diodes. Under these conditions, the bridge is balanced
of T6 is rectified by CR2; the dc is routed from terminal
X on CR2 through the lower winding of T4 to F+ and
and no current flows between points X and Y through
the field.
the control winding (CW). In view B, line voltage has
risen to 210 volts. Since the voltage drop across VR1
The amount of power T6 may provide to the exciter
field is governed by the impedance of the series load
and VR2 cannot change, the increase of applied
windings 3-4 in L1. The impedance of load windings
voltage alone has caused an imbalance to the bridge;
3-4 is, in turn, regulated by the bridge-powered dc
current flows from X to Y through CW. In view C,
control winding 1-2 in L1.
where line voltage drops to 190 volts, the Zener diodes
The complete voltage regulating loop can now
continue to have voltage drops of 100 volts. The
be seen. AC line voltage at T1, T2, and T3 acts
decrease of applied voltage appears across R1 and R2;
through CR1 and BRDG 1 into the control field of
the bridge is again unbalanced but in the opposite
the magnetic amplifier. This governs the output of
direction. This causes current to flow from Y to X
T6 through L1, CR2, and lower winding T4 into the
through CW.
exciter control field. The exciter control field then
You can see that the essential function of the bridge
regulates the exciter armature voltage, thereby
controlling generator field strength, and finally the ac
is to translate a variation of line voltage into a current
output at T1, T2, and T3.
through the control winding.
The other path for dc from terminal X of CR2 is
Q7-1. What are the two types of materials used to
through T4, T5, temperature-compensator TC1, down
make semiconductors?
through L1's winding 5-6, upward through winding
1.
Silicone and Titanium
8-7, and back to the rectifier source, terminal Y. This
2.
Titanium and plastic
circuit is used to bias the magnetic amplifier at the
3.
Germanium and Silicone
proper operating level.
4.
Germanium and plastic
Voltage adjustment of this regulator is done by
Q7-2. Which of the following components is
positioning R1. This sets the bridge circuit BRDG 1,
considered to be the simplest kind of
and, in turn, sets the ac line output voltage to 120, 117,
semiconductor?
etc. To understand the function of R1 and BRDG 1,
refer to figure 7-20.
1.
A transistor
2.
A diode
Figure 7-20 illustrates the primary features of a
3.
A Zener diode
voltage-sensing bridge circuit. The line ac voltage is
4.
A silicone-controlled rectifier
rectified to a nominal value of 200 volts dc and applied
across a bridge consisting of R1, VR1, R2, and VR2.
Q7-3. Which of the following components is
Resistors R1 and R2 are fixed resistors. Zener diodes
commonly referred to as a voltage reference
VR1 and VR2 are used as the voltage reference points.
diode?
They maintain a constant voltage drop across
themselves of 100 volts. When line voltage is at the
1.
A Zener diode
proper value, the dc bridge voltage is 200 volts. The
2.
A regulator diode
ohmic value of R1 and R2 is such that their voltage
3.
A NPN diode
drops are the same as the voltage drops of the Zener
4.
A PNP diode
ADJ
ADJ
ADJ
100V
100V
100V
90V
11 0 V
11 0 V
VR2
VR2
VR2
R1
R1
30
R1
L1
L1
210
200
L1
A-C
190
VDC X
VDC X
Y
X
Y
Y
ICW2
VDC
1
2
2
1
R2
R2
R2
11 0 V
VR1
VR1
90V
100V
VR1
100V
100V
100V
A
B
C
ASf07020
Figure 7-20.--Views of line voltage-sensing bridge (BRDG).
7-12
