3-38 Figure 3-41.—Series voltage regulator. You should be able to see that as the input voltage decreases, the resistance of the variable resistor R_{v}decreases almost simultaneously, thereby compensating for the voltage drop. Since there is a smaller voltage drop across R_{v}, the output voltage remains almost constant. Voltage fluctuations within the circuit occur in microseconds. Shunt Voltage Regulator The diagram in figure 3-42 represents a shunter voltage regulator. Notice that variable resistor R_{v} is in parallel with the load resistance R_{L} and that fixed resistor R_{S} is in series with the load resistance. You already know the voltage drop across a fixed resistor remains constant unless there is a variation (increase or decrease) in the current through it. In a shunt regulator as shown in figure 3-42, output voltage regulation is determined by the current through the parallel resistances of the regulating device (R_{v}), the load resistance (R_{L}), and the series resistor (R_{S}). For now, assume that the circuit in figure 3-42 is operating under normal conditions, that the input is 120 volts dc, and that the desired regulated output is 100 volts dc. For a 100-volt output to be maintained, 20 volts must be dropped across the series resistor (R_{S}). If you assume that the value of R_{S} is 2 ohms, then you must have 10 amperes of current through R_{v} and R_{L}. (Remember: E = IR.) If the values of the resistance of R_{v} and R_{L} are equal, then 5 amperes of current will flow through each resistance (R_{v}and R_{L}). Figure 3-42.—Shunt voltage regulator. Now, if the load resistance (R_{L}) increases, the current through R_{L} will decrease. For example, assume that the current through R_{L} is now 4 amperes and that the total current through R_{S} is 9 amperes. With this drop in current, the voltage drop across R_{S} is 18 volts; consequently, the output of the regulator has increased to 102 volts. At this time, the regulating device (R_{v}) decreases in resistance, and 6 amperes of current flows through this resistance (R_{v}). Thus, the total current through R_{S} is once again 10 amperes (6 amperes across R_{v}, 4 amperes through R_{L}); therefore, 20 volts will be dropped across R_{S} causing the output to decrease back to 100 volts.