REGULATORS

You know that the output of a power supply varies with changes in input voltage and circuit load current requirements. Because many military electronic equipments require operating voltages and currents that must remain constant, some form of regulation is necessary. The circuits that maintain power supply voltage or current outputs within specified limits, or tolerances, are called regulators. They are designated as dc voltage or dc current regulators, depending on their specific application.

Voltage regulator circuits are additions to basic power supply circuits and are made up of rectifier and filter sections. The purpose of the voltage regulator is to provide an output voltage with little or no variation. Regulator circuits sense changes in output voltages and compensate for the changes. Regulators that maintain voltages within plus or minus () 0.1 percent are quite common. The diagram in figure 3-39 clearly illustrates the purpose of the voltage regulator.

Figure 3-39. - Block diagram of a power supply and regulators

There are two basic types of voltage regulators, series and shunt. Whether a voltage regulator is classified as series or shunt depends on the location or position of the regulating element(s) in relation to the circuit load resistance.

Figure 3-40 illustrates the two basic types of voltage regulators. In actual practice the circuitry of regulating devices may be quite complex. We use the simplified drawings in the figure to emphasize that there are two basic types of voltage regulators. Broken lines highlight the differences between the series and shunt regulators.

Figure 3-40. - Series and shunt regulators.

The schematic in view (A) is that of a shunt-type regulator. It is called a shunt-type regulator because the regulating device is connected in parallel with the load resistance. This is a characteristic of all shunt-type regulators. The schematic in view (B) is that of a series regulator. It is called a series regulator because the regulating device is connected in series with the load resistance.

Series Voltage Regulator

Figure 3-41 illustrates the principle of series voltage regulation. As you study the figure, notice that the regulator is in series with the load resistance and that all current passes through the regulator. In this example, variable resistor R_v is used for regulation. Examine the circuit to determine how the regulator functions. When the input voltage increases, the output voltage also increases. However, since the voltage regulator device (R_v) senses this change, the resistance of the regulating device increases and results in a greater voltage drop through R_v. This causes the output voltage to decrease to normal or, for all practical purposes, to remain constant.

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.

You should know by now that if the load resistance (R_L) increases, the regulating device (R_v) decreases its resistance to compensate for the change. If R_L decreases, the opposite effect will occur and R_v will increase. Now take a look at the circuit when a decrease in load resistance takes place.

When R_L decreases, the current through R_L subsequently increases to 6 amperes. This action causes a total of 11 amperes to flow through R_s which now drops 22 volts. As a result, the output is now 98 volts. However, the regulating device (R _v) senses this change and increases its resistance so that less current (4 amperes) flows through R_v. 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 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 R_v automatically decreases to maintain the correct voltage division between R_v and R_S. 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.

Q.36 The purpose of a voltage regulator is to provide an output voltage with little or no ____.
Q.37 The two basic types of voltage regulators are _______ and _______.
Q.38 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.
Q.39 A shunt type voltage regulator is connected in series/parallel with the load resistance.