Adjustable Output Voltage Regulator

The basic circuit of Adjustable Output Voltage Regulator is essentially the same as for fixed output voltage regulators, but there is provision for an external resistive divider or potentiometer to adjust the output voltage. Positive or negative polarity output voltage can be obtained by the same type of connections described in Fixed Output Voltage Regulator.

Most of the adjustable type voltage regulators also contain one or more protective features such as over-voltage and over-temperature shutdown. Manufacturers data contain specific resistance values and/or potentiometer connections to provide the desired output voltage for a specified input voltage.

Key parameters of Adjustable Output Voltage Regulator

Essentially the same as for the fixed output voltage regulator, except for the following:

Output Voltage: The regulated output voltage maintained over the range of voltage from 1.2V to 37 V in both polarities i.e. positive as well as negative DC voltage.

Input voltage: Input voltage must greater then output voltage by V_REF. Where V_REF= 1.25V. Therefore we can say that input voltage = 1.25+ output voltage.

Safe operation area (SOA): Combination of maximum output current and the voltage difference between input and output voltage. Manufacturers data contain coordinates and a curve that must be used by the designer to locate the input and output voltage range, and output current, within the safe operating area.

Load Regulation: It indicates the change in the output voltage when the load current changes from no load to full load condition. It is denoted by LR.

Load regulation (LR) = V_NL – V_FL

Where, V_NL = output voltage at no load

V_FL = output voltage at full load

Percentage (%) regulation = $\dfrac{V_{NL}-V_{FL}}{V_{FL}}$

A typical value for variable voltage regulator is 0.1% over the specified range of load current.

Line Regulation: It indicates the change in load voltage for a specified range of input voltage. It is denoted by SR.

Line regulation (SR) = V_LH – V_LL

Where,

V_LH = Load voltage with high line voltage

V_LL = Load voltage with low line voltage

Percentage (%) Regulation = $\dfrac{V_{LH}-V_{LL}}{V_{NOM}}$

Where, V_NOM is the normal load voltage under the typical condition. Line regulation is also sometimes called as source regulation. Often stated in millivolts for 1-volt changes, a typical line regulation is 0.1% per volt for variable voltage regulator.

Applications of Adjustable Output Voltage Regulator

In many types of analog equipment, the regulated power supply output voltage must be carefully adjusted for optimum circuit performance. Photo tubes and transistors are typical of devices for which adjustable, regulated, power supplies are required.

What is the advantage of adjustable voltage regulators over fixed voltage regulators?

The adjustable voltage regulators offer variety of performance and reliability advantages over fixed voltage regulators i.e. improved system performance by having line and load regulation of a factor of 10 or better. Also, it improves the system reliability and performance.

Representative Part Number: National Semiconductor LM317 (positive adjustable voltage regulator) National Semiconductor LM337 (negative voltage regulator circuit).

Figure: Author Prototype of Adjustable Voltage Regulator

Previously, we had already posted the Adjustable Bipolar Voltage Regulator Circuit using LM317 and LM337. This circuit provide variable output voltage of 1.2V to 20V in both polarities i.e. $\pm1.2V$ to $\pm 20V$ .

Solved Numerical:

Design a dc voltage regulator for V_OUT = 6V to 18V.

Ans: here, we have to design a voltage regulator with adjustable output requirement. Therefore, we shall use LM317 adjustable IC voltage regulator.

For the circuit,

$V_O = V_{REF}\times (1 + \dfrac{R_2}{R_1}) + I_{ADJ} \times R_2$

Typically, $V_{REF} = 1.25V$ and $I_{ADJ} = 100 \mu A$

Since, the value of I_ADJ is negligible small we shall ignore its value in our design and calculation.

$V_O = V_{REF}\times (1 + \dfrac{R_2}{R_1}) = 1.25\times (1 + \dfrac{R2}{R1})$

Rearranging the equation,

$R_2 = R_1\times (\dfrac{V_O}{1.25}-1)$

When V_O = 6V,

$R_1\times (\dfrac{6}{1.25}-1) = 3.8R_1$

Therefore, $R_2 = 3.8R_1$

When V_O = 18V,

$R_2 = R_1(\dfrac{18}{1.25}-1) = 13.3R_1$

Therefore, $R_2 = 13.4 R$

Taking $R_1 = 10K\Omega$

For $V_0 = 6V$ ,

$R_2 = 3.8 R_1 = 3.8 \times 10 = 38 k\Omega$

For $V<sub>O</sub> = 18V$ ,

$R_2 = 13.4 \times R_1 = 13.4 \times 10 = 134 K\Omega$ .

Thus, if we take $R_1 = 10 k\Omega$ , then the output voltage can be obtained in the range V_O = 6V to 18V by taking a variable resistor R₂ having resistance in the range $R_2 = 38K\Omega$ to $134 k\Omega$ .