Showing posts with label MICROWAVE OVEN. Show all posts
Showing posts with label MICROWAVE OVEN. Show all posts
Tuesday, June 28, 2016
PANASONIC NN-SE992S - NN-SD982S - NN-ST962S – Inverter power supply working – Testing magnetron – Measuring the microwave output – Energy leakage test – Troubleshooting – Microwave oven repair and service
Category: Microwave Oven Repair and Service
Contents of this article
- Measuring the microwave output
- Measuring energy level
- Magnetron testing
PANASONIC NN-SE992S - NN-SD982S - NN-ST962S
Inverter power supply circuit working
The Inverter Power Supply circuit powered from the line voltage, 120V 60Hz 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 120V 60Hz 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.
The Inverter Power Supply circuit powered from the line voltage, 120V 60Hz 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 120V 60Hz 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.
COMPONENT TEST
Primary, Secondary Interlock Switch & Power Relay RY1
1. Unplug lead connectors to Power Relay RY1 and verify open circuit of the Power Relay RY1 1-2 terminals.
2. Unplug lead connectors to Primary Interlock Switch and Secondary Interlock Switch.
3. Test the continuity of switches at door opened and closed positions with ohm meter (low scale). Normal continuity readings should be as follows.
1. Unplug lead connectors to Power Relay RY1 and verify open circuit of the Power Relay RY1 1-2 terminals.
2. Unplug lead connectors to Primary Interlock Switch and Secondary Interlock Switch.
3. Test the continuity of switches at door opened and closed positions with ohm meter (low scale). Normal continuity readings should be as follows.
Primary Interlock Switch 0Ω (Close) Ω(Open)
Secondary Interlock Switch 0Ω (Close) Ω(Open)
Power Relay RY1 Ω (Open) Ω(Open)
Power Relay RY1 Ω (Open) Ω(Open)
Monitor Switch
1. Unplug lead wires from Inverter Power Supply (U) primary
terminals.
2. Connect test probes of ohm meter to the disconnected leads that were connected to Inverter Power Supply (U).
3. Test the continuity of Monitor Switch with door opened and closed positions using lowest scale of the ohm meter. Normal continuity readings should be as follows.
1. Unplug lead wires from Inverter Power Supply (U) primary
terminals.
2. Connect test probes of ohm meter to the disconnected leads that were connected to Inverter Power Supply (U).
3. Test the continuity of Monitor Switch with door opened and closed positions using lowest scale of the ohm meter. Normal continuity readings should be as follows.
Door Opened Door Closed
0Ω (Close) Ω (Open)
0Ω (Close) Ω (Open)
Magnetron
Continuity checks can only indicate an open filament or a shorted magnetron. To diagnose for an open filament or shorted magnetron.
1. Isolate magnetron from the circuit by disconnecting the
leads.
2. A continuity check across magnetron filament terminals should indicate one ohm or less.
3. A continuity check between each filament terminal and magnetron case should read open.
Continuity checks can only indicate an open filament or a shorted magnetron. To diagnose for an open filament or shorted magnetron.
1. Isolate magnetron from the circuit by disconnecting the
leads.
2. A continuity check across magnetron filament terminals should indicate one ohm or less.
3. A continuity check between each filament terminal and magnetron case should read open.
Key board membrane (Membrane switch assembly)
Check continuity between switch terminals, by tapping an appropriate pad on the key board. The contacts assignment of the respective pads on the key board is as shown in digital programmer circuit.
Check continuity between switch terminals, by tapping an appropriate pad on the key board. The contacts assignment of the respective pads on the key board is as shown in digital programmer circuit.
Inverter power supply
DO NOT try to REPAIR H.V. Inverter power supply (U).
Replace complete H.V. Inverter(U) Unit.
DO NOT try to REPAIR H.V. Inverter power supply (U).
Replace complete H.V. Inverter(U) Unit.
WARNING: HIGH VOLTAGE
Test if failure codes H95, H97 or H98 appear when performing the following procedure. It is recommended to use an AC line input current ammeter for testing.
Test 1
1. With the oven unit’s AC power supply cord is unplugged from the wall outlet, unplug the 2 pin H.V. connector CN703 from the magnetron tube.
2. Place 1 liter of water load into oven cavity.
3. Plug in the oven’s AC power supply cord into outlet.
4. Program DPC.
a. Press Timer/Clock button twice.
b. Press Start button once.
c. Press Power Level button once.
5. Program oven at High power for 1 minute and press [Start] button.
a. After approximately 23 seconds, oven stops operating.
b. During oven operation, the input current is approximately 0.5 to 1A. If both a and b are OK, proceed to test 2.
Test if failure codes H95, H97 or H98 appear when performing the following procedure. It is recommended to use an AC line input current ammeter for testing.
Test 1
1. With the oven unit’s AC power supply cord is unplugged from the wall outlet, unplug the 2 pin H.V. connector CN703 from the magnetron tube.
2. Place 1 liter of water load into oven cavity.
3. Plug in the oven’s AC power supply cord into outlet.
4. Program DPC.
a. Press Timer/Clock button twice.
b. Press Start button once.
c. Press Power Level button once.
5. Program oven at High power for 1 minute and press [Start] button.
a. After approximately 23 seconds, oven stops operating.
b. During oven operation, the input current is approximately 0.5 to 1A. If both a and b are OK, proceed to test 2.
Unplug CN703 0.5 to 1A Oven stops in 23 seconds after started.
Test 2
Continued from Test 1
1. Unplug the oven’s AC power supply cord from outlet.
2. Unplug 3 pin connector CN701. CN703 remains unplugged.
3. Plug in the oven’s AC power supply cord into outlet.
4. Program DPC.
a. Press Timer/Clock button twice.
b. Press Start button once.
c. Press Power Level button once.
Continued from Test 1
1. Unplug the oven’s AC power supply cord from outlet.
2. Unplug 3 pin connector CN701. CN703 remains unplugged.
3. Plug in the oven’s AC power supply cord into outlet.
4. Program DPC.
a. Press Timer/Clock button twice.
b. Press Start button once.
c. Press Power Level button once.
Measurement of microwave output
The output power of the magnetron can be determined by performing IEC standard test procedures. However, due to the complexity of IEC test procedures, it is recommended to test the magnetron using the simple method outlined below.
The output power of the magnetron can be determined by performing IEC standard test procedures. However, due to the complexity of IEC test procedures, it is recommended to test the magnetron using the simple method outlined below.
Necessary Equipment:
*1 liter beaker *Glass thermometer
*Wrist watch or stopwatch
NOTE:
Check the line voltage under load. Low voltage will lower the magnetron output. Take the temperature readings and heating time as accurately as possible.
1. Fill the beaker with exactly one liter of tap water. Stir the water using the thermometer and record the water’s temperature. (recorded as T1).
2. Place the beaker on the center of glass tray. Set the oven for High power and heat it for exactly one minute.
3. Stir the water again and read the temperature of the water. (recorded as T2).
4. The normal temperature rise at High power level for each model is as shown in table.
*1 liter beaker *Glass thermometer
*Wrist watch or stopwatch
NOTE:
Check the line voltage under load. Low voltage will lower the magnetron output. Take the temperature readings and heating time as accurately as possible.
1. Fill the beaker with exactly one liter of tap water. Stir the water using the thermometer and record the water’s temperature. (recorded as T1).
