TM 11-6625-2735-14-1
Turn-Off Circuit
Q1622 is off under normal operating conditions until
the dc source drops below 22 V and causes Q1622 to
conduct. Q1622 does not conduct during 12 V dc opera-
tion, since the turn-off level circuit is disabled, CR1625,
CR1626, CR1627, and CR1628 form a bridge rectifier. The
inverter waveform is rectified to provide operating power
for the turn-off circuit. C1626 filters the inverter spikes to
keep them from firing Q1626 (scr). R1623 prevents C1626
from charging to the peak-to-peak inverter spikes.
Q1622 saturates when it is turned on. C1622 provides
the high current path for feedback current via CR1625 or
CR1626. Once the inverter is shut down, R1622 es-
tablishes a path to discharge C1622.
If 24 V dc is accidentally applied when the mode switch
is in the 12 V position, transformer T1400 attempts to
produce two times the correct feedback. This is sufficient
to cause VR1622 to conduct. VR1622 provides the firing
current for the scr, Q1626. Scr Q1626 fires and shorts out
the bridge rectifier and the primary of T1631, stopping the
inverter. R1625 prevents Q1626 from being fired by
inverter noise, R1624 and C1626 provide holding current
for Q1626, keeping it conducting until the surge current
created by the over-voltage conditions have terminated.
CR1624 permits rapid charging of C1626.
Start circuit
When S1601 is closed, the external dc source is applied
to C1614, VR1641, and R1645. The initial surge is coupled
to Q1642 through C1614, VR1639, and R1641. Transistor
Q1642 saturates until Q1614 charges through R1639 to the
value determined by VR1639 and the base-emitter junc-
tion of Q1642 (about 5.7 volts), then Q1642 is cut off.
R1641 limits the base current in Q1642. Zener diode
VR1639, once C1614 is charged, makes Q1642 insensitive
to input variations. R1642 limits Q1642 collector current.
Q1644, R1645, and VR1641 provide a constant current
during the time Q1642 is saturated, regardless of the dc
source voltage. CR1643 is reverse biased by this starting
current. The starting current is applied to the inverter
transistors through T1631.
Inverter Circuit
The starting surge is applied to the bases of Q1652,
Q1662, Q1654, and Q1664 trough T1631, R1652, R1662,
R1654, and R1664. Since the transistors do not have
identical parameters, one pair will conduct before the
other, and start the inverter. Operating base current is
provided through CR1643.
R1626, R1631, and T1631 primary and secondary are
the main frequency-determining components for the
inverter, Four base resistors, R1652, R1662, R1654, and
Change 1 6-2.3
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