Figure 1-45.-Generator field rectifier, simplified schematic.

resistance is controlled by the current through resistor

relaxation oscillator that initiates controlled rate pulses

R6.) An increase in the error signal across R6 decreases

to trigger the field rectifiers. Q4 turns on when the

the collector-emitter resistance of Q3 and thus decreases

voltage across C4 and the emitter current of Q4 reach

the charge time of C4. If the error signal increases, the

preset values. When Q4 conducts, trigger pulses are

charge time of C4 is increased.

applied to the trigger amplifier Q5.

Capacitor C4 discharges when the voltage across it

is approximately 9 volts (the peak point voltage of

Trigger Amplifier Circuit

unijunction transistor Q4).

A synchronizing circuit (discussed later) clamps C4

The trigger amplifier (fig. 1-44) amplifies and

to ground, thus delaying the RC time cycle. A rate

shapes the trigger pulses. The circuit is a common

feedback signal is also applied by the rate circuit to the

emitter amplifier with RC input (R13 and C5) and

collector of Q2. This signal modifies the error signal,

transformer output (T2).

thus stabilizing the voltage regulator.

Transistor Q5 is protected against the inductive

kickback voltage of T2 by diode CR8. Resistors R14 and

Rate Circuit

R15 with capacitors C6 and C7 include a pulse-shaping

network to prolong the life of the SCRs in the field

The function of the rate circuit is to dampen the

rectifier.

generator output voltage distortion about a set point.

Otherwise, the high gain of the voltage regulator would

GENERATOR FIELD RECTIFIER

cause the generator output voltage to hunt. The method

used for damping the voltage distortion is feeding back

an inverted signal (opposite to the error signal),

Both the ac and dc field rectifiers are similar in

proportional to the rate of voltage change.

operation. The function of the generator field rectifier is

to provide controlled dc power to the generator field to

The rate circuit (fig. 1-44) consists of a common

regulate the generator input voltage with the field power

emitter amplifier (Q6, R19, and R20) and an integrating

being proportional to the conduction line of the SCRs.

circuit (R16 and C8). Resistor R17 is a discharge resistor

for C8, and CR9 and CR10 are common rectifiers.

Transformer T3 (fig. 1-45) transfers voltage from

the generator that is rectified by the bridge rectifier

The input is supplied by the generator field rectifier

(CR11 through CR14). The conduction of the bridge is

(described later), integrated, and applied as forward bias

controlled by SCRs, CR13, and CR14. One series

to the base of Q6. Any change in the base is amplified

combination of diode and SCR (CR11, CR13, or CR12,

and passed by C9 to the collector of the error detector,

CR14) may conduct for alternate half cycles. The dc

Q2. As this signal is opposite to the error signal, it will

output is the controlled generator field power.

decrease conduction and stabilize the circuit.

Diode CR 15 is used as an inductive kickback diode

Unijunction Trigger Circuit

to provide a path for the current generated by the

collapsing magnetic field of the generator during the idle

The trigger function is performed by the unijunction

portion of each cycle. The amount of field power can be

trigger circuit (fig. 1-44). Unijunction transistor Q4 is a

adjusted by R21.

Figure 1-45.-Generator field rectifier, simplified schematic.

1-41