2. Place the beaker on the center of glass tray. Set the oven for High power and heat it for exactly one minute.
3. Stir the water again and read the temperature of the water. (recorded as T2).
4. The normal temperature rise at High power level for each model is as shown in table.
1200W Min. 18.5°F(10.3°C)
MEASURING MICROWAVE ENERGY LEAKAGE
Equipment
# Electromagnatic radiation monitor
# Glass thermometer 212°F or 100°C
# 600cc glass beaker
# Electromagnatic radiation monitor
# Glass thermometer 212°F or 100°C
# 600cc glass beaker
Procedure for measuring radiation leakage
Note before measuring:
# Do not exceed meter full scale deflection. Leakage monitor should initially be set to the highest scale.
# To prevent false readings, the test probe should be held by the grip portion of the handle only and moved along the shaded area in Figure no faster than 1 inch/sec (2.5cm/sec).
# Leakage with the outer panel removed: less than 5mW/cm2.
# Leakage for a fully assembled oven with door normally closed: less than 2mW/cm 2.
# Leakage for a fully assembled oven [Before the latch switch (primary) is interrupted] while pulling the door: less than 2mW/cm 2.
1. Pour 275 ± 15cc (9ozss± 1/2oz) of 20°C ± 5°C (68° ± 9°F) water in a beaker which is graduated to 600cc, and place in the center of the oven.
2. Set the radiation monitor to 2450MHz and use it following the manufacturer´s recommended test procedure to assure correct results.
3. When measuring the leakage, always use the 2 inch (5cm) spacer supplied with the probe.
4. Tap the start button or set the timer and with the magnetron oscillating, measure the leakage by holding the probe perpendicular to the surface being measured.
Note before measuring:
# Do not exceed meter full scale deflection. Leakage monitor should initially be set to the highest scale.
# To prevent false readings, the test probe should be held by the grip portion of the handle only and moved along the shaded area in Figure no faster than 1 inch/sec (2.5cm/sec).
# Leakage with the outer panel removed: less than 5mW/cm2.
# Leakage for a fully assembled oven with door normally closed: less than 2mW/cm 2.
# Leakage for a fully assembled oven [Before the latch switch (primary) is interrupted] while pulling the door: less than 2mW/cm 2.
1. Pour 275 ± 15cc (9ozss± 1/2oz) of 20°C ± 5°C (68° ± 9°F) water in a beaker which is graduated to 600cc, and place in the center of the oven.
2. Set the radiation monitor to 2450MHz and use it following the manufacturer´s recommended test procedure to assure correct results.
3. When measuring the leakage, always use the 2 inch (5cm) spacer supplied with the probe.
4. Tap the start button or set the timer and with the magnetron oscillating, measure the leakage by holding the probe perpendicular to the surface being measured.
TROUBLESHOOTING
H97 & H98 error code display
If 3 times H97 or 2 times H98 exist, microwave oven can not be used any more, even if the defective parts already be replaced & un-plug and plug-in again.
How to reset for the service:
Please take the following steps
1. Plug-in oven.
2. Press Stop/Reset button once.
3. Open the door.
4. Keep pressing Start button for more than 2 seconds until buzzer beeps.
5. Press Stop/Reset button three times, oven will show the total usage time of magnetron.
6. Press Power Level button once, oven will show the total number of occurrence of H97 & H98.
If 3 times H97 or 2 times H98 exist, microwave oven can not be used any more, even if the defective parts already be replaced & un-plug and plug-in again.
How to reset for the service:
Please take the following steps
1. Plug-in oven.
2. Press Stop/Reset button once.
3. Open the door.
4. Keep pressing Start button for more than 2 seconds until buzzer beeps.
5. Press Stop/Reset button three times, oven will show the total usage time of magnetron.
6. Press Power Level button once, oven will show the total number of occurrence of H97 & H98.
7. Keep pressing Start button for more than 2 seconds (until buzzer beeps) to reset.
8. While finish resetting, one beeps, and show “end”.
9. Press Stop/Reset to return to initial stage.
Magnetron usage time display
Oven has magnetron usage time display function but it will not be activated in normal operation mode.
To show magnetron usage time result, please take the following steps:
1. Plug-in oven.
2. Press Stop/Reset button once.
3. Open the door.
4. Keep pressing Start button for more than 2 seconds until buzzer beeps.
5. Press Stop/Reset button three times, oven will show the total usage time of magnetron.
(note1:the magnetron usage time is kept in L.S.I. memory.)
(note2:Magnetron usage time display is in hours.)
NOTE:
To delete the recorder of L.S.I. memory.
a. Press Start button for more than 2 seconds until buzzer beeps.
b. While finish deleting, one beeps and show “end”.
c. Press Stop/Reset to return to initial stage.
8. While finish resetting, one beeps, and show “end”.
9. Press Stop/Reset to return to initial stage.
Magnetron usage time display
Oven has magnetron usage time display function but it will not be activated in normal operation mode.
To show magnetron usage time result, please take the following steps:
1. Plug-in oven.
2. Press Stop/Reset button once.
3. Open the door.
4. Keep pressing Start button for more than 2 seconds until buzzer beeps.
5. Press Stop/Reset button three times, oven will show the total usage time of magnetron.
(note1:the magnetron usage time is kept in L.S.I. memory.)
(note2:Magnetron usage time display is in hours.)
NOTE:
To delete the recorder of L.S.I. memory.
a. Press Start button for more than 2 seconds until buzzer beeps.
b. While finish deleting, one beeps and show “end”.
c. Press Stop/Reset to return to initial stage.
Wednesday, June 22, 2016
PANASONIC Microwave oven NN-CS596A - NN-CS596S - Troubleshooting - Test procedure - Magnetron test - Output power test - self diagnostic
Category: Microwave Oven Repair and Service
Contents of this article
- Magnetron test
- Output Power test
- Self diagnostics
PANASONIC NN-CS596A - NN-CS596S
CAUTIONS TO BE OBSERVED WHEN TROUBLESHOOTING
Unlike many other appliances, the microwave oven is a high voltage, high current device. It is free from danger in ordinary use, though extreme care should be taken during repair.
CAUTION
Servicemen should remove their watches whenever working close to or replacing the magnetron.
Check the grounding
Do not operate on a two wire extension cord. The microwave oven is designed to be grounded when used. It is imperative, therefore, to ensure the appliance is properly grounded before beginning repair work.
Inverter warnings
DANGER, HIGH VOLTAGE AND HIGH TEMPERATURE (HOT/LIVE) OF THE INVERTER POWER SUPPLY
The High Voltage Inverter Power Supply handles very high voltage and current for the magnetron tube. Though it is free from danger in ordinary use, extreme care should be taken during repair.
The aluminum heat sink is also energized with high voltage (HOT), do not touch when the AC input terminals are energized. The power device Collector is directly connected to the aluminum heat sink. The aluminum heat sink may be HOT due to heat energy, therefore, extreme care should be taken during servicing.
CAUTION
Servicemen should remove their watches whenever working close to or replacing the magnetron.
Check the grounding
Do not operate on a two wire extension cord. The microwave oven is designed to be grounded when used. It is imperative, therefore, to ensure the appliance is properly grounded before beginning repair work.
