4-39
drop in current, the voltage drop across RS is 18 volts; consequently, the output of the regulator has
increased to 102 volts. At this time, the regulating device (RV) decreases in resistance, and 6 amperes of
current flows through this resistance (RV). Thus, the total current RS is once again 10 amperes (6 amperes
through RV; 4 amperes through RL). Therefore, 20 volts is dropped across RS causing the output to
decrease back to 100 volts. You should know by now that if the load resistance (RL) increases, the
regulating device (RV) decreases its resistance to compensate for the change. If RL decreases, the opposite
effect occurs and RV increases.
Now consider the circuit when a decrease in load resistance takes place. When RL decreases, the
current through R
L
subsequently increases to 6 amperes. This action causes a total of 11 amperes to flow
through RS which then drops 22 volts. As a result, the output is 98 volts. However, the regulating device
(RV) senses this change and increases its resistance so that less current (4 amperes) flows through RV. The
total current again becomes 10 amperes, and the output is again 100 volts.
From these examples, you should now understand that the shunt regulator maintains the desired
output voltage first by sensing the current change in the parallel resistance of the circuit and then by
compensating for the change.
Again refer to the schematic shown in figure 4-33 and consider how the voltage regulator operates to
compensate for changes in input voltages. You know, of course, that the input voltage may vary and that
any variation must be compensated for by the regulating device. If an increase in input voltage occurs, the
resistance of R
V
automatically decreases to maintain the correct voltage division between RV and RS. You
should see, therefore, that the regulator operates in the opposite way to compensate for a decrease in input
voltage.
So far only voltage regulators that use variable resistors have been explained. However, this type of
regulation has limitations. Obviously, the variable resistor cannot be adjusted rapidly enough to
compensate for frequent fluctuations in voltages. Since input voltages fluctuate frequently and rapidly, the
variable resistor is not a practical method for voltage regulation. A voltage regulator that operates
continuously and automatically to regulate the output voltage without external manipulation is required
for practical regulation.
Q28. Circuits which maintain constant voltage or current outputs are called dc voltage or dc current
___.
Q29. The purpose of a voltage regulator is to provide an output voltage with little or no ___.
Q30. The two basic types of voltage regulators are ___ and ___.
Q31. When a series voltage regulator is used to control output voltages, any increase in the input
voltage results in an increase/a decrease (which one) in the resistance of the regulating device.
Q32. A shunt-type voltage regulator is connected in serial/parallel (which one) with the load resistance.
The schematic for a typical series voltage regulator is shown in figure 4-34. Notice that this regulator
has a transistor (Q1) in the place of the variable resistor found in figure 4-32. Because the total load
current passes through this transistor, it is sometimes called a "pass transistor." Other components which
make up the circuit are the current limiting resistor (R1) and the Zener diode (CR1).
