TM 11-8125-240-34
Section V. CIRCUIT ANALYSIS, REGULATOR 4B93-1-A.
2 - 9 . Voltage Regulation (4B93-1-A)
will bias Q1 off. During the positive half-cycle when Q1
is off, capacitor C1 is permitted to charge through R4
(fig. FO-1)
from the reference voltage created by Zener diodes
a. Ac generator single-phase output voltage is
CR2 and CR5. When Q1 turns on, during the negative
sensed, half-wave rectified, reduced by a voltage
half-cycles, Cl is discharged and will not start charg-
divider and changed to a pulsating dc voltage with a
ing again until the beginning of the next negative half-
ripple, through CR11, R24, R25 and C5. The superim-
cycle. Diode CR1 protects the base-emitter junction of
posed ripple is further filtered into a sawtooth
Q1 during the positive half-cycles.
waveshape. This dc voltage with the superimposed
b. During the low frequency conditions, the time
sawtooth ripple varies directly with the ac voltage out-
that Cl is allowed to discharge is longer than the time
put of the ac generator and is compared with a Zener
allowed during higher frequency operation. This means
reference composed of CR2, CR3, CR4, CR5, R7 and
that for each half-cycle that Q1 is on, at low frequency
R17.
conditions, Cl will discharge to a lower potential.
b. Since the top of the resultant applied dc voltage
Transistor Q10 is an emitter-follower whose base
signal is a constantly varying sawtooth wave, it will
voltage is adjust by R5. The voltage across R5 is held
automatically straddle the Zener voltage reference line
constant by the reference voltage. The adjustment of
such that the width of the pulse formed will, just sus-
R5 determines the voltage which Q10 applies across R6
tain it in a steady state, closed loop condition. Max-
and C7. The emitter of Q2 is also connected to R6 and
imum excitation will be amplified to the generator
C7.
when the pulse is maximum width or full-on and
minimum excitation occurs at zero width or when full-
C. The voltage across Cl is connected to the base of
off occurs. The average regulated voltage will then de-
Q2, thus the voltage level to which Cl must charge to
pend on the time ratio of the on and off condition or
bias Q2 off is adjusted by R5. When the voltage across
pulse width.
Cl reaches the level required to bias Q2 off, collect cur-
c. When the ac generator output voltage increases, a
rent decreases through R8, R9 and the base-emitter
larger negative voltage is seen at the wiper arm of R2
junction of Q3. When Q3 turns off, a path is provided
causing Q9 to turn off (decrease pulse width). With Q9
for Q4 base current to flow through R11 and result in
off, current amplifier transistors Q7, Q8 and Q11 turn
Q4 being on. With Q4 on, current amplifier transistors
off, causing less current to flow through the alternat-
Q5 and Q6 turn on allowing more current to flow
ing field winding, thus decreasing the ac voltage to
through the shunt field to the dc motor, decreasing its
nominal value.
speed, thus lowering the frequency of the ac generator
d. When the ac generator output voltage decreases,
to nominal value.
the less negative voltage is seen at wiper arm of R2
d. During the high frequency conditions, the time
causing Q9 to turn on (increase pulse width). With Q9
that Cl is allowed to discharge is faster than the time
on, Q7, Q8 and Q11 turn on allowing more current to
allowed at low frequency operation. This means that
flow through the alternating field winding, thus in-
for each half-cycle that Q1 is off, Cl will charge to a
creasing the ac voltage to nominal value. Variable
higher potential, biasing Q2 on. With Q2 on, base cur-
resistor R2 is used to set the ac generator output
rent is provided through R8 to turn Q3 on. As Q3 turns
voltage.
on, the base of Q4 is reversed biased and Q4 turns off,
2 - 1 0 . Frequency Regulation (4B93-1-A)
and in turn current amplifier transistors Q5 and Q6
turn off. With Q5 and Q6 off, less current flows
(fig. FO-l)
through the shunt field of the dc motor increasing the
a. Ac generator output voltage is connected to the
frequency of the ac generator to a nominal value,
base of transistor Q1 through resistors R2 and R27.
Variable resistor R5 is used to set the ac generator out-
This signal overrides Q1 so that the negative half-cycle
put frequency.
will bias Q1 into saturation and the positive half-cycle
2-8
