Three Terminal Fixed Voltage Regulators As the name suggests, three terminal voltage regulators have three terminals namely input which is unregulated (V in ), regulated output (V o ) and common or a ground terminal. These regulators do not require any feedback connections. Figure shows the basic the 3 terminal voltage regulator. Block diagram of Basic Three Terminal IC Regulator The figure shows the functional block diagram of basic three terminal IC regulator. This is basically a series voltage regulator circuit. A part of output voltage is taken with the help of potential divider formed by R1 and R2. This is compared with reference voltage, V ref internally generated with the help of zener diode.
After comparison, a control signal is generated which is applied through protective circuit to the series pass transistor working as control element. This element works as a variable resistance. The control signal adjusts the control element in such a way that output voltage remains constant Thermal shutdown means that the chip will automatically turn itself off if the internal temperature exceeds, typically, 175 C. The current limiting circuit will protect the chip from excessive load current. Because of the thermal shutdown and current limiting, the IC voltage regulator chip is almost indestructible. IC Series of Three Terminal Fixed Voltage Regulators The popular IC series of three terminal regulators is μa78xx and μa79xx. The series μa 78XX is the series of three terminal positive voltage regulators while μa79xx is the series of three terminal negative voltage regulators. The last two digits denoted as XX, indicate the output voltage rating of the IC. These ICs are provided with adequate heat sinking and can deliver output currents more than 1A. These ICs do not require external components. These are provided with internal thermal protection, overload and short circuit protection. Typical Connection of IC 7805 Regulator The Figure shows the typical connection of IC μa 7805 regulator. The maximum input voltage which can be applied to IC 7805 is 35 V. The maximum dropout voltage is 2.5V. The output of 7805 regulator is 5 V fixed. Hence minimum input voltage required is 5 + 2.5 = 7.5. So input range of IC 7805 regulator is 7.5 V to 35 V to get fixed 5 V as regulated output. This is indicated in the Figure
Positive 5 V Power Supply using IC 7805 A 5V output voltage supply system using full wave bride rectifier, capacitor filter and IC regulator 7805 is shown in the Figure. The a.c. line voltage is 230 V which is stepped down to 15 V using a transformer. A full wave rectifier along with the capacitor voltage provides the unregulated voltage input to IC 7805 regulator. This input contains a.c. ripple of few volts. The IC 7805 regulator provides the regulated output of 5V. Applications of IC 78XX and 79XX These ICs are regulator ICs and are basically used to provide constant d.c. voltages to various components in complex electronic circuits. The IC 7805 is typically used to provide constant 5V supply to the digital circuits. V R = 5V for 7805 IC
Three Terminal Adjustable Regulator: 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 and 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 40V. The following table shows the various types of adjustable regulators and the corresponding output voltage range and maximum output current rating.
Connection Diagram of LM 317 Regulators The typical connection diagram of LM 317 regulator is shown in the Figure.
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 W. 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 diodes are necessary if output voltage is higher than 25 V. The functional diagram of LM 317 along with the capacitors and the protecting diodes is shown in the figure.
Limitations of Linear Voltage Regulators The regulators discussed up till now are called as linear regulators because the series pass transistor used in such regulators operates in the linear region. But such linear regulated power supplies have following limitations: 1. The required input step down transformer is bulky and expensive. 2. Due to low line frequency (50 Hz), large values of filter capacitors are required. 3. The efficiency is very low. 4. Input must be greater than the output voltage. 5. As large is the difference between input and output voltage, more is the power dissipation in the series pass transistor. 6. For higher input voltages, efficiency decreases. 7. The need for dual supply is not economical and feasible to achieve with the help of linear regulators. The switching regulators overcome all these limitations.