Inverter warnings
DANGER, HIGH VOLTAGE AND HIGH TEMPERATURE (HOT/LIVE) OF THE INVERTER POWER SUPPLY
The High Voltage Inverter Power Supply handles very high voltage and current for the magnetron tube. Though it is free from danger in ordinary use, extreme care should be taken during repair.
The aluminum heat sink is also energized with high voltage (HOT), do not touch when the AC input terminals are energized. The power device Collector is directly connected to the aluminum heat sink. The aluminum heat sink may be HOT due to heat energy, therefore, extreme care should be taken during servicing.
WARNING FOR INVERTER POWER SUPPLY GROUNDING
Check the High Voltage Inverter Power Supply circuit grounding. The high voltage inverter power supply circuit board must have a proper chassis ground. The inverter grounding bracket must be connected to the chassis. If the inverter board is not grounded it will expose the user to very high voltages and cause extreme DANGER! Be sure that the inverter circuit is properly grounded via the inverter grounding bracket.
Check the High Voltage Inverter Power Supply circuit grounding. The high voltage inverter power supply circuit board must have a proper chassis ground. The inverter grounding bracket must be connected to the chassis. If the inverter board is not grounded it will expose the user to very high voltages and cause extreme DANGER! Be sure that the inverter circuit is properly grounded via the inverter grounding bracket.
WARNING DISCHARGE THE HIGH VOLATGE CAPACITORS
For about 30 seconds after the oven is turned off, an electric charge remains in the high voltage capacitors in the Inverter Power Supply circuit board. When replacing or checking parts, remove the power plug from the outlet and short the inverter output terminal of the magnetron filament terminals to the chassis ground with an insulated handle screwdriver to discharge. Please be sure to touch the chassis ground side first and then short to the output terminals.
For about 30 seconds after the oven is turned off, an electric charge remains in the high voltage capacitors in the Inverter Power Supply circuit board. When replacing or checking parts, remove the power plug from the outlet and short the inverter output terminal of the magnetron filament terminals to the chassis ground with an insulated handle screwdriver to discharge. Please be sure to touch the chassis ground side first and then short to the output terminals.
Discharging the high voltage capacitors
WARNING
There is high voltage present with high current capabilities in the circuits of the primary and secondary windings, choke coil and heat sink of the inverter. It is extremely dangerous to work on or near these circuits with the oven energized. DO NOT measure the voltage in the high voltage circuit including the filament voltage of the magnetron. WARNING
Never touch any circuit wiring with your hand or with an insulated tool during operation.
Confirm before repair
1. Before repair or replacement of parts, ensure to take out the water tank from microwave oven. Further more, use drainage function, to drain the water remaining in the water pipes and tubes into oven cavity.
2. Wipe up the oven cavity.
WARNING
Before beginning repair work, make sure that there is no water in microwave oven, otherwise the water might invade the electric parts and will cause short circuit even result in electric shock.
Part replacement
When any part or component is to be replaced, always ensure that the power cord is removed from the wall outlet.
When the 10A fuse is blown due to the operation of the short switch
WARNING
When the 10A 220V fuse is blown due to the operation of the interlock monitor switch, replace all of the components (primary latch switch, short switch and power relay B (RY1)).
1. This is mandatory. Refer to “adjustments and measurements” for the location of these switches.
2. When replacing the fuse, confirm that it has the appropriate rating for these models.
3. When replacing faulty switches, be sure the mounting tabs are not bent, broken or deficient in their ability to hold the switches.
Avoid inserting nails, wire etc. through any holes in the unit during operation
Never insert a wire, nail or any other metal object through the lamp holes on the cavity or any holes or gaps, because such objects may work as an antennaand cause microwave leakage.
WARNING
There is high voltage present with high current capabilities in the circuits of the primary and secondary windings, choke coil and heat sink of the inverter. It is extremely dangerous to work on or near these circuits with the oven energized. DO NOT measure the voltage in the high voltage circuit including the filament voltage of the magnetron. WARNING
Never touch any circuit wiring with your hand or with an insulated tool during operation.
Confirm before repair
1. Before repair or replacement of parts, ensure to take out the water tank from microwave oven. Further more, use drainage function, to drain the water remaining in the water pipes and tubes into oven cavity.
2. Wipe up the oven cavity.
WARNING
Before beginning repair work, make sure that there is no water in microwave oven, otherwise the water might invade the electric parts and will cause short circuit even result in electric shock.
Part replacement
When any part or component is to be replaced, always ensure that the power cord is removed from the wall outlet.
When the 10A fuse is blown due to the operation of the short switch
WARNING
When the 10A 220V fuse is blown due to the operation of the interlock monitor switch, replace all of the components (primary latch switch, short switch and power relay B (RY1)).
1. This is mandatory. Refer to “adjustments and measurements” for the location of these switches.
2. When replacing the fuse, confirm that it has the appropriate rating for these models.
3. When replacing faulty switches, be sure the mounting tabs are not bent, broken or deficient in their ability to hold the switches.
Avoid inserting nails, wire etc. through any holes in the unit during operation
Never insert a wire, nail or any other metal object through the lamp holes on the cavity or any holes or gaps, because such objects may work as an antennaand cause microwave leakage.
Confirm after repair
1. After repair or replacement of parts, make sure that the screws of the oven, etc. are neither loose or missing. Microwave might leak if screws are not properly tightened.
2. Make sure that all electrical connections are tight before inserting the plug into the wall outlet.
3. Check for microwave energy leakage. (Refer to procedure for measuring microwave energy leakage).
CAUTION MICROWAVE RADIATION USE CAUTION NOT TO BECOME EXPOSED TO RADIATION FROM THE MICROWAVE MAGNETRON OR OTHER PARTS CONDUCTING MICROWAVE ENERGY. IMPORTANT NOTICE
The following components have potentials above 2000V while the appliance is operated.
# Magnetron
# High voltage transformer (Located on inverter (U))
# High voltage diodes (Located on inverter (U))
# High voltage capacitors (Located on inverter (U))
Pay special attention to these areas.
When the appliance is operated with the door hinges or magnetron installed incorrectly, the microwave leakage can exceed more than 5mW/cm2. After repair or exchange, it is very important to check if the magnetron and the door hinges are correctly installed.
IMPORTANT NOTICE
After repair or replacement of parts, make sure that all the water pipes and tubes are properly connected, otherwise the water might invade the electric parts and will cause short circuit even result in electric shock.
1. After repair or replacement of parts, make sure that the screws of the oven, etc. are neither loose or missing. Microwave might leak if screws are not properly tightened.
2. Make sure that all electrical connections are tight before inserting the plug into the wall outlet.
3. Check for microwave energy leakage. (Refer to procedure for measuring microwave energy leakage).
CAUTION MICROWAVE RADIATION USE CAUTION NOT TO BECOME EXPOSED TO RADIATION FROM THE MICROWAVE MAGNETRON OR OTHER PARTS CONDUCTING MICROWAVE ENERGY. IMPORTANT NOTICE
The following components have potentials above 2000V while the appliance is operated.
# Magnetron
# High voltage transformer (Located on inverter (U))
# High voltage diodes (Located on inverter (U))
# High voltage capacitors (Located on inverter (U))
Pay special attention to these areas.
When the appliance is operated with the door hinges or magnetron installed incorrectly, the microwave leakage can exceed more than 5mW/cm2. After repair or exchange, it is very important to check if the magnetron and the door hinges are correctly installed.
