Variable Voltage Regulator

Topics: Voltage regulator, Resistor, Capacitor Pages: 5 (1148 words) Published: May 4, 2010


In order to achieve several small scale and large scale objectives in various electronic circuits, it is highly essential to regulate the voltage supply, given to electronic circuits. This project, namely VARIABLE VOLTAGE REGULATOR, aims at fulfilling such small goals, using the 3-terminal voltage regulator LM317.

The circuit consists of an IC LM317 and a set of resistors and capacitors, in addition to the transformer and the rectifying element.

The AC voltage, from the mains supply is initially stepped down to the desired voltage, which is then rectified to be applied to the input terminal of the voltage regulator IC. As mentioned earlier the voltage regulator then takes care of supplying a constant voltage at the output terminal. This voltage can be varied according to our requirements with the help of a variable resistor. This project designed has the ability to vary the voltage values from 1.5V to 25V at an output current of 1A.


The LM317 is versatile in its applications, including uses in programmable output regulation and local on-card regulations

Typical other applications of the voltage regulator circuits include the following: • 5V Logic Regulator with Electronic Shutdown
• Slow Turn-On 15V Regulator
• High Stability 10V Regulator
• 0 to 30V Regulator
• Power Follower
• High Gain Amplifier
• 4A Switching Regulator with Overload Protection • Precision Current Limiter
• Tracking Preregulator
• AC Voltage Regulator
• Digitally Selected Outputs

Besides replacing fixed regulators, the LM317 is useful in a wide variety of other applications. Since the regulator is “floating” and sees only the input-to-output differential voltage, supplies of several hundred volts can be regulated as long as the maximum input to output differential is not exceeded, i.e., avoid short-circuiting the output.




IC - LM317
D1-D4 – 1N4007 diode
C1 – 2200µF, 50V electrolytic capacitor
C2 – 0.1µF, ceramic disc capacitor
C3 – 10µF, 40V electrolytic capacitor
C4 – 22µF, 35V electrolytic capacitor
R1 - 47Ω, 1/4W resistor
R2 - 10Ω, 1/2W resistor
R3, R4 - 100Ω, 1/2W resistor
VR1 – 1kΩ, 1/2W wire wound potentiometer
LEDs – green, red

1. IC LM317: An Adjustable Linear Voltage Regulator


LM317 is the standard part number for an integrated three-terminal adjustable linear voltage regulator. LM317 is a positive voltage regulator supporting input voltage of 3V to 40V and output voltage between 1.25V and 37V. A typical current rating is 1.5A although several lower and higher current models are available. Variable output voltage is achieved by using a potentiometer or a variable voltage from another source to apply a control voltage to the control terminal. LM317 also has a built-in current limiter to prevent the output current from exceeding the rated current, and LM317 will automatically reduce its output current if an overheat condition occurs under load. LM317 is manufactured by many companies, including National Semiconductor, Fairchild Semiconductor, and STMicroelectronics. Although LM317 is an adjustable regulator, it is sometimes preferred for high-precision fixed voltage applications instead of the similar LM78xx devices because the LM317 is designed with superior output tolerances. For a fixed voltage application, the control pin will typically be biased with a fixed resistor network, a Zener diode network, or a fixed control voltage from another source. Manufacturer datasheets provide standard configurations for achieving various design applications, including the use of a pass transistor to achieve regulated output currents in excess of what the LM317 alone can provide....
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