
Turntable...........................................................................................................5
Primary/secondary latch switches.....................................................................6
Short switch.......................................................................................................6
Temperature Sensors........................................................................................8
Steam Sensor....................................................................................................9
How to check the steam-sensor function.........................................................10
Inverter Circuit.................................................................................................11
Test and Measuring Procedures......................................................................15
Procedure to check the Inverter using an Amp. Meter ....................................15
Measurement of microwave output .................................................................16
Procedure for measuring microwave energy leakage .....................................17
Safety Tips.........................................................................................................19
Safety tips for operation of microwave ovens..................................................19
Safety tips for installation and maintenance of microwave ovens....................19
Models line-up.................................................................................................20
Features.............................................................................................................21
Functions...........................................................................................................27
Understanding Ionizing & Non-Ionizing Radiation.........................................29
Troubleshooting................................................................................................31
Inverter Circuit and Magnetron Troubleshooting .............................................33
Glossary of Electronic & Microwave Oven Related Terms ...........................34

Microwave
What are Microwaves?
Microwaves, like visible light, are a part of the electromagnetic radiation
spectrum. They are extremely high frequency radio waves. As the frequency of
radiation increases, its wavelength decreases. The very high frequencies
correspond to very short wavelengths; hence the name microwaves. Infrared
radiation, ultraviolet light and X-rays are also electromagnetic radiations, but
have even shorter wavelengths than microwaves.
Microwaves absorption and reflectivity are functions of the matter subjected to
them. Metallic surfaces are good reflectors of microwaves. However, electrically
non-conductive materials allow microwave to pass through them with very little
absorption. All other materials that fall between metals and electrical isolators
absorb microwave at different rates. Materials containing moisture, such as food
and even people, absorb microwave energy. If energy is absorbed at a rate
greater than the rate at which the material looses energy (rate of cooling), its
temperature increases.
How do microwaves cook?
In microwave ovens, magnetrons are used to produce the microwaves. These
microwaves have a frequency of 2,450 MHz, which is transmitted into the
enclosed metal oven cavity. In the cavity, they are reflected by the oven walls
and absorbed in food or drink placed in the oven.
Microwaves continuously reverse the polarity of the food molecules a
tremendous number of times (2,450,000,000 times per second). This causes
molecular agitation and thus friction, which produces heat and results in a rapid
rise in temperature. Cooking time is usually much shorter than in a conventional
oven.
The rate of heating depends on the type of food, its shape, volume, and mass.
The oven walls are not directly heated by microwaves, as they do not absorb
microwave energy. However, the inside of the oven may feel warm due to the
presence of the hot food and the heat generated by the electrical circuits.
1

Inverter Technology
Panasonic uses proprietary Inverter technology in most of its microwave ovens.
Difference Between Traditional and Inverter Microwave Ovens
Traditional microwave ovens
Conventional microwaves ovens use power transformers to increase the
household line voltage (120 VAC at 60 Hz) to a level high enough to operate the
magnetron. The magnetron generates the microwaves that cook the food. This
technique has its drawbacks.
Operating at a low frequency of 60 Hz, the transformer is relatively inefficient:
• Power is lost (through heat dissipation) in converting the line voltage to the
higher magnetron level.
• The transformer operates at a constant power (cooking level) that can only
be changed by switching the power on or off repeatedly.
Inverter Technology
In inverter-equipped microwave ovens, the power transformer is replaced by a
circuit board, which converts the 60Hz incoming line frequency to a variable rate
of 20 KHz to 45 KHz. A relatively small transformer is then required to increase
the voltage to the level required by a magnetron.
By varying the pulse width, the output power can be linearly controlled for more
precise cooking and defrosting levels. The bulky power transformer is replaced
by a small, lightweight circuit board; and, because less heat is dissipated, power
efficiency is increased.
Conventional technology uses just a single power level, which is regulated by
switching pulses. In contrast, inverter technology directly controls the power
output. This constant soft penetration of microwave energy prevents the common
problems of shrinkage, overcooking, and loss of nutrients. The result is even food
temperature and textures throughout.
2

Power Level Comparison
Traditional microwaves send out a single level of power in small bursts to cook
food at different speeds. For example, when set at 60% power, the microwave
energy would be on 60% of the time and idle 40% of the time.
Inverter microwaves, however, give accurate, true multiple power levels. When
you ask for 60% power, the oven delivers 60% power (e.g. they don’t just operate
60% of the time). This applies no matter what power level is selected. True
power levels give you better cooking results and your food would have an even
texture and temperature after cooking. The constant soft penetration of
microwave energy into the center of the food helps prevent overcooking on
edges and surfaces.
Difference Between Pulsing and Linear Power Control
Figure 1
Figure 2
3

