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 will occur and
Rv will increase. Now take a look at the circuit when a decrease in load resistance takes place.
When RL decreases, the current through RL subsequently increases to 6 amperes. This action causes a
total of 11 amperes to flow through RS which now drops 22 volts. As a result, the output is now 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
From these examples, you should now understand that the shunt regulator maintains the desired
output voltage by sensing the current change that occurs in the parallel resistance of the circuit.
Again refer to the schematic shown in figure 3-42 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. Consider an increase in input voltage.
When this happens the resistance of Rv 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 we have explained the operation of voltage regulators that use variable resistors; however, this
type of regulation has limitations. Obviously, the variable resistor cannot be adjusted rapidly enough to
compensate for frequent fluctuations in voltage. 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.
Q36. The purpose of a voltage regulator is to provide an output voltage with little or no ____.
Q37. The two basic types of voltage regulators are _______ and _______.
Q38. When a series voltage regulator is used to control output voltages, any increase in the input
voltage results in an increase/a decrease in the resistance of the regulating device.
Q39. A shunt type voltage regulator is connected in series/parallel with the load resistance.
Basic VR Tube Regulator Circuit
Although we covered the electrical characteristics of the VR tube in chapter 2 of this module, we
now need to cover the capabilities and limitations of the VR tube itself.
Figure 3-43 shows a basic VR tube regulating circuit. The voltage produced by the source is 150
volts. The VR 90 will provide a constant 90 volts across the load resistance (RL) if the tube is operated in
the normal glow discharge region. This means that 60 volts is dropped across RS, which is the series
limiting resistance used to limit the current through the VR tube.