4-42
The schematic shown in figure 4-37 is that of a shunt voltage regulator. Notice that Q1 is in parallel
with the load. Components of this circuit are identical with those of the series voltage regulator except for
the addition of fixed resistor RS. As you study the schematic, you will see that this resistor is connected in
series with the output load resistance. The current limiting resistor (R1) and Zener diode (CR1) provide a
constant reference voltage for the base-collector junction of Q1. Notice that the bias of Q1 is determined
by the voltage drop across RS and R1. As you should know, the amount of forward bias across a transistor
affects its total resistance. In this case, the voltage drop across RS is the key to the total circuit operation.
Figure 4-37.—Shunt voltage regulator.
Figure 4-38 is the schematic for a typical shunt-type regulator. Notice that the schematic is identical
to the schematic shown in figure 4-37 except that voltages are shown to help you understand the functions
of the various components. In the circuit shown, the voltage drop across the Zener diode (CR1) remains
constant at 5.6 volts. This means that with a 20-volt input voltage, the voltage drop across R1 is 14.4
volts. With a base-emitter voltage of 0.7 volt, the output voltage is equal to the sum of the voltages across
CR1 and the voltage at the base-emitter junction of Q1. In this example, with an output voltage of 6.3
volts and a 20-volt input voltage, the voltage drop across RS equals 13.7 volts. Study the schematic to
understand fully how these voltages are developed. Pay close attention to the voltages shown.
Figure 4-38.—Shunt voltage regulator (with voltages).