Major components.
Magnetron
In a microwave oven, the magnetron is used to produce the high frequency
required for cooking. The frequency of microwaves for general cooking is
2,450MHz.
Magnetron components
Cooling
Fins
Filament Terminals Antenna Gasket
Antenna
Figure 3
How to diagnose for an open filament or a shorted magnetron
To check for open filament:
• Disconnect the filament leads.
• Measure the resistance across the filament terminals. The normal
resistance is approximately 1 ohm or less.
To check for a shorted magnetron:
• Disconnect the filament leads.
• Measure the resistance between each filament terminal and the
magnetron case. Normally it should read open.
Note:
Do not rely on continuity check alone to determine if a magnetron is defective.
Normal continuity reading can be obtained from a magnetron that is defective.
4

Whenever you replace the magnetron, measure for radiation leakage before the
outer panel is installed and after all necessary components are replaced or
adjusted.
Warning:
Special care should be taken in measuring around the magnetron.
Avoid contact with any of the high voltage parts while conducting the radiation
leakage test.
Oven cavity
The cavity is a multimode cavity resonator
designed to resonate the microwaves
emitted from the magnetron. Here is where
the food is placed for cooking or heating.
The oven cavity is made with stainless
steel, aluminum, or painted steel plate to
reflect the microwaves.
Oven Cavity
Figure 4
Turntable
The microwaves come into the oven’s cavity
from the side of the oven to ensure that the
microwave field evenly covers the top, sides,
and bottom of the food. The turntable then
exposes all parts of the food to the field, for
perfect results every time.
Turntable
Figure 5
5

Primary / secondary latch switches
These are safety switches, and their basic function is to interrupt the power
supply to the magnetron when the door starts to open. They are open when the
door is opened.
Switch position
Switch Door Opened Door Closed
Primary Open Close
Secondary Open Close
Table 1
Short switch
This switch is used to prevent the operation of the microwave oven while the
door is opened.
The short switch creates a short circuit to blow the line fuse and stop microwave
oven’s operation if the door switches fail to open when the door is opened.
This switch is normally open when the door is closed.
Switches Location
Primary Switch
Short Switch (Lower)
Secondary Switch (Upper)
Side view of Mic
rowave Oven
Figure 6
Note: When the fuse is blown due to operation of the short switch, replace the
primary switch, the short switch, the secondary switch, and the power relay.
6

Adjustment of the primary latch switch, the secondary latch switch,
and the short switch
1. Mount the primary latch switch, the secondary latch switch, and the short
switch to the door hook assembly as shown in the figure 7.
NOTE:No specific individual adjustments during installation of the primary
latch switch, secondary latch switch or short switch to the door hook are
required.
2. When mounting the door hook assembly to the oven assembly, adjust the
door hook assembly by moving it in the direction of the arrows in figure 7,
so that the oven door will not have any play in it.
3. Check for play in the door by pulling the door assembly.
4. Make sure that the latch keys move smoothly after adjustment is
completed. Tighten the screws holding the door hook assembly to the
oven assembly.
5. Reconnect the short switch and check the continuity of the monitor circuit
and all latch switches again by following the component test procedures.
Interlock switches
Switch
Actuator
The gap should be
less than 0.7mm
Figure 7
7

Temperature Sensors
This microwave oven uses 2 different types of temperature sensors. One is a
thermal cutout sensor located on top of the oven cavity, and the other is a
thermistor, which is mounted on the side of the magnetron. See Figure 8.
The thermal cutout sensor is used to stop the flow of AC to the oven, if the cavity
surface overheats for any reason.
The thermistor will shut the oven down and reset the display when the magnetron
overheats and reaches a temperature of 257° F.
The cooking program can be re-started after the magnetron cools down.
The value of this thermistor is 30K to 120K at 50°F ~ 86°F.
Temperature Sensors Location
Thermal cutoff
Thermistor
Figure 8
8

