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## DC Variable Power supply (1.25V To 6.5V)

INTRODUCTION: A power supply is a circuit that converts Alternating current (AC) to direct Current (Dc).we supply Dc to all Circuits where we need it. Dc supply is very important for the proper biasing of circuit components.with the help of a variable Dc power supply we give different voltages to a circuit for biasing because different electronic component works on different voltages.

DEFINITION:
A Dc regulated power supply (with variable output voltages from 1.25v to 6.5v) is a circuit that converts alternating current to variable output direct current.
1.AC SOURCE:
AC stands for alternating current.AC source is a simple voltage source which is available in every home. We apply 220v ac to the circuit as an input. The AC source has a sinusoidal waveform.
2.TRANSFORMER:

A transformer is simple an electric device or also a transformer is an ac machine. There are two types of the transformer (1)Step up transformer and (2)Step down transformer.
(1)Step up transformer:

It is the type of transformer that converts low voltage to high voltage. Such types of transformer are used in powerhouses where the need for high voltage. In a step up transformer number of turns in the primary side is less than a number of turns in the secondary Np<Ns. #### (2)Step down transformer:

It also a type of transformer which converts high voltage to low voltage .we use a step-down transformer in our circuit it converts 220v AC to 12v AC because we need 12v AC for the circuit. In the step-down transformer number of turns in the primary side is more than the number of turns in the secondary side Ns>Np. 3.RECTIFIER.
The rectifier is a circuit that converts alternating current to pulsating direct current (DC). There are two types of rectifier circuit (1) Half wave rectifier and (2) Full wave rectifier. We use a full wave rectifier in our circuit.

### BRIDGE RECTIFIER:

The bridge rectifier is a full wave rectifier. In the bridge rectifier, four diodes are used.When the input cycle is positive the diode D1 and D2 are forward biased and conduct the current at this time diode D3 and D4 are reverse biased. When the input cycle is negative the diode D3 and D4 are forward biased and conduct current while diode D1 and D2 are reversed biased. So these processes convert alternating current to pulsating direct current. ### 4. FILTER :

A filter is a circuit that covert pulsating dc to a ripple dc. This circuit is connected after the bridge rectifier. In a filter circuit, only a capacitor of high value is used. The positive end of a capacitor is connected to the output of a bridge circuit while the negative end is grounded. 5. VOLTAGE REGULATOR:

In this circuit, we use the LM317 voltage regulator. The LM317 voltage regulator has three terminals, the three terminals are Vout, Vin and ADJUSTMENT (ADJ). The LM317 series regulator is available in standard transistor packages that are easily mounted and handled. The main function of the regulator circuit is to convert ripple dc to regulated ripple dc to regulated DC. ### INTRODUCTION TO BLOCK DIAGRAM: Block diagram of dc regulated the power supply

### INTRODUCTION TO THE WORKING PRINCIPAL:

First of all, we give 220 v AC supply to a step-down transformer convert it to 12.6 v. After this 12.6 v  is then passed through a bridge rectifier. The bridge rectifier converts these AC voltages to a pulsating DC voltage. After the bridge rectifier, we connect a filter circuit. The filter circuit removes pulsation from pulsating DC and gives ripple DC voltage. After the ripple part DC, the output of the circuit is passed through the voltages regulator circuit which gives the regulated DC output which is our required.

### OPERATION:

In the operation of a DC regulated power supply circuit. First, we give 220v AC from the main supply or main switch to a step-down transformer. the function of a step-down transformer is to convert high voltage to a low voltage. In the step-down transformer, the number of primary turns is more than the number of secondary turns. The step-down transformer gives the output 12.6vAC. Then this 12.6v AC is passed through the full wave bridge rectifier. In the bridge rectifier, four diodes are used. The four diodes are (IN4001) rectifier diodes. When the positive side of the AC voltage into the upper side of the bridge. The Diode D1 and D3 are forward Biased while D2 and D4 are reversed Biased and in the next cycle when the Negative cycle comes then the diodes D1 and D3 are reversed bias while the Diode D2 and D4 are forward Biased. The Bridge rectifier gives the pulsating DC output. After this, we connect a capacitor of value 1000uF. which works as a filter. when the pulsating DC goes to the peak the capacitor C1 becomes recharge and when the pulsating DC becomes decrease. At this time the capacitor becomes discharged. Due to this recharging and discharging of a capacitor.it converts pulsating DC to ripple DC. After this, we connect a voltage regulator. This regulator circuit converts ripple DC to a regulated DC. Here a three-terminal regulator is used which is as adjustable output controlled by a variable resistor R2. Note that R2 is adjustable from 0 to 1Kohms. The LM317 regulator keeps a constant 1.25v between the output and adjusts terminal. This produces a constant current in R1 of 1.25v/240ohm=52mA.
Neglecting the very small current through the adjust terminal the current in R2 is the same as the current in R1.the output is taken across both R1 and R2 and is found from the Equation.
Vout=1.25v(R1+R2)/R1
Notice that the output voltage from the power supply is the regulator’s 1.25v multiplied by the ratio of the resistance. from the case shown in the circuit.when R2 is set to the minimum 0 resistance the output is 1.25v.when R2 is set to the maximum the output is nearly 6.5v. Resistor R1 and R2 are connected as a voltage divider circuit. Capacitor C2 and C3 are used for more Filtration output DC.so at the end we get a regulated DC from 1.25v to 6.5v.
CIRCUIT  DIAGRAM: ### MULTIMETER  OUTPUT  VOLTAGES:  OSCILLOSCOPE  INPUT  AND  OUTPUT WAVEFORM:  