High-Voltage Transformers; Wiring The High-Voltage Transformers; Testing A Filament Or High-Voltage Transformer; High-Voltage Diodes - TurboChef ngo Service Manual

24 OVEN SYSTEMS

High-Voltage Transformers

High-voltage transformers are ferro-resonant, which
limits faulty currents and minimizes magnetron power
changes due to input voltage changes. The high-
voltage transformer supplies the high voltage for the
voltage doubler circuit. They are controlled via the
K8 relay.
NOTE: The Single Mgnetron Sota uses only one high
voltage transformer.

Wiring the High-Voltage Transformers

DANGER: Never attempt to wire or
measure the secondary voltage values of the
high-voltage transformers. Lethal voltage
will be present.
The proper reinstallation of a high-voltage transformer
is critical. Upon removing a high-voltage transformer,
make sure to note where each wire was installed. Refer
to the oven schematic (page 45) for wiring detail.
As shown in the schematic, transformers are installed
mirror opposite and wired 180 º out-of-phase. It is
essential for longevity that the high-voltage trans-
formers remain 180 º out-of-phase. This can be checked
by placing a volt meter across terminals T1-1 and T2-1
(primary voltage) .
NOTE: The Single Mgnetron Sota uses only one high
voltage transformer.
With the microwave system energized, the volt meter
will read the incoming voltage (different readings for
different electrical installations). If the meter reads 0
VAC , the high-voltage transformers are most likely
wired in-phase. As a last check, energize the microwave
system and verify the voltages between the taps on each
high-voltage transformer.
The wiring issue must be corrected prior to returning
the oven to service, as the voltages must be:
- north america: 208 VAC between 1 & 2 and
240 between 1 & 3.
- international: 230 VAC
NOTE: The orange wire always goes to terminal 3 on
US models .

Testing a Filament or High-Voltage Transformer

DANGER: Never attempt to measure the
secondary voltage values of the HV
transformers. Lethal voltage will be present.
1. Disconnect the AC power source and discharge
the high-voltage capacitors.
2. Disconnect all the wires in question going to and
from the transformer.
3. Use an ohmmeter to check the impedance of the
primary and secondary winding. Refer to Figure 31,
page 25 to determine if the transformer is okay. If
the resistance is different than the table indicates,
replace the transformer.

High-Voltage Diodes

The high-voltage diode (Figure 30) is assembled by
connecting several 1000-1500 volt semi-conductor
diodes in a series to increase the reverse voltage
capability. In the circuit, the high-voltage diode
conducts to prevent the filament voltage from
becoming positive, thus as the high-voltage winding
of the transformer goes to a peak of 2400 volts, the
high-voltage capacitor is charged to 2400 volts.
Figure 30: High-voltage Diode
When the high-voltage winding starts to go toward
negative, the high-voltage diode becomes non-
conducting with the charged high-voltage capacitor
in series with the high-voltage winding. When the
transformer gets to its negative peak of -2400 volts,
the voltage applied to the filament is -4500 volts. The
high-voltage diodes are rated at 16 kVDC.
NOTE: The Single Mgnetron Sota uses only one diode.