IMPORTANT NOTICE
After repair or replacement of parts, make sure that all the water pipes and tubes are properly connected, otherwise the water might invade the electric parts and will cause short circuit even result in electric shock.
COMPONENT TEST PROCEDURE
Primary latch switch (door switch and power relay B) interlocks
1. Unplug lead connectors to Power Relay B and verify open circuit of the power relay B 1-2 terminals.
2. Unplug lead connectors to Primary Latch Switch and Door Switch.
3. Test the continuity of switches at door opened and closed positions with ohm meter (low scale). Normal continuity readings should be as follows.
1. Unplug lead connectors to Power Relay B and verify open circuit of the power relay B 1-2 terminals.
2. Unplug lead connectors to Primary Latch Switch and Door Switch.
3. Test the continuity of switches at door opened and closed positions with ohm meter (low scale). Normal continuity readings should be as follows.
| | Door Closed | Door Opened |
| Primary Latch switch | 0 ohm (Close) | Infinite ohm (Open) |
| Door Switch | 0 ohm (Close) | Infinite ohm (Open) |
| Power Relay B | (Close) | Infinite ohm (Open) |
Short switch & monitor
1. Unplug lead wires from Inverter Power Supply primary terminals.
2. Connect test probes of ohm meter to the disconnected leads that were connected to Inverter Power Supply .
3. Test the continuity of short switch with door opened and closed positions using lowest scale of the ohm meter. Normal continuity readings should be as follows.
2. Connect test probes of ohm meter to the disconnected leads that were connected to Inverter Power Supply .
3. Test the continuity of short switch with door opened and closed positions using lowest scale of the ohm meter. Normal continuity readings should be as follows.
Door open 0 ohms
Door closed infinite ohms
Magnetron
Continuity checks can only indicate an open filament or a shorted magnetron. To diagnose for an open filament or shorted magnetron.
1. Isolate magnetron from the circuit by disconnecting the leads.
2. A continuity check across magnetron filament terminals should indicate one ohm or less.
3. A continuity check between each filament terminal and magnetron case should read open.
Continuity checks can only indicate an open filament or a shorted magnetron. To diagnose for an open filament or shorted magnetron.
1. Isolate magnetron from the circuit by disconnecting the leads.
2. A continuity check across magnetron filament terminals should indicate one ohm or less.
3. A continuity check between each filament terminal and magnetron case should read open.
Measurement of microwave output
The output power of the magnetron can be determined by performing IEC standard test procedures. However, due to the complexity of IEC test procedures, it is recommended to test the magnetron using the simple method outlined below.
The output power of the magnetron can be determined by performing IEC standard test procedures. However, due to the complexity of IEC test procedures, it is recommended to test the magnetron using the simple method outlined below.
Necessary Equipment:
# 1 litre beaker
# Glass thermometer
# Wrist watch or stopwatch
NOTE:
Check the line voltage under load. Low voltage will lower the magnetron output. Take the temperature readings and heating time as accurately as possible.
1. Fill the beaker with exactly one litre of tap water. Stir the water using the thermometer and record the water’s temperature. (recorded as T1).
2. Place the beaker on the center of glass tray. Set the oven for High power and heat it for exactly one minute.
3. Stir the water again and read the temperature of the water. (recorded as T2).
4. The normal temperature rise at High power level for each model is as shown bellow.
# 1 litre beaker
# Glass thermometer
# Wrist watch or stopwatch
NOTE:
Check the line voltage under load. Low voltage will lower the magnetron output. Take the temperature readings and heating time as accurately as possible.
1. Fill the beaker with exactly one litre of tap water. Stir the water using the thermometer and record the water’s temperature. (recorded as T1).
2. Place the beaker on the center of glass tray. Set the oven for High power and heat it for exactly one minute.
3. Stir the water again and read the temperature of the water. (recorded as T2).
4. The normal temperature rise at High power level for each model is as shown bellow.
RATED OUTPUT - 1000W
TEMPERATURE RISE - Min.8.5°C
Self diagnostic display
Oven has self diagnostic function but it will not be activated in normal operation mode.
To show self diagnostic result, please take the following steps.
1. Firstly, you must program the DPC into TEST MODE (Plug-in oven > press Timer/Clock pad twice > press Start pad once >press Micro Power pad once.)
2. Keep pressing Timer/Clock pad for more than 2 seconds until buzzer beeps.
3. Press Start pad twice, oven will show error code.
NOTE:
1. If any error was observed, it will be kept in memory up to 3 errors in the past. If there are more than 4 cases, the memory will renew the latest 3 errors codes.
2. Press Start pad again, one more older error code will be displayed.
3. If the oven is ok, it will show “000” and blinking.
4. Error cod list
H** Hardware problem, oven itself has problem.
U** Usage problem such as run out of water and oven itself works well.
Oven has self diagnostic function but it will not be activated in normal operation mode.
To show self diagnostic result, please take the following steps.
1. Firstly, you must program the DPC into TEST MODE (Plug-in oven > press Timer/Clock pad twice > press Start pad once >press Micro Power pad once.)
2. Keep pressing Timer/Clock pad for more than 2 seconds until buzzer beeps.
3. Press Start pad twice, oven will show error code.
NOTE:
1. If any error was observed, it will be kept in memory up to 3 errors in the past. If there are more than 4 cases, the memory will renew the latest 3 errors codes.
2. Press Start pad again, one more older error code will be displayed.
3. If the oven is ok, it will show “000” and blinking.
4. Error cod list
H** Hardware problem, oven itself has problem.
U** Usage problem such as run out of water and oven itself works well.
Monday, June 20, 2016
PANASONIC Microwave oven NN-GD576 - NN-GT546 - NN-SD556 - NN-ST556 – Measurement and Adjustment procedure - Microwave Oven Repair and service
Category: Microwave over Repair and Service
Contents of this article
- Safety Precisions
- Measurement and Adjustment
PANASONIC NN-GD576 - NN-GT546 - NN-SD556 - NN-ST556
CAUTIONS TO BE OBSERVED WHEN TROUBLESHOOTING
Unlike many other appliances, the microwave oven is a high voltage, high current device. It is free from danger in ordinary use, though extreme care should be taken during repair.
Caution
Servicemen should remove their watches whenever working close to or replacing the magnetron.
Check the grounding
Do not operate on a two wire extension cord. The microwave oven is designed to be grounded when used. It is imperative, therefore, to ensure the appliance is properly grounded before beginning repair work.
Inverter warnings
DANGER, HIGH VOLTAGE AND HIGH TEMPERATURE (HOT/LINE) OF THE INVERTER POWER SUPPLY
The high voltage inverter power supply handles very high voltage and current for the magnetron tube. Though it is free from danger in ordinary use, extreme care should be taken during repair.
The aluminum heat sink is also energized with high voltage (HOT), do not touch when the AC input terminals are energized. The power device Collector is directly connected to the aluminum heat sink. The aluminum heat sink may be HOT due to heat energy, therefore, extreme care should be taken during servicing.
Caution
Servicemen should remove their watches whenever working close to or replacing the magnetron.
Check the grounding
Do not operate on a two wire extension cord. The microwave oven is designed to be grounded when used. It is imperative, therefore, to ensure the appliance is properly grounded before beginning repair work.
