Figure 1-28.-Zener reference bridge.

Consider the input voltage

to be 22 volts; then

voltage output that varies linearly with changes in

the output voltage will be 1 volt. Next, consider the input

generator output frequency rather than generator output

voltage

to be 24 volts; then the output voltage will

voltage. The signal voltage obtained from the

be 2 volts. Although the input voltage is 22 volts to 24

frequency-sensing transformer is rectified, filtered, and

volts, the output voltage is only 1 volt to 2 volts. There-

compared in a Zener reference voltage divider, all

fore, without adding additional components to lower the

contained within the detector circuit. This circuit

voltage to the point where it can be used as abase drive

provides an interesting application of Zener diodes, as

for a transistor, the output voltage of the bridge can be

shown in figure 1-28. The purpose of the Zener

used as abase drive for a transistor.

reference bridge is to compare a high-supply voltage

with a reference voltage and to provide a low-voltage

The signal leaving the Zener bridge is amplified by

two dc transistor amplifiers, in the detector, before going

amplitude output signal voltage to be used as a base

to the preamp and trigger (fig. 1-27).

drive for a transistor.

The purpose of the preamp and trigger is to amplify

The Zener reference bridge consists of resistors R1,

and convert the varying dc input voltage into controlled

R2, R3, and Zener diode D1, as shown in figure 1-28.

pulses of sufficient amplitude to fire the SCRs.

Resistors R1, R2, and R3 are equal, and the Zener diode

is equal

D1 has a breakdown rating of 10 volts. When

In the trigger circuit, the signal (pulse) amplitude

to or less than 10 volts, negligible current will flow

controls the tiring point of the SCRs in the motor rotor

through R1, and the bridge is operating in mode I, as

control circuit (which are in series with large, approx-

shown on the graph in figure 1-28. As

rises above 10

imately 3,000-watt resistors). Thus, control is exerted

volts, the voltage drop across D1 remains constant at 10

on the motor rotor.

volts, and the current through R2 and R3 increases,

increasing the voltage drop across R2 and R3. When

VOLTAGE-REGULATING

equals 20 volts, the drop across resistors R1, R2, and R3

SYSTEM

is 10 volts, so

is zero. When

is between 10 volts

and 20 volts, the bridge is in mode H, as shown on the

The voltage-regulating system is composed of the

graph in figure 1-28. As

rises above 20 volts, the

voltage regulator and the static exciter (fig. 1-27). The

voltage at point B will rise above 10 volts; however, the

voltage regulator receives its signal from the generator

voltage at point A will remain at 10 volts, and potential

output. The static exciter receives its signal input from

the power section in the voltage regulator.

differences between points B to A will increase. For

greater than 20 volts, the bridge is in mode III, which is

The operation of the detector in the voltage reg-

the normal operating mode.

ulator is similar to that of the frequency regulator, in that

the detector senses a change in generator output; how-

ever, the change is in voltage rather than frequency. The

increase or decrease in voltage is rectified, faltered, and

compared prior to amplification. Again the comparison

is made on a Zener reference bridge before amplification

in dc amplifiers.

The preamp and trigger operate essentially as

described in the section under frequency regulation,

except that in this case the signal is fed to a power

section.

The power circuit provides an application of SCR

operation. This section (fig. 1-29) consists of three

diodes (D1, D2, and D3) and two SCRs (SCR1 and

SCR2). D2, D3, SCR1, and SCR2 are connected in the

normal full-wave rectifier bridge manner. No current

will flow out of the bridge (between points E and F) until

the SCRs receive a trigger pulse at the gates that will

turn the SCRs on. Assume that during the first half cycle

Figure 1-28.-Zener reference bridge.

of applied ac voltage (time 0 to 1), SCR 1 has its anode

1-24