TM 11-6125-261-30
signal for this circuit is similar to the circuit for Q112.
2-25. Overcurrent Amplifier
Normally, the transistor is turned "ON". The three over- -
Transistor A220112 (0B-A21Q101, 0C-A21Q1) serves as
current signals are tied together at the output of the filter
the overcurrent-amplifier. Under normal load conditions,
and diode OR gate, A24U29. The gate does not have
resistor A22R124 (0B-A21R108, 0C-A21R8) provides
enough voltage drop across it to conduct. When the
just enough bias current to keep Q112 turned on. The
overcurrent feedback is sufficiently negative (as in a
voltage at the collector of Q112 is 0.6 volts. An alternate
shorted output) the gate is forward biased and Q113 is
input occurs through A22CR112 (0B-A21CR102) when
turned "OFF". The voltage at the collector may now go
the unit goes into an overcurrent mode. A resistor divider
high.
consisting of A22R125 (0B-A21R121, 0C-A21R21) and
2-27. Overcurrent Delay Timer
A22R 126(0B-A21R114, 0C-A21R14) determines the trip
point. The overcurrent feedback signal is applied to one
The delay time is determined by the resistor-capacitor
end of R125. Approximately +16volts is at the other end of
combination of A22L130 and A22C109 which work with
the divider. At the junction of the two resistors the voltage
transistor A22Q115. When Q113 is turned "Off", the
is normally slightly above ground (approximately +0.1
voltage at the collector of Q113 will begin to rise as Q115 is
volts). This leaves diode CR112 with too low a voltage drop
set up so that Q115 will turn "ON" when the voltage at the
across it to conduct. But when the overcurrent feedbaek
base exceeds approximately 5 volts. When the RC network
goes more negative because of an overload, the voltage at
is charged sufficiently, Q115 will turn "ON". This takes
R125/R126 junction also goes more negative. The current
approximately 5 seeonds.
through R124 is enough to keep Q112 turned "ON", but if
2-28. Timer Amplifier
CR112 shunts any current away, Q112 will turn off slightly.
Transistor A22Q114 is an emitter follower which feeds the
As the current sense voltage goes more negative, the
control voltage to the overcurrent summing network. The
junction voltage also goes more negative, the current
voltage at the emitter will be 0.6 volts below the voltage at
through CR112 increases and Q112 is turned "OFF" even
the base. The base voltage is normally high (approximately
more.
+16.0 volts) but when Q115 turns "ON" the voltage at the
b. When Q112 is turned "OFF" the voltage at the
base of Q114 goes low and the emitter voltage follows the
collector will increase to several volts. This signal will
base. The emitter voltage feeds the overcurrent summing
forward bias A22CR108 (B-A21CR102 C-A21CR3)
network. With the high end of this network now at +2.0
and cause the transformer input current feedback signal to
volts instead of + 16.0 volts, the summing signal is pulled
be too high as it feeds the pulser drive summing network.
even more negative and swamps the overcurrent amplifier
The pulser drive will then turn off in proportion to the
"OFF". The pulser drive is recduced to a safe level of
amount of overcurrent. The output voltage will reduce to
operation that can be maintained by the inverter for several
50 to 60 V ac when the output current is twice the rated
minutes. This fold back is easily detected by observing
current of 2.18 amps per phase. If the inverter output is
either the inverter inpur current or the output current. The
shorted, this initial protection will limit the output to
initial output current of 5.45 amps per phase wil;will reduce
approximately 5.45 amps of output current per phase.
to about one half or about 2.7 amps per phase. There is a
2-26. Short Circuit Detector
corresponding reduction in the input current. When the
short or overload is removed the conditions discussed in
The inverter is not designed to maintain the initial limit of
paragraphs 2-25 through 2-28 will reverse themselves
5.45 amps per phase. A separate circuit sets up a delay
automatically and the inverter will resume full output
which will reduce the output current after five seconds. The
capacity in less than two seconds.
detector for this circuit is transistor A22Q113. The base
2-4
