Because many military electronic equipment require operating voltages and currents that must remain constant, regulators become necessary. You know that the output of a power supply varies with changes in input voltage and circuit load current requirements. 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 control devices, depending on their specific application.

Voltage control circuits are additions to basic power supply circuits and are made up of rectifier and filter sections. The purpose of this voltage control circuit is to provide an output voltage with little or no variation. These circuits sense changes in output voltages and compensate for the changes. The circuits that maintain voltages within plus or minus (±) 0.1 percent are quite common. The diagram shown below clearly illustrates the purpose of this governing circuitry.

Block diagram of a power supply and regulators

Block diagram of a power supply and regulators.

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

The next picture below illustrates the two basic types of voltage controllers. In actual practice the circuitry of the controlling device may be quite complex. We use the simplified drawings in the figure to emphasize that there are two basic types of voltage controllers. Broken lines highlight the differences between the series and shunt types.

Series and shunt regulators

Series and shunt regulators.

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

Series Voltage Regulator

The following picture illustrates the principle of series voltage governance. As you study the figure, notice that the device is in series with the load resistance and that all current passes through the circuit. In this example, variable resistor Rv 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 (Rv) senses this change, the resistance of the regulating device increases and results in a greater voltage drop through Rv. This causes the output voltage to decrease to normal or, for all practical purposes, to remain constant.

Series voltage regulator

Series voltage regulator.

You should be able to see that as the input voltage decreases, the resistance of the variable resistor Rv decreases almost simultaneously, thereby compensating for the voltage drop. Since there is a smaller voltage drop across Rv, the output voltage remains almost constant. Voltage fluctuations within the circuit occur in microseconds.

Shunt Voltage Regulator

The diagram below represents a shunter voltage controller. Notice that variable resistor Rv is in parallel with the load resistance RL and that fixed resistor RS 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 type controller, as shown in the picture below, output voltage is determined by the current through the parallel resistances of the governing device (Rv), the load resistance (RL), and the series resistor (RS). For now, assume that the circuit in the picture is operating under normal conditions, that the input is 120 volts dc, and that the desired controlled output is 100 volts dc. For a 100-volt output to be maintained, 20 volts must be dropped across the series resistor (RS). If you assume that the value of RS is 2 ohms, then you must have 10 amperes of current through Rv and RL. (Remember: E = IR.) If the values of the resistance of Rv and RL are equal, then 5 amperes of current will flow through each resistance (Rv and RL)

Shunt voltage regulator

Shunt voltage regulator.

Now, if the load resistance (RL) increases, the current through RL will decrease. For example, assume that the current through RL is now 4 amperes and that the total current through RS is 9 amperes. With this drop in current, the voltage drop across RS is 18 volts; consequently, the output of the control circuit has increased to 102 volts. At this time, the governing device (Rv) decreases in resistance, and 6 amperes of current flows through this resistance (Rv). Thus, the total current through RS is once again 10 amperes (6 amperes across Rv, 4 amperes through RL); therefore, 20 volts will be dropped across RS causing the output to decrease back to 100 volts.

You may 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 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 the picture above and consider how the voltage controller 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 controlling circuit 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 governance 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 control the output voltage without external manipulation is required.

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