Three Terminal Adjustable Regulators: LM 317

Three Terminal Adjustable Regulators : LM 317

Besides fixed voltage regulators, IC voltage regulators are available which allow the adjustment of the output voltage. The output voltage can be adjusted from 1.2 V to as high as 57 V with the help of such regulators. In such regulator ICs the common terminal plays the role of control input and hence called as ADJUSTMENT (ADJ) terminal. The LM317 series is the most commonly used three terminal adjustable regulators. These devices are available in a variety of packages which can be easily mounted and handled. The power rating of such regulators is 1.5 A. The maximum input voltage of LM317 is 40 V. The Table 5.11.1 shows the various types of adjustable regulators and the corresponding output voltage range and maximum output current rating.

Adjustable voltage regulators have the following advantages over fixed voltage regulators,

1) Improved line and load regulation by a factor of 10 or more.

2) Because of improved overload protection, greater load current can be drawn over the given operating temperature range.

3) Improved reliability for the power supply using these regulators.

Thus, adjustable voltage regulators have become more popular because of their versatility, performance and reliability. The LM 317 is the most widely used adjustable voltage regulator series.

1. Connection Diagram of LM 317 Regulator

The typical connection diagram of LM 317 regulator is shown in the Fig. 5.11.1.

The LM 317 requires only two external resistances R₁ and R₂, to set the required output voltage. Internally it develops a reference voltage of 1.25 V between OUT and ADJ terminals, which is denoted as V_REF. This voltage is impressed across the resistance 

R₁. So for constant V_REF and R₁ current I_R1 is also constant. So resistance R₁ which sets the current I_R1 is called current set resistor or program resistor.

The current I_ADJ flows out from the ADJ terminal and is quiescent current I_Q voltage regulators. This current, along with I_R1, flow through the resistance R₂. The design of LM 317 is such that Fi3- 5-11-1 The schematic diagram of LM 317 the current I_ADJ is very small and remains constant with line and load changes. The maximum value of I_ADJ is 100 µA which is very small.

By KVL, we can write,

The current I_ADJ is very small and hence the drop I_ADJ R₂ is also very small and can be neglected.

V_o = 1.25[ 1 + R₂ / R₁]  … (5.10.4)

Thus the output voltage is a function of R₁ and R₂. Keeping R1 fixed and varying R₂, the output voltage can be adjusted. The program resistor R1 is generally 240 Ω.

Generally no capacitors are required. But if LM 317 is located far from the power supply filter then the capacitors C₁ and C₂ are required. The input capacitor C1 is generally 0.1 µF disc or 1 µF tantalum while the output capacitor C₂ is in the range of 1 to 1000 µF. To get high ripple rejection, the capacitor C₃ is used. To avoid the discharging of the capacitors through low current points into the regulator, the protecting diodes are used. The Fig. 5.11.2 shows the functional diagram of LM 317 alongwith the capacitors and the protecting diodes.

Key Point The diodes are necessary if output voltage is higher than 25 V.

Applications

1. Various electronic circuits. 2. Medical and fitness applications

3. Printers 4. Various appliances 5. T.V. sets and set top boxes

Example 5.11.1 Determine the regulated output voltage for the LM 317 voltage regulator shown in the Fig. 5.11.3.

Solution : The resistances used are,

Example 5.11.2 Find the range in which output voltage can be varied with the help of LM 317 regulator using R₁ = 820 Ω and R₂ as 10 k Ω potentiometer.

Solution : For LM 317, the current I_ADJ = 100 µA

When R₂ is minimum i.e. R₂ = 0 then,

Thus the output voltage can be varied in the range 1.25 V to 17.49 V

Review Questions

1. Draw and explain the functional block diagram of the LM 317 three terminal adjustable regulator.

Dec,-03, Marks 12

2. What do you mean by variable voltage regulator ? List its applications.

May-07, Dec.-16, Marks 8