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The Magnetron Tube Used In Microwave Ovens
Excerpts from the book
The Complete Microwave Oven Service Handbook
Available on CD-ROM (CLICK
Copyright © 1989-2013 by J.
and from the DVD video You
Can Fix Microwave Ovens
Structure and Operation
The heart of every microwave oven is the high
voltage system . Its purpose is to generate microwave energy. The high-voltage
components accomplish this by stepping up AC line voltage to high voltage, which
is then changed to an even higher DC voltage. This DC power is then converted
to the RF energy that cooks the food.
Basic Magnetron Structure
The nucleus of the high-voltage system is the magnetron
tube . The magnetron is a diode-type electron tube which 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.
While the external configurations of different magnetrons will vary, the basic
internal structures are the same. These include 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 (see
Fig. 2). The open trapezoidal shaped areas between each of the vanes are resonant
cavities that 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 become easier to understand as the description
of operation is considered.
The FILAMENT (also called 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 is a probe or loop that
is connected to the anode and extends into one of the tuned cavities. The antenna
is coupled to the waveguide , a hollow metal enclosure,
into which the antenna transmits the RF energy.
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.
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 fundamental laws that govern their trajectory:
- The force exerted by an electric field on an
electron is proportional to the strength of the field. Electrons tend to move
from a point of negative potential toward a positive potential. Figure
3-A shows the uniform and direct movement of the electrons in an electric
field with no magnetic field present, from the negative cathode to the positive
- The force exerted on an electron in a magnetic
field is at right angles to both the field itself, and to the path of the
electron. The direction of the force is such that the electron proceeds to
the anode in a curve rather than a direct path.
Effect of the Magnetic
In Figure 3-B two permanent
magnets are added above and below the tube structure. In Figure 3-C, assume
the upper magnet is a north pole and you are viewing from that position. The
lower, south pole magnet, is located underneath the page, so that the magnetic
field appears to be coming right through the page. Just as electrons flowing
through a conductor cause a magnetic field to build up around that conductor,
so an electron moving through space tends to build up a magnetic field around
itself. On one side (left) of the electron's path, this self induced magnetic
field adds to the permanent magnetic field surrounding it. On the other side
(right) of its path, it has the opposite effect of subtracting from the permanent
magnetic field. The magnetic field on the right side is therefore weakened,
and the electron's trajectory bends in that direction, resulting in a circular
motion of travel to the anode.
The process begins with a low voltage being applied
to the filament, which causes it to heat up (filament voltage is usually 3
to 4 VAC, depending on the make and model). Remember, in a magnetron tube,
the filament is also the cathode. The temperature rise causes increased molecular
activity within the cathode, to the extent that it begins to "boil off" or
emit electrons. Electrons leaving the surface of a heated filament wire might
be compared to molecules that leave the surface of boiling water in the form
of steam. Unlike steam, though, the electrons do not evaporate. They float,
or hover, just off the surface of the cathode, waiting for some momentum.
Continued on Page
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Copyright © 1989-2013 by
J. Carlton Gallawa
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