Inverter warnings
DANGER, HIGH VOLTAGE AND HIGH TEMPERATURE (HOT/LINE) OF THE INVERTER POWER SUPPLY
The high voltage inverter power supply handles very high voltage and current for the magnetron tube. Though it is free from danger in ordinary use, extreme care should be taken during repair.
The aluminum heat sink is also energized with high voltage (HOT), do not touch when the AC input terminals are energized. The power device Collector is directly connected to the aluminum heat sink. The aluminum heat sink may be HOT due to heat energy, therefore, extreme care should be taken during servicing.
WARNING FOR INVERTER POWER SUPPLY GROUNDING
Check the high voltage inverter power supply circuit grounding. The high voltage inverter power supply circuit board must have a proper chassis ground. The inverter grounding bracket must be connected to the chassis. If the inverter board is not grounded it will expose the user to very high voltages and cause extreme DANGER! Be sure that the inverter circuit is properly grounded via the inverter earth bracket.
Check the high voltage inverter power supply circuit grounding. The high voltage inverter power supply circuit board must have a proper chassis ground. The inverter grounding bracket must be connected to the chassis. If the inverter board is not grounded it will expose the user to very high voltages and cause extreme DANGER! Be sure that the inverter circuit is properly grounded via the inverter earth bracket.
WARNING DISCHARGE THE HIGH VOLATGE CAPACITORS
For about 30 seconds after the oven is turned off, an electric charge remains in the high voltage capacitors in the inverter power supply circuit board. When replacing or checking parts, remove the power plug from the outlet and short the inverter output terminal of the magnetron filament terminals to the chassis ground with an insulated handle screwdriver to discharge. Please be sure to touch the chassis ground side first and then short to the output terminals.
For about 30 seconds after the oven is turned off, an electric charge remains in the high voltage capacitors in the inverter power supply circuit board. When replacing or checking parts, remove the power plug from the outlet and short the inverter output terminal of the magnetron filament terminals to the chassis ground with an insulated handle screwdriver to discharge. Please be sure to touch the chassis ground side first and then short to the output terminals.
WARNING
There is high voltage present with high current capabilities in the circuits of the primary and secondary windings, choke coil and heat sink of the inverter. It is extremely dangerous to work on or near these circuits with the oven energized.
DO NOT measure the voltage in the high voltage circuit including the filament voltage of the magnetron.
WARNING
Never touch any circuit wiring with your hand or with an insulated tool during operation
There is high voltage present with high current capabilities in the circuits of the primary and secondary windings, choke coil and heat sink of the inverter. It is extremely dangerous to work on or near these circuits with the oven energized.
DO NOT measure the voltage in the high voltage circuit including the filament voltage of the magnetron.
WARNING
Never touch any circuit wiring with your hand or with an insulated tool during operation
Part replacement.
When any part or component is to be replaced, always ensure that the power cord is removed from the wall outlet.
When the 10A fuse is blown due to the operation of the short switch:
WARNING
When the 10A 250V fuse is blown due to the operation of the interlock monitor switch, replace all of the components (primary latch switch, secondary latch switch, short switch and power relay B (RY1)).
1. This is mandatory. Refer to “adjustments and measurements” for the location of these switches.
2. When replacing the fuse, confirm that it has the appropriate rating for these models.
3. When replacing faulty switches, be sure the mounting tabs are not bent, broken or deficient in their ability to hold the switches.
Avoid inserting nails, wire etc. through any holes in the unit during operation.
Never insert a wire, nail or any other metal object through the lamp holes on the cavity or any holes or gaps, because such objects may work as an antenna and cause microwave leakage.
Confirm after repair
1. After repair or replacement of parts, make sure that the screws of the oven, etc. are neither loose nor missing. Microwaves might leak if screws are not properly tightened.
2. Make sure that all electrical connections are tight before inserting the plug into the wall outlet.
3. Check for microwave energy leakage. (Refer to procedure for measuring microwave energy leakage).
CAUTION MICROWAVE RADIATION
USE CAUTION NOT TO BECOME EXPOSED TO RADIATION FROM THE MICROWAVE MAGNETRON OR OTHER PARTS CONDUCTING MICROWAVE ENERGY
IMPORTANT NOTICE
The following components have potentials above 2000V while the appliance is operated.
# Magnetron
# High voltage transformer (Located on inverter.
# High voltage diodes (Located on inverter).
# High voltage capacitors (Located on inverter)
Pay special attention to these areas. When the appliance is operated with the door hinges or magnetron installed incorrectly, the microwave leakage can exceed more than 5mW/cm2. After repair or exchange, it is very important to check if the magnetron and the door hinges are correctly installed.
When any part or component is to be replaced, always ensure that the power cord is removed from the wall outlet.
When the 10A fuse is blown due to the operation of the short switch:
WARNING
When the 10A 250V fuse is blown due to the operation of the interlock monitor switch, replace all of the components (primary latch switch, secondary latch switch, short switch and power relay B (RY1)).
1. This is mandatory. Refer to “adjustments and measurements” for the location of these switches.
2. When replacing the fuse, confirm that it has the appropriate rating for these models.
3. When replacing faulty switches, be sure the mounting tabs are not bent, broken or deficient in their ability to hold the switches.
Avoid inserting nails, wire etc. through any holes in the unit during operation.
Never insert a wire, nail or any other metal object through the lamp holes on the cavity or any holes or gaps, because such objects may work as an antenna and cause microwave leakage.
Confirm after repair
1. After repair or replacement of parts, make sure that the screws of the oven, etc. are neither loose nor missing. Microwaves might leak if screws are not properly tightened.
2. Make sure that all electrical connections are tight before inserting the plug into the wall outlet.
3. Check for microwave energy leakage. (Refer to procedure for measuring microwave energy leakage).
CAUTION MICROWAVE RADIATION
USE CAUTION NOT TO BECOME EXPOSED TO RADIATION FROM THE MICROWAVE MAGNETRON OR OTHER PARTS CONDUCTING MICROWAVE ENERGY
IMPORTANT NOTICE
The following components have potentials above 2000V while the appliance is operated.
# Magnetron
# High voltage transformer (Located on inverter.
# High voltage diodes (Located on inverter).
# High voltage capacitors (Located on inverter)
Pay special attention to these areas. When the appliance is operated with the door hinges or magnetron installed incorrectly, the microwave leakage can exceed more than 5mW/cm2. After repair or exchange, it is very important to check if the magnetron and the door hinges are correctly installed.
Sharp edges
Caution
Please use caution when unpacking, installing or moving the unit, as some exposed edges may be sharp to the touch and cause injury if not handled with care.
Caution
Please use caution when unpacking, installing or moving the unit, as some exposed edges may be sharp to the touch and cause injury if not handled with care.
MEASUREMENTS AND ADJUSTMENTS
Adjustment of primary latch switch, secondary latch switch and short switch.
1. Mount the Primary latch switch, the secondary latch switch and the short switch to the door hook assembly as shown in ILL.
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 the illustration, so that the oven door will not have any play in it. Check for play in the door by pulling the door assembly. Make sure that the latch keys move smoothly after adjustment is completed. Completely tighten the screws holding the door hook assembly to the oven assembly.
3. Reconnect the short switch and check the continuity of the monitor circuit and all latch switches again by following the component test procedures.
1. Mount the Primary latch switch, the secondary latch switch and the short switch to the door hook assembly as shown in ILL.
