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Operation, Maintenance, Troubleshooting and Repair

Copyright © 1989-2013 J. Carlton Gallawa . All Rights Reserved Worldwide

7.1 INTRODUCTION

The purpose of the high-voltage system (Fig. 7-1) is to generate microwave energy. The heart of the microwave oven, it steps up AC line voltage to high voltage, changes the high AC voltage to an even higher DC voltage, and then converts the DC power to RF energy.

The nucleus of the high-voltage system is the MAGNETRON TUBE.

7.2 MAGNETRON TUBE

The magnetron tube is a diode-type electron tube that is used to produce the required 2450 MHz of microwave energy. It is classed as a diode because it has no grid, as does an ordinary electron tube. A magnetic field imposed on the space between the anode (plate) and the cathode serves as the grid. Figure 7-2 is a sectional view of a typical magnetron tube. While the outer configurations of different type magnetrons will vary by make and model, the basic internal structures are the same. These are the anode, the filament/cathode, the antenna and the magnets.

The ANODE (or plate) is a hollow cylinder of iron from which an even number of anode vanes extend inward, as shown in Figure 7-3 and Figure 7-3A. The open trapezoidal shaped areas between each of the vanes are resonant cavities, which serve as tuned circuits, and determine the output frequency of the tube. The anode operates in such a way that alternate segments must be connected, or strapped, so that each segment is opposite in polarity to the segment on either side. In effect, the cavities are connected in parallel with regard to the output. This will be become easier to understand as the description of operation is considered.

The FILAMENT (also referred to as the heater), which also serves as the CATHODE of the tube, is located in the center of the magnetron and is supported by the large and rigid filament leads which are carefully sealed into the tube and shielded.

The ANTENNA, a probe or loop connected to the anode and extending into one of the tuned cavities, is coupled to the waveguide into which it transmits the RF energy.

The other parts of the magnetron assembly may vary in their relative positions, size and shape, depending on the manufacturer. To keep the following explanation of operation as simple as possible, only the terms that are not self-explanatory as to their purpose will be elaborated on.

The MAGNETIC FIELD is provided by strong permanent magnets, which are mounted around the magnetron so that the magnetic field is parallel with the axis of the cathode.

7.3 BASIC MAGNETRON OPERATION
The theory of magnetron operation is based on the motion of electrons under the combined influence of electric and magnetic fields. For the tube to operate, electrons must flow from the cathode to the anode. There are two basic laws that govern this motion: (continued next page)

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Copyright © 1989-2013 J. Carlton Gallawa . All Rights Reserved Worldwide