Magnetron Theory

                                                So far nosotros direct hold discussed microwave unloose energy in addition to its characteristics.If yous haven't seen this click hither to meet article nearly microwave energy. In this department nosotros volition hold off at how the microwave unloose energy is generated. The ingredient used to generate microwave unloose energy inward a microwave oven is called a Magnetron. It is a thermionic device similar inward some respects to a thermionic diode. To sympathise the basic performance of the magnetron, the performance of a thermionic diode valve is discussed below.

THERMIONIC DIODE OPERATION:
                                               A diode consists of 2 electrodes which are

•                                                                    the Anode
• the Cathode

                                           

                                                      These electrodes are enclosed inward an evacuated drinking glass or metallic envelope. The cathode is coated amongst a textile that, when heated, volition emit electrons (sub-atomic particles). The cathode has to live heated inward social club to gratis these electrons. In a magnetron the cathode is straight heated in addition to is normally referred to equally the filament. The anode is used to collect electrons that are produced yesteryear the filament. To do this, the anode has to live positive amongst honor to filament. Electrons are negatively charged particles, hence they are attracted towards the positive anode. Electrons volition period of time constantly equally long equally the potential divergence is maintained, providing electrical current period of time through the device.

MAGNETRON OPERATION:

                                          The magnetron is a peculiarly designed type of thermionic diode, which is made to self oscillate. The major differences beingness the shape in addition to construction of the anode in addition to the add-on of 2 strong external magnets, 1 inward a higher house in addition to 1 below the anode chamber. The resultant magnetic plain is real high, equally together amongst the anode voltage, it determines the path the electrons volition take. 

                                          Without the magnets inward seat the electrons would move straight to the anode inward the normal agency inward a straight line. With the magnets inward position, the strong magnetic plain exerted across the magnetron envelope volition drive the electrons emitted yesteryear the filament to direct hold a spiral path equally they displace towards the anode structure.

                                            The shape of the anode forms an fifty-fifty publish of structures called cavity resonators, which shape private tuned circuits. These tuned circuits volition oscillate equally the passing of the electrons induces charges into them. All the tuned circuits are connected together inward stage in addition to the resultant ability is transmitted via the antenna, which is connected to the anode structure, into the cavity. Influenza A virus subtype H5N1 to a greater extent than detailed explanation of this concept follows.

MAGNETRON CONSTRUCTION:

                                                     The diagram inward below is the diagram of a typical magnetron used inward a Sharp microwave oven.  The left-hand side shows the exterior appearance spell the right mitt side shows a ‘cut away’ views

                                                      When examining a magnetron it would look that at that spot are entirely 2 terminals for connection. These are inward fact for the filament in addition to cathode. However, it should live noted that the anode construction is electrically connected to the outer instance of the magnetron, this hence comprises a tertiary connection. 

                                                       As discussed inward the Basic Thermionic Diode Operation section, the anode is at a positive potential amongst honor to the filament. The anode of a magnetron is connected to its outer metallic case, which is inward plough connected to ground.  It hence becomes necessary to apply a negative potential to the filament. 

                                                       Whilst the magnetron is operating, it runs quite hot at some 96 degrees Celsius. For this argue it has to live cooled, air is continually beingness blown over it yesteryear a fan. Cooling fins are fitted to the magnetron to allow the gratis period of time of air about the anode structure, maximising the dissipation of excess heat.

PRINCIPLES OF MAGNETRON OPERATION:

                                                         The magnetron has a peculiarly shaped Anode cavity resonator structure, equally tin live seen from the diagram below, which creates twelve cavity resonators formed yesteryear the anode vanes.

                                       Each cavity resonator forms a conventional parallel tuned circuit, which consists of a capacitor connected inward parallel amongst an inductor. In the instance of the cavity resonator the capacitance is created yesteryear the vanes, which are seen equally the 2 plates of the capacitor in addition to the gap betwixt the vanes is the dielectric. The length of each vane forms the inductance. The diagram below shows the magnetron anode equally conventional components for repose of understanding

                                                            Influenza A virus subtype H5N1 conventional parallel tuned circuit required to oscillate at 2450MHz would require real pocket-size values of inductance in addition to capacitance.  These tin live calculated yesteryear using the next equation.

