Circuit Description-Type 422 AC-DC
(C)
___
___________
Pulse
I | Duration
I I
Pulse-*-!
I duration I
Fig. 3-14. Illustration of energy-storage principle: (A) Simplified diagram of energy-storage circuit, (B) graph of charge stored in inductor
versus source voltage, (C) idealized waveforms.
the collector of Q1120. A third winding of T l 120 provides
a negative going output pulse to the Multivibrator stage.
This pulse is of short duration due to the action of Q1120.
(See waveform 1, Fig. 3-15.)
The operating frequency of the Blocking Oscillator stage
is determined by Cl 121-R1123-R1124-R1125 in the
emitter circuit of Q1120. When Q1120 conducts, its emit
ter quickly goes positive along with the signal applied to the
base from Tl 120. Cl 121 quickly discharges to the level on
the emitter of Q1120. When Q1120 shuts off, its emitter
level is determined by the charge rate of Cl 121. The charge
current for Cl 121 is supplied through R1124-R1125 and
through R1123-D1155. As Cl 121 charges toward the zero-
reference voltage level, the emitter level of Q1120 follows.
However, when the emitter level of Q1120 drops to where
it is about 0.5 volt more negative than the base level set by
diodes D1117-D1118, Q1120 is forward biased to start a
new cycle. Frequency of operation is about seven kilohertz.
The Oscillator Frequency adjustment R1125 varies the
charge time of Cl 121 and thus determines the frequency at
which the blocking oscillator operates. The signal at the
emitter of Q1120 is connected to the Pulse Width Control
stage through R1123 (see waveform 2, Fig. 3-15).
Multivibrator. Transistors Q1105 and Q1115 form a
bistable miltivibrator. The negative-going output pulses
from the Blocking Oscillator stage are connected to the
junction of D1105 and D ll 15 through R1118. These
pulses switch the Multivibrator as follows: assume that
Q1105 is conducting and Q1115 is off. The current through
Q1105 raises its emitter more positive than the emitter of
Q1115 and also pulls the collector of Q1105 more negative
than the collector of Q1115. The negative-going pulses
from the Blocking Oscillator stage are connected to the
bases of both Q1105 and Q1115 through diodes D1105 and
D ll 15. This pulse momentarily interrupts the conduction
of both transistors. When the pulse ends, the stored charge
on commutating capacitor Cl 106 determines which transis
tor conducts first. While Q1105 is conducting, Cl 106
charges positive on the Q1105 side to the level at the
emitter of Q1105. This charge holds the emitter of 01115
more negative than the emitter of Q1105 while they are
momentarily off. Then as the Blocking Oscillator pulse
ends, both Q1105 and Q1115 attempt to come into con
duction. However, since the emitter of Q1115 is the more
negative, this transistor achieves the greater base emitter
bias and conducts the more heavily. The collector of Q1115
goes negative and pulls the base of Q1105 negative also, to
reverse bias Q1105. Thus, Q1115 has gained control of the
stage. The action is similar but opposite when the next
trigger pulse is received from the Blocking Oscillator stage.
The output pulses at the collectors of Q1105 and Q1115
are connected to the Steering Switch stage. Waveform 3
(Fig. 3-15) shows the Multivibrator output from Q1105.
The output from Q1115 is similar except that it is 180° out
of phase with the waveform shown.
3-25
To Next Page
Loading...