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 the illustration, so that the oven door will not have any play in it. Check for play in the door by pulling the door assembly. Make sure that the latch keys move smoothly after adjustment is completed. Completely tighten the screws holding the door hook assembly to the oven assembly.
3. Reconnect the short switch and check the continuity of the monitor circuit and all latch switches again by following the component test procedures.
Measurement of microwave
output
The output power of the magnetron can be determined by performing IEC standard test procedures. However,due to the complexity of IEC test procedures, it is recommended to test the magnetron using the simple method outlined below.
Necessary Equipment:
*1 liter beaker *Glass thermometer
*Wrist watch or stopwatch
NOTE:
Check the line voltage under load.Low voltage will lower the magnetron output. Take the temperature readings and heating time as accurately as possible.
1. Fill the beaker with exactly one liter of tap water.Stir the water using the thermometer and record the water’s temperature. (recorded as T1).
2. Place the beaker on the center of glass tray. Set the oven for High power and heat it for exactly one minute.
3. Stir the water again and read the temperature of the water. (recorded as T2).
4. The normal temperature rise at High power level for each model, is as shown in table.
output
The output power of the magnetron can be determined by performing IEC standard test procedures. However,due to the complexity of IEC test procedures, it is recommended to test the magnetron using the simple method outlined below.
Necessary Equipment:
*1 liter beaker *Glass thermometer
*Wrist watch or stopwatch
NOTE:
Check the line voltage under load.Low voltage will lower the magnetron output. Take the temperature readings and heating time as accurately as possible.
1. Fill the beaker with exactly one liter of tap water.Stir the water using the thermometer and record the water’s temperature. (recorded as T1).
2. Place the beaker on the center of glass tray. Set the oven for High power and heat it for exactly one minute.
3. Stir the water again and read the temperature of the water. (recorded as T2).
4. The normal temperature rise at High power level for each model, is as shown in table.
| RATED OUTPUT | TEMPERATURE RISE |
| 1000W | Min. 8.6°C |
| 1100W | Min. 9.4°C |
Thursday, June 9, 2016
Microwave Working principle – Sharp Microwave oven - How magnetron works – Microwave oven Repair and Service
Category: Microwave Oven Repair and Servcie
Contents of this article
- How Magnetron works
- Magnetron Construction
- Energy and Standing waves
Sharp Microwave oven
Distribution of Energy within the Oven Cavity
The microwave energy produced by the magnetron is fed to the oven cavity through a waveguide. The waveguide shape and size is designed to enable the energy to be transferred with very little loss. When entering the cavity, the energy will, if left unmodified set up 'Standing Waves'. These standing waves are produced by reflected energy from the wall, floor and ceiling of the cavity. A regular pattern is established, with energy present along the standing waves. Some areas within the cavity will have no microwave energy p
The microwave energy produced by the magnetron is fed to the oven cavity through a waveguide. The waveguide shape and size is designed to enable the energy to be transferred with very little loss. When entering the cavity, the energy will, if left unmodified set up 'Standing Waves'. These standing waves are produced by reflected energy from the wall, floor and ceiling of the cavity. A regular pattern is established, with energy present along the standing waves. Some areas within the cavity will have no microwave energy p
The difference in energy levels throughout the cavity, caused by the standing waves, would produce uneven heating of the food during the cooking process. However, there are two main ways in which this problem can be alleviated. That is by using either a mode stirrer system, or by the use of a turntable.
The Turntable
This method leaves the heating pattern produced by the cavity design unaltered. The food is placed on a turntable during cooking, the turntable is rotated so that all parts of the food pass through several standing waves of energy during each rotation of the turntable.
The most effective way of using a turntable system is to place the food close to the outside of the turntable allowing the maximum travel through the standing wave energy field. Note that if food is placed too near to the edge of the turntable, it may become unbalanced and not rotate correctly
This method leaves the heating pattern produced by the cavity design unaltered. The food is placed on a turntable during cooking, the turntable is rotated so that all parts of the food pass through several standing waves of energy during each rotation of the turntable.
The most effective way of using a turntable system is to place the food close to the outside of the turntable allowing the maximum travel through the standing wave energy field. Note that if food is placed too near to the edge of the turntable, it may become unbalanced and not rotate correctly
Mode Stirrers
Mode stirrers, stirrer fans and devices known as rotating antennae, work in such a way as to constantly change the energy wave pattern within the cavity during the cooking process; These units are made of materials that will reflect microwave energy. Therefore when they are placed in the energy field and made to spin, they will randomly affect the pattern. This sets up a continuously changing energy field in the cavity. These devices are usually motor driven, but can be air driven, usually the food remains static within the cavity.
Mode stirrers, stirrer fans and devices known as rotating antennae, work in such a way as to constantly change the energy wave pattern within the cavity during the cooking process; These units are made of materials that will reflect microwave energy. Therefore when they are placed in the energy field and made to spin, they will randomly affect the pattern. This sets up a continuously changing energy field in the cavity. These devices are usually motor driven, but can be air driven, usually the food remains static within the cavity.
Microwave ovens for domestic use employ the turntable method to produce even cooking, whereas microwave ovens designed for commercial use generally incorporate mode stirrers. Commercial ovens also use two magnetrons, which gives a double advantage of more power with two heating patterns. A detailed description of differences of a commercial oven compared with the domestic models is included in the Sharp Commercial Microwave Ovens section at the end of this book. The magnetron, waveguide and cavity can be thought of as a 'matched' or 'tuned' circuit. The oven cavity is a multi-mode cavity resonator, designed to resonate at the frequency generated by the magnetron. The whole system requires a load to work into. So when running a microwave oven in the cook condition, there should always be at least a small load in the cavity. The most convenient load is probably a glass of water, this will enable the oven to be run long enough to carry out most tests.
If an oven is run for any length of time without a load, then the magnetron will be stressed. This is caused by a back heating effect, and if left too long, eventual damage causing low output will result. Modern magnetrons are fairly tolerant of the no load condition, so a sudden catastrophic failure is unlikely, however the effective life expectancy could be greatly reduced.
Another way in which a magnetron may be damaged is by the use of an excessive amount of metal foil or large metal utensils within the oven cavity. The effect can be that energy is reflected back to the magnetron where it will be dissipated as heat. Small amounts of foil can be tolerated, as called for in certain recipe books, and devices such as temperature probes, which are part metal in their construction. It should be remembered though, when using metal in an oven to keep it well away from the cavity walls. if metal objects are placed in the energy field and then come close to an earthed surface, arcing will occur and the surface of the metal could be marked. In the case of the temperature probe, it could be rendered inoperative. Metal racks and turntables, designed for use in the cavity, have good insulation or make good contact each other, Therefore it is expected that most magnetrons will last for the lifetime of the oven.
If an oven is run for any length of time without a load, then the magnetron will be stressed. This is caused by a back heating effect, and if left too long, eventual damage causing low output will result. Modern magnetrons are fairly tolerant of the no load condition, so a sudden catastrophic failure is unlikely, however the effective life expectancy could be greatly reduced.