                                             F=1➗2ℼ√(LC)

Therefore possible values could be:

                                    C (Capacitance) 64.95 x 10-12Farad (64.95pF)

                                    L (Inductance) 64.95 x 10-12 Henry (64.95pH)

                              

                                  The inward a higher house examples are non practical values, merely they do illustrate that the values of capacitance in addition to inductance created inside a magnetron yesteryear the cavity resonators are real small. 

                                  By inter-connecting every other anode vane, using agency or strap rings, it is possible to ensure that next cavity resonators oscillate 180 degrees out of stage when the magnetron is active.  This configuration is shown inward the diagram below.

                                               The diagram below shows the anode construction of the magnetron in addition to the seat of the magnets. Influenza A virus subtype H5N1 strong magnetic plain is acquaint about the chamber. The outcome of the magnetic plain causes the electrons to direct hold a spiral path equally they move towards the anode.

                                                  For the magnetron to operate correctly, a real high potential divergence betwixt the filament in addition to anode is needed, the anode beingness positive amongst honor to the filament. In practise this is achieved yesteryear connecting the anode to Earth in addition to applying a high negative voltage to the filament. 

                                                     When the filament is heated, the electrons larn excited in addition to get down to trammel from the filament. These gratis electrons shape a cloud or 'space charge' about the filament. The electrons are in addition to so 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 plain that is created yesteryear the magnets inward a higher house in addition to below the anode sleeping room (Lorentz's law). As the electrons displace closer to the cavity resonators they cause a accuse inside the resonator in addition to this sets upward 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 volition live inward a negative dry reason in addition to some positive state, each cavity resonator beingness 180 degrees out of stage amongst its neighbour. These weather contrary equally the cycle of oscillation is completed, that is the resonators that were positive larn negative in addition to those that were negative larn positive. This has a farther outcome on the paths taken yesteryear the electrons.

                                                      Any electron inward the expanse of the negatively charged resonator vane is repelled because of their 'like charges', negative electrons in addition to negatively charged resonator. The velocity of these electrons causes them to provide to the  filament, where they acquit upon upon it, causing 'back heating' in addition to 'secondary emission'. Conversely electrons inward the vicinity of a positively charged resonator are attracted farther towards the anode where they volition lastly land.

                                                      As shown inward the diagrams below, these 2 weather do a pattern of electrons inside the magnetron chamber. This pattern is normally referred to equally the ‘spoked cycle effect’; the 'spokes' are formed because of the positively charged cavity resonators attracting electrons towards the anode. The spaces betwixt the spokes are caused yesteryear electrons beingness repelled due to the negatively charged resonators. 

                                                       It is of import to retrieve that the polarity of accuse is constantly changing inside the cavity resonators. As the oscillation continues, during 1 one-half cycle of performance electrons are attracted yesteryear alternate resonators in addition to repelled yesteryear the others. On the next one-half cycle the polarities volition change. This outcome together amongst the magnetic plain causes the 'spoked wheel' to rotate so that the 'spokes' are ever pointing to the positively charged cavity resonators, in addition to hence the gaps are aligned amongst the negatively-going cavity resonators. As the oscillation continues the 'spoked wheel' volition progressively turn.

                                                 Diagram inward a higher house exhibit the 'Spoked wheel' pattern formed yesteryear the electron cloud inward the 2 maximum states of oscillation.

                                                  It tin live seen from the diagram below, all twelve cavity resonators are effectively connected inward parallel, hence the ability available from each 1 is added together. As the cavity resonators are inward parallel, it is possible to connect an antenna (aerial) to whatever of the anode vanes, enabling the full sum of microwave unloose energy produced to live transmitted through the waveguide into the oven cavity.

When replacing a magnetron attention should live taken on the next points:

• There is a RF gasket fitted about the antenna to preclude microwave unloose energy escaping from the seal betwixt the magnetron in addition to the waveguide.  Always ensure the gasket is non distorted when fixing the magnetron inward place.

• When handling, direct hold attention non to go out greasy deposits either about or on the antenna, which may carbonise, causing arcing at a after date.

• Ensure that the connections to the magnetron terminals are tight.  If they are loose, overheating in addition to impairment volition occur.

• Always retrieve the 3D checks when working about the magnetron in addition to high voltage circuit.

                                          Several Sharp microwaves may purpose the same type of magnetron, merely direct hold dissimilar output RF powers. This is due to the RF output ability beingness straight proportional to the anode current, which tin live controlled inward the HIGH VOLTAGE circuit design. The filament potential is altered to give the required ability for private models.

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