Variable Voltage Regulator

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:

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• 5V Logic Regulator with Electronic Shutdown
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• 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 - 47? , 1/4W resistor R2 - 10? , 1/2W resistor R3, R4 - 100? , 1/2W resistor VR1 – 1k? 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. LM317 is available in a wide range of package forms for different applications including heatsink mounting and surface-mount applications.

Common form factors for high-current applications include TO-220 with part number LM317T and TO-3 with part number LM317K. LM317 is capable of dissipating a large amount of heat at medium to high current loads and the use of a heatsink is recommended to maximize the lifep and power-handling capability. LM337 is the negative voltage complement to LM317 and the specifications and function are essentially identical, except that the regulator must receive a control voltage and act on an input voltage that are below the ground reference point instead of above it

These voltage regulators are exceptionally easy to use and require only two external resistors to set the output voltage. Further, both line and load regulation is better than standard fixed regulators. Also, the LM117 is packaged in standard transistor packages which are easily mounted and handled. The general circuit of a voltage regulator circuit containing LM317 is as given below:  Normally, no capacitors are needed unless the device is situated more than 6 inches from the input filter capacitors in which case an input bypass is needed.

An optional output capacitor can be added to improve transient response. The adjustment terminal can be bypassed to achieve very high ripple rejection ratios which are difficult to achieve with standard 3-terminal regulators. Here we have designed the circuit according to the required specifications of the output current and output range of voltage. Before analyzing the design and calculation part, let us look into the general operation of the circuit for the variable voltage regulator.

1 is the mains step down transformer (12-0-12), with a current rating of 500mA. It supplies a voltage of 12V or 24V to the regulator circuit. C1 is the main filter capacitor. The unregulated DC voltage across C1 at no load forms the input to LM317 regulator. The LM317 is a complete regulator. It has internal feedback, regulating voltage and current passing elements. In operation an accurate reference voltage, typically 1. 25V is developed between the output and adjust terminals.

This reference voltage, when impressed across the resistor R1, sets up a current I1 that equals Vref/R1 in R1. I1 together with the quiescent current Iadj from the adjust terminal of the IC flows in the output set resistor R2, so that the voltage across R2 is V2 = (Vref/R1 + Iadj)*R2 The output voltage Vo at the output terminal of the IC is hence Vo=Vref + V2, given by Vo = Vref + (Vref/R1 + Iadj)*R2 Or Vo = Vref(1+R2/R1) + Iadj. R2 he LM317 is designed to minimize Iadj, this being an error term and also to make Iadj independent of line and load changes.

To achieve this quiescent current is returned to the output terminal, thus establishing a minimum load current requirement. If the Iadj is neglected then the output voltage is then given by: Vo = Vref(1+R2/R1) In our project, this output voltage is available from a range of 1. 5V to 25V. This is attained with the help of the variable resistor, whose value can be varied to change the Iadj value and consequently get the desired voltage value at the output .

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