Another way in which a magnetron may be damaged is by the use of an excessive amount of metal foil or large metal utensils within the oven cavity. The effect can be that energy is reflected back to the magnetron where it will be dissipated as heat. Small amounts of foil can be tolerated, as called for in certain recipe books, and devices such as temperature probes, which are part metal in their construction. It should be remembered though, when using metal in an oven to keep it well away from the cavity walls. if metal objects are placed in the energy field and then come close to an earthed surface, arcing will occur and the surface of the metal could be marked. In the case of the temperature probe, it could be rendered inoperative. Metal racks and turntables, designed for use in the cavity, have good insulation or make good contact each other, Therefore it is expected that most magnetrons will last for the lifetime of the oven.
Cavity and Waveguide Practical Problems
A motor usually rotates mode stirrers and turntables and therefore occasional motor failure is possible. It is important that the turntable couplings are kept clean otherwise food debris may find their way down to the motor below and this may cause premature failure because of seizure. It is also possible for turntable couplings to become damaged due to the repeated heating and subsequent carbonising of food debris beneath the turntable. This may cause the plastic coupling and 'spider' to melt and the excessive heat may crack the glass turntable. It should be noted that the turntable motor is asymmetric, which means that it is possible for the motor to turn in either a clockwise or anticlockwise direction.
Another item that may give problems from time to time, if it is not kept clean, is the waveguide cover, or the stirrer cover. Waveguide and stirrer covers are made of materials that are inert to microwave energy. However, if the cover is not kept clean, food debris will build up and be repeatedly cooked. This will then carbonise, and arcing will occur, finally the waveguide cover will have holes burnt in it. The cure is a new waveguide cover and a tactful reminder to the customer to keep the oven clean. Although this problem is simple to cure, and the customer can replace most waveguide covers, when the fault occurs it can nevertheless be disconcerting.
The oven should not be operated with a damaged waveguide cover or with the cover removed. In these situations food splashes could enter the waveguide, causing arcing and eventually corrosion. The result is that a new cavity is required to remedy the problem.
Very occasionally an engineer may encounter an oven that has a broken glass turntable. Apart from the obvious possibility of its having been dropped, the damage could have been caused by the use of a browning dish. To safeguard against possible damage when using a browning dish, use an upturned oven proof plate on the turntable, so that the turntable is insulated from the heat source generated by the browning dish. Periodically an engineer may come into contact with an oven that has been damaged because of severe overcooking of food. The amount of damage can vary between smudging that can be cleaned off, to extensive damage requiring the replacement of the cavity and other components. Sharp ovens incorporate temperature fuses, which will operate and stop further magnetron output in the event of severe overheating within the cavity.
Magnetron Theory
So far we have discussed microwave energy and its characteristics. In this section we will look at how the microwave energy is generated. The component used to generate microwave energy in a microwave oven is called a Magnetron; this is a thermionic device similar in some respects to a thermionic diode. To understand the basic operation of the magnetron, the operation of a thermionic diode valve will be discussed.
Thermionic Diode Operation
A diode consists of two electrodes, the Anode and the Cathode, which are contained within an evacuated glass or metal envelope. The cathode is coated with a material that, when heated, will emit electrons (sub-atomic particles). The cathode has to be heated in order to free these electrons. In a magnetron the cathode is directly heated and is usually referred to as the filament.
The anode is used to collect electrons given off by the filament. To do this the anode has to be positive with respect to filament. Electrons are negatively charged particles, therefore they are attracted towards the positive anode. Electrons will flow constantly as long as the potential difference is maintained, providing current flow through the device.
A motor usually rotates mode stirrers and turntables and therefore occasional motor failure is possible. It is important that the turntable couplings are kept clean otherwise food debris may find their way down to the motor below and this may cause premature failure because of seizure. It is also possible for turntable couplings to become damaged due to the repeated heating and subsequent carbonising of food debris beneath the turntable. This may cause the plastic coupling and 'spider' to melt and the excessive heat may crack the glass turntable. It should be noted that the turntable motor is asymmetric, which means that it is possible for the motor to turn in either a clockwise or anticlockwise direction.
Another item that may give problems from time to time, if it is not kept clean, is the waveguide cover, or the stirrer cover. Waveguide and stirrer covers are made of materials that are inert to microwave energy. However, if the cover is not kept clean, food debris will build up and be repeatedly cooked. This will then carbonise, and arcing will occur, finally the waveguide cover will have holes burnt in it. The cure is a new waveguide cover and a tactful reminder to the customer to keep the oven clean. Although this problem is simple to cure, and the customer can replace most waveguide covers, when the fault occurs it can nevertheless be disconcerting.
The oven should not be operated with a damaged waveguide cover or with the cover removed. In these situations food splashes could enter the waveguide, causing arcing and eventually corrosion. The result is that a new cavity is required to remedy the problem.
Very occasionally an engineer may encounter an oven that has a broken glass turntable. Apart from the obvious possibility of its having been dropped, the damage could have been caused by the use of a browning dish. To safeguard against possible damage when using a browning dish, use an upturned oven proof plate on the turntable, so that the turntable is insulated from the heat source generated by the browning dish. Periodically an engineer may come into contact with an oven that has been damaged because of severe overcooking of food. The amount of damage can vary between smudging that can be cleaned off, to extensive damage requiring the replacement of the cavity and other components. Sharp ovens incorporate temperature fuses, which will operate and stop further magnetron output in the event of severe overheating within the cavity.
Magnetron Theory
So far we have discussed microwave energy and its characteristics. In this section we will look at how the microwave energy is generated. The component used to generate microwave energy in a microwave oven is called a Magnetron; this is a thermionic device similar in some respects to a thermionic diode. To understand the basic operation of the magnetron, the operation of a thermionic diode valve will be discussed.
Thermionic Diode Operation
A diode consists of two electrodes, the Anode and the Cathode, which are contained within an evacuated glass or metal envelope. The cathode is coated with a material that, when heated, will emit electrons (sub-atomic particles). The cathode has to be heated in order to free these electrons. In a magnetron the cathode is directly heated and is usually referred to as the filament.
The anode is used to collect electrons given off by the filament. To do this the anode has to be positive with respect to filament. Electrons are negatively charged particles, therefore they are attracted towards the positive anode. Electrons will flow constantly as long as the potential difference is maintained, providing current flow through the device.
Overview of Magnetron Operation
The magnetron is a specially designed type of thermionic diode, which is made to self oscillate. The major differences being the shape and structure of the anode and the addition of two strong external magnets, one above and one below the anode chamber. The resultant magnetic field is critical, as together with the anode voltage, it determines the path the electrons will take.
Without the magnets in position the electrons would travel directly to the anode in the normal way in a straight line. With the magnets in position, the strong magnetic field exerted across the magnetron envelope will cause the electrons emitted by the filament to take a spiral path as they move towards the anode structure. The shape of the anode forms an even number of structures called cavity resonators, which form individual tuned circuits. These tuned circuits will oscillate as the passing of the electrons induces charges into them. All the tuned circuits are connected together in phase and the resultant power is transmitted via the antenna, which is connected to the anode structure, into the cavity. A more detailed explanation of this concept follows.
The magnetron is a specially designed type of thermionic diode, which is made to self oscillate. The major differences being the shape and structure of the anode and the addition of two strong external magnets, one above and one below the anode chamber. The resultant magnetic field is critical, as together with the anode voltage, it determines the path the electrons will take.
