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78
SPECIAL
DEVICES
115~
± 1 : . . . . . 3
~
10
Compensating
flux lines to travel around the core and up through
the center instead of across the area with the air spaces.
The flux lines through the lower center part of the
core induce a voltage into the resonant and secondary
115~
±~
o
Resonant
--
~
0
6.3 Volt
Secondary
s ±5%
windings. Connected in series with one of these wind-
ings is a capacitor (matched in rated size to the in-
ductance of the coil), that causes a high current to
flow (resonant). Only the
DC
resistance of the circuit
opposes the resonating current. The high current flow
in the resonant winding causes a magnetic field that
saturates the lower core area with magnetic lines of
flux.
Figure 58. Filament Voltage Transformer (Schematic)
FILAMENT VOLTAGE TRANSFORMER
A regulating transformer is used in the power sup-
ply to provide voltage regulation for the 2D21 tube
filaments (Figure 58). The regulating transformer is
designed to deliver an output of 6.3 volts + 5 percent
over a ± 10 percent variation of rated line _ voltage
on the primary. It is especially constructed with a
compensating winding wound with the primary at one
end of the core (Figure 59). At the other end of the
core, a resonant winding and the secondary winding
are wound. In the design of the transformer, a space
is provided between the primary and secondary wind-
ings to change the reluctance of the transformer.
When the input voltage is applied to the primary
windings, magnetic lines of flux set up in the trans-
former core. In the path of the flux linkages, the air
space makes the normal magnetic path higher in re-
luctance (magnetic resistance )
~han
that of a solid
core arrangement. The result of this design causes the
Primary
Calls (2)
115V
AC
Resonant Coil
Compensating Coil
Secondary Coil
6.3 V
II
Figure 59. Schematic
of
Filament Voltage Transformer
With the resonant winding saturating the lower core,
more lines of flux are available than can pass through
the lower part of the core. Consequently, not all of
the primary flux linkages can go through the lower
core area because it is saturated. Part of the flux crosses
the air space as a path of less resistance.
Saturation is maintained at the lower core area by
compensating for the resistive losses of the resonant
coil. When the losses tend to cause the magnetic field
to fall below saturation, more of the primary flux can
enter through the lower core area. The additional pri-
mary flux induces voltage into the resonant winding
which increases the magnetic field and maintains sat-
uration of the lower core area.
By maintaining saturation, the secondary winding
output
is
maintained because the flux changes at the
secondary winding are not in proportion to variations
in primary voltage. Because of inherent conditions in
the transformer, however, there are small changes of
output voltage when the input voltage varies. The slight
changes are regulated by the compensating winding.
The compensating winding is designed to cancel the
slight variations in secondary output voltage. It is
wound (positioned) with the primary windings and
wired in series with the secondary winding. The com-
pensating winding is wound so that its induced voltage
opposes that of the secondary winding (Figure 59).
Within the range of regulation, if the primary input
voltage rises, the induced secondary voltage rises
slightly. Also the voltage induced into the compen-
sating coil increases in proportion to the increase over
normal output of the secondary winding. The two
voltages oppose each other with the result that 6.3
volts remain impressed on the filament circuit (Figure
61). When the input voltage on the primary decreases,
the induced voltage of the secondary is lower, with a
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