Specifications

Ver SG 9/200 27 Sharp Electronics Corporation
HIGH VOLTAGE SECTION
The high voltage section of the microwave oven is the section responsible for developing
the voltages required by the magnetron. These voltages are the 3 volts AC and the -4000
volts DC. Refer to the graphic on page 29 for the following explanation.
The voltage induction principle of a transformer is used to develop the volts from the 120
volt supply. The 3 volts AC is developed on the voltage step down winding which steps
the voltage down by a 40 to 1 ratio. For every 40 volts applied to the primary winding
we will get 1 volt on the secondary of the step down winding. This winding is often
referred to as the filament winding. Due to the induction principle the output of the
transformer must be alternating voltage (AC). This voltage is applied directly across the
heating element that heats the cathode surface of the magnetron.
The -4000 volts required to charge the electrons within the magnetron’s vacuum space is
developed on the step up winding. This winding steps the voltage up on a ratio of 1 to
20. For every 1 volt applied to the primary winding, 20 volts will be developed on the
secondary. On a unit that has 120 volts applied to the primary the output on the high
voltage winding will be about 2400 volts AC. This voltage must now be rectified
(converted into DC voltage) and increased. This is accomplished with the use of a silicon
rectifier and a capacitor. As the transformer makes its positive and negative swings along
the sine wave, the rectifier will conduct only during the negative swing. During this
period of time the rectifier will totally shunt the magnetron and charge the capacitor.
During the positive swing the rectifier will not conduct. This will now give us the 2400
volt potential within the secondary winding along with the 2400 volts stored in the
capacitor. The voltage is in series aiding which gives the 4800 volt potential available for
the magnetron.
The voltages referred to in our explanation are not real voltages that you would measure
on a working microwave. In practice the voltages may be lower and will vary from
model to model. The transformer secondary windings are matched to an output rating of
the magnetron. The size of the capacitor is also matched to the magnetron and
transformer output. The rectifier is current rated for the individual circuit it is used in.
Never replace a rectifier, capacitor, transformer, or magnetron with a generic part. You
can compromise the circuit operation and reliability of the unit.
If you suspect a problem in the magnetron power supply the following test procedure
should be used.
Disconnect the 120 volt power source from the oven (unplug the unit). Discharge the
capacitor. A capacitor is nothing more than a storage device for electricity. Even with
the power disconnected the capacitor can hold a charge of around 2000 volts. This
voltage must be discharged to render the unit safe to work on. To discharge the
capacitor, short across the terminals. An arc may occur when the terminals are shorted.
This would be a normal situation.