Without the magnets in position the electrons would travel directly to the anode in the normal way in a straight line. With the magnets in position, the strong magnetic field exerted across the magnetron envelope will cause the electrons emitted by the filament to take a spiral path as they move towards the anode structure. The shape of the anode forms an even number of structures called cavity resonators, which form individual tuned circuits. These tuned circuits will oscillate as the passing of the electrons induces charges into them. All the tuned circuits are connected together in phase and the resultant power is transmitted via the antenna, which is connected to the anode structure, into the cavity. A more detailed explanation of this concept follows.
Magnetron Construction
Shown below is a diagram of a typical magnetron used in a Sharp microwave oven. The left-hand side shows the outside appearance while the right hand side shows a ‘cut away’ views
Shown below is a diagram of a typical magnetron used in a Sharp microwave oven. The left-hand side shows the outside appearance while the right hand side shows a ‘cut away’ views
When examining a magnetron it would appear that there are only two terminals for connection. These are in fact for the filament and cathode. However, it should be noted that the anode structure is electrically connected to the outer case of the magnetron, this therefore comprises a third connection. As discussed in the Basic Thermionic Diode Operation section, the anode is at a positive potential with respect to the filament. The anode of a magnetron is connected to its outer metal case, which is in turn connected to ground. It therefore becomes necessary to apply a negative potential to the filament. Whilst the magnetron is operating, it runs quite hot at approximately 96 degrees Celsius. For this reason it has to be cooled, air is continually being blown over it by a fan. Cooling fins are fitted to the magnetron to allow the free flow of air around the anode structure, maximising the dissipation of excess heat.
Principles of Magnetron Operation
The magnetron has a specially shaped Anode cavity resonator structure, as can be seen from the diagram below, which creates twelve cavity resonators formed by the anode vanes.
The magnetron has a specially shaped Anode cavity resonator structure, as can be seen from the diagram below, which creates twelve cavity resonators formed by the anode vanes.
Each cavity resonator forms a conventional parallel tuned circuit, which consists of a capacitor connected in parallel with an inductor. In the case of the cavity resonator the capacitance is created by the vanes, which are seen as the two plates of the capacitor and the gap between the vanes is the dielectric. The length of each vane forms the inductance. The diagram below shows the magnetron anode as conventional components for ease of understanding.
A conventional parallel tuned circuit required to oscillate at 2450MHz would require very small values of inductance and capacitance. These can be calculated by using the following equation.
Resonant frequency = 1/2π Root LC
Therefore possible values could be:
C (Capacitance) 64.95 x 10-12 Farad (64.95pF)
L (Inductance) 64.95 x 10-12 Henry (64.95pH)
The above examples are not practical values, but they do illustrate that the values of capacitance and inductance created within a magnetron by the cavity resonators are very small.
By inter-connecting every other anode vane, using mode or strap rings, it is possible to ensure that adjacent cavity resonators oscillate 180 degrees out of phase when the magnetron is active. This configuration is shown in the diagram below.
C (Capacitance) 64.95 x 10-12 Farad (64.95pF)
L (Inductance) 64.95 x 10-12 Henry (64.95pH)
The above examples are not practical values, but they do illustrate that the values of capacitance and inductance created within a magnetron by the cavity resonators are very small.
By inter-connecting every other anode vane, using mode or strap rings, it is possible to ensure that adjacent cavity resonators oscillate 180 degrees out of phase when the magnetron is active. This configuration is shown in the diagram below.
The diagram below shows the anode structure of the magnetron and the position of the magnets. A strong magnetic field is present around the chamber. The effect of the magnetic field causes the electrons to take a spiral path as they travel towards the anode.
For the magnetron to operate correctly, a very high potential difference between the filament and anode is
Needed, the anode being positive with respect to the filament. In practice this is achieved by connecting the anode to ground and applying a high negative voltage to the filament. When the filament is heated, the electrons become excited and begin to jump from the filament. These free electrons form a cloud or 'space charge' around the filament. The electrons are then attracted towards the anode due to its positive polarity. However they are forced into taking a spiral path due to the influence of the external magnetic field that is created by the magnets above and below the anode chamber (Lorentz's law). As the electrons move closer to the cavity resonators they induce a charge within the resonator and this sets up the initial oscillation. Their movement over the gaps of the vanes creates a positive feedback effect, which causes the oscillation to continue.
As the oscillation develops some resonators will be in a negative state and some positive state, each cavity resonator being 180 degrees out of phase with its neighbour. These conditions reverse as the cycle of oscillation is completed, that is the resonators that were positive become negative and those that were negative become positive. This has a further effect on the paths taken by the electrons. Any electron in the area of the negatively charged resonator vane is repelled because of their 'like charges', negative electrons and negatively charged resonator. The velocity of these electrons causes them to return to the filament, where they impact upon it, causing 'back heating' and 'secondary emission'. Conversely electrons in the vicinity of a positively charged resonator are attracted further towards the anode where they will finally land. As shown in the diagrams below, these two conditions create a pattern of electrons within the magnetron chamber. This pattern is usually referred to as the ‘spoked wheel effect’; the 'spokes' are formed because of the positively charged cavity resonators attracting electrons towards the anode. The spaces between the spokes are caused by electrons being repelled due to the negatively charged resonators. It is important to remember that the polarity of charge is constantly changing within the cavity resonators. As the oscillation continues, during one half cycle of operation electrons are attracted by alternate resonators and repelled by the others. On the next half cycle the polarities will change. This effect together with the magnetic field causes the 'spoked wheel' to rotate so that the 'spokes' are always pointing to the positively charged cavity resonators, and therefore the gaps are aligned with the negatively-going cavity resonators. As the oscillation continues the 'spoked wheel' will progressively turn.
The two diagrams above show the 'Spoked wheel' pattern formed by the electron cloud in the two maximum states of oscillation.
It can be seen from the diagram below, all twelve cavity resonators are effectively connected in parallel, therefore the power available from each one is added together.
As the cavity resonators are in parallel, it is possible to connect an antenna (aerial) to any of the anode vanes, enabling the total amount of microwave energy produced to be transmitted through the waveguide into the oven cavity.
As the cavity resonators are in parallel, it is possible to connect an antenna (aerial) to any of the anode vanes, enabling the total amount of microwave energy produced to be transmitted through the waveguide into the oven cavity.
When replacing a magnetron care should be taken on the following points:
There is a RF gasket fitted around the antenna to prevent microwave energy escaping from the seal between the magnetron and the waveguide. Always ensure the gasket is not distorted when fixing the magnetron in place.
# When handling, take care not to leave greasy deposits either around or on the antenna, which may carbonise, causing arcing at a later date.
# Ensure that the connections to the magnetron terminals are tight. If they are loose, overheating and damage will occur.
# Always remember the 3D checks when working around the magnetron and high voltage circuit. Several Sharp microwaves may use the same type of magnetron, but have different output RF powers. This is due to the RF output power being directly proportional to the anode current, which can be controlled in the HIGH VOLTAGE circuit design. The filament potential is altered to give the required power for individual models.
# When handling, take care not to leave greasy deposits either around or on the antenna, which may carbonise, causing arcing at a later date.
# Ensure that the connections to the magnetron terminals are tight. If they are loose, overheating and damage will occur.
# Always remember the 3D checks when working around the magnetron and high voltage circuit. Several Sharp microwaves may use the same type of magnetron, but have different output RF powers. This is due to the RF output power being directly proportional to the anode current, which can be controlled in the HIGH VOLTAGE circuit design. The filament potential is altered to give the required power for individual models.
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