Steam Sensor
This sensor detects the presence of steam emitted by the food being heated and
then, based on how long it took to reach the steam stage, it gauges how much
longer it should cook, before shutting off.
The steam sensor works just like the effect called piezoelectricity. The
piezoelectricity effect generates electricity when mechanical shock is applied to
the general dielectrics material.
In the case of the steam sensor, this effect is called pyroelectricity, where
electricity is generated when heat shock (hot steam) is applied to the general
dielectrics material. See the figure below.
Principle of Steam Sensor
Dielectrics (Pyroelectricitic) Material
Steam
Sensor
Figure 9
When the food is heated by the microwave oven, the food temperature gradually
increases and steam is generated from the food. The steam sensor, which is
located near the cooling fan, detects the steam from the food.
The fan keeps one side of the sensor cool and the other side, which has the
element, receives and feels the hot steam from the oven cavity.
Steam Sensor Location
Figure 10
9

Side view of the microwave oven illustrating the steam operation
Door
Hot Steam
Turntable
Fan
Steam Sensor
Figure 11
How to check the steam-sensor function.
To determine if this function is working ok, do the following:
1. Place a water load of 150 cc in the oven.
2. Press the sensor re-heat pad.
3. Press start.
Note: Steam is normally detected approximately 1.5 to 4 minutes after
pressing power. This period is named “The detection period T1”.
After going through this period, the unit automatically jumps to another
period called “The remaining cooking time period T2”. The T2 time ranges
from 8 seconds to 23 seconds.
4. “The steam sensor function” is normal, if after jumping to the T2 period,
the “remaining cooking time” (8sec. ~ 23 sec.) appears in the display
window.
10

Inverter Circuit
For information about the inverter power supply turn to pages 2 and 3.….
Inverter Power supply
CN702
AC Input
CN701
Control
Signals line
High Voltage
Transformer
CN703
High Voltage
output to the
magnetron
Heat sink
Figure 12
11

Inverter Power Supply Circuit Explanation
The inverter power supply circuit uses the AC line 120V, 60Hz to supply 4,000V
DC to the magnetron tube.
The AC input voltage is rectified by the bridge rectifier DB701
DC voltage is applied to the Switching IGBT (Insulated Gate Bipolar Transistor)
circuit.
Note: The IGBT is a cross between the bipolar and MOSFET transistors.
The high voltage transformer is driven by a PWM (Pulse Width Modulated) signal
generated by the microprocessor in the DPC (Digital Programmer Circuit). The
transformer is a component in the resonance circuit of the oscillator. Therefore,
a change in load or the power level affects the frequency of the drive signal.
Typically, the frequency ranges between 20KHz to 40KHz.
The high voltage transformer generates approximately 2,000V DC or more in the
secondary winding and approximately 3V AC in the filament winding.
The half wave rectifier circuit, (D701, D702) generates the necessary 4,000V DC
needed to drive the magnetron.
A signal from the current-sensing transformer CT701 in the inverter circuit is
used to monitor the power output from the magnetron. This signal is applied to
the microprocessor in the DPC to determine the working condition and the output
necessary to control the PWM signal supplied to the inverter to control the power
output.
Warning:
It’s neither necessary nor advisable to attempt measurement of the high voltage.
12

Inverter Power Supply Circuit
IGBT
Circui
DB701
D702
D701
CT701
Figure 13
Warning
1. Always unplug the microwave oven from the electricity supply, before
removing the outer panel.
2. Never touch the inverter PCB with the microwave oven plugged into the
electricity supply. The inverter circuit board handles voltages up to 5000
volts and is very dangerous.
3. Do not touch the heat sink during operation of the microwave oven. The
heat sink handles high voltage and becomes very hot.
4. Always discharge the high voltage capacitors located on the inverter
circuit board before beginning any troubleshooting.
5. Only test the inverter circuit board by installing it completely into the oven
and refitting the outer panel.
6. Always connect the inverter circuit to earth via the earth plate. It is very
dangerous to operate the inverter circuit when it is not connected to earth.
13

Inverter Power Supply
Figure 14
14

Test and Measuring Procedures
Procedure to check the Inverter using an Ammeter
Equipment needed:
1-liter beaker
An AC Ammeter.
1. Place the beaker with one liter of water into the oven cavity.
2. Unplug the 2 pins high voltage connector from the plug CN703 on the
Inverter power supply.
3. Set the oven at high power for 1 minute and press start. The Oven
operates for approximately 15 seconds and then it stops showing the error
code H98.
During operation, the AC line current should be between 1A and 1.7A.
4. Unplug the 3 pins connector from CN701 on the inverter power supply.
5. Set the oven at high power for 1 minute and press start. The Oven
operates for approximately 27seconds and then it stops showing the error
code H97.
During operation, the AC line current should be between 0.4A and 0.8A.
15