TM 11-6125-261-30
2-16. Summing Network
Two resistors form a current summing network. One side is
and R33 in series to maintain the ESR (effective series
fed by the negative going full wave rectified sine wave. The
resistance). The 40 kilohertz switching rate of the pulser is
other side of the network is fed by a positive going full wave
too fast to feed to the center tap of the main transformer.
rectified signal which is fed back from the center tap of the
The transformer needs a current source that is operating at
main transformer A3T1. The two resistors A22R113 and
400 hertz sinusoidal. Since the 28 V dc pulsed current
A22R114 (0B-A21R104 and A 21R112, 0C-A21R4 and
source is positive only, the result is a wave form that is full
A21R12) are selected so that the result of the two opposing
wave rectified. The inductor and capacitor combine to
signals is close to zero at their junction. Since both signals
smooth out the high speed 40 kilohertz signal into the 400
are continously changing, this is not true and some changes
hertz sinusoidal waveform.
to the signal can be seen. The negative going input to the
2-20. Main Transformer
summing network is the reference signal. The positive
The positive voltage pulsed current source in transformed
by main transformer A3T1 to the alternating current that is
going signal is the feedback of what is actually being fed to
desired at the output. The pulsed current source is
the main transformer. The desired result is for the pulser
alternately switched by the inverter switches. The trans-
section (which the junction signal controls) to approximate
former couples the now alternating current to the secondary
the reference signal as closely as possible.
winding which then puts out the complete sine wave signal.
2-17. Schmidt Trigger
The transformer output receives more filtering before
The difference signal from the summing network is fed to
being put out at connector J2. The main transformer is
the base of A22Q108 (0B-A21Q103, 0C-A21Q3). Since
specially designed for 3 phase operation. It includes a
the emitter of Q108 is near ground, the difference signal will
tertiary (third) winding to eliminate the third harmonic
be biased approximately 0.7 volts signal. If the pulser
noise.
feedback is greater than the reference, the signal at the base
2-21. Fan Transformer
of Q108 is biased more positive and turns Q108 "ON". If
Transformer A3T7 taps off a small amount of the output
the pulser feedback is less than the reference, the signal at
from the main transformer to drive the cooling fan.
the base of Q108 is more negative and Q108 turns "OFF".
Q108 drives A22Q109 (0B-A21Q102, 0C-A21Q2) in the
2-22. Turning Capacitors
standard common emitter ciruit. The signal at the collector
For correct operation, the capacitance must be selected so
of Q109 is the inverse of the signal of Q108.
that the secondary winding and capacitors A27C24 with
A8C24A, A27C25 with A8C25A, and A27C26 with
2-18. Pulser Switch Amplifier
A8C26A, form resonant tanks for 400 hertz. This action is
a. The signal from the schmidt trigger is further
a very effective filter for eliminating the harmonic signals
amplified before being applied to the pulser switches. Like
that are apart of any oscillator/amplifier. The capacitance
the inverter switches, the pulser switches are divided into
required is approximately 10 mfd for each phase. A27C25
three identical sections. Transistors A1Q13, Q14, and Q15
and A8C25A are the tuning capacitors for phase B.
receive the signal from the three schmidt triggers. Phase A
A27C26 and A27C26A are the tuning capacitors for phase
drives A1Q14, phase B drives A1Q13, and phase C drive
A, and A27C24 and A27C24A are the tuning capacitors
A1Q14. They boost the switching signal current to drive
for phase C.
transistors A1Q1 through A1Q12. In each phase, the first
2-23. Output RFI Filter
transistor in a complementary darlington circuit. This is
This circuit is the final filter. The series inductors and
necessary to insure that the switching signal generated by
parallel capacitors in filter A5 have values chosen to
the Schmidt trigger has been boosted sufficiently to drive
eliminate the possibility of any radio frequency energy
the very high power transistor switches.
being on the output power line. This includes RFI which
b. The pulser switching is responsible for controlling the
may be emitted by the inverter and RFI being reflected
current being applied to the center taps of the main
back from external sources.
transformer A3T1. In operation, the switches are turned
"ON" and "OFF" at approximately a 40 kilohertz rate.
2-24. Over Current Detector
This switching changes as the load increases or decreases.
The overcurrent circuit is designed to protect the inverter
When the inverter is operating with a very small load on the
when an overload situation occurs. The overcurrent may
output, only 4 or 5 pulses per half cycle are necessary to
exist if the unit is simply overloaded (the output is greater
maintain the output at the proper voltage. As the load
than 3.27 amps per phase) or if the unit is completely
requirement increases, the pulser will increase the number
shorted. The basic sense point for overcurrent of phase A is
of pulses per half cycle to supply the increased current
at resistor A2R26 mounted on A2T3, phase B overcurrent
requirement.
is sensed by A2R25 and phase C overcurrent is sensed by
A2R27. This resistor senses the amount of base drive
2-19. Pulse Integrator
current that is being required by the inverter switches. The
The pulse integrator shapes the current from the pulser.
voltage at this point is normally about -0.2 volts at full
Each power switch transistor in the pulser is tied to an
load. As the load increases, this voltage will become more
inductor A1L2-L10 and a capacitance A28C18, C 19,
negative.
A28C20, C21, A28C22, C23 with resistors A28R32, R33,
2-3
