Introduction: Variable Voltage Power Supply

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About: Just another person curious for nature and its governing laws! Always have my own particular ways of building, designing, and performing on things! Exploring science and technology !

Hello guys, this is my first Instructable ! We all are surrounded with electrical appliances of various specifications.Most of them run directly on 220V AC supply. But what if you made any personalized appliance or a project which requires specific voltage and that too in DC! So the aim is to make a basic Power Supply providing variable voltage using lm317 voltage regulator IC .

Step 1: What Does the Power Supply Do?

One needs to understand the purpose the Power Supply serves.

  • It will convert AC to DC by taking supply from the AC Mains.
  • It will provide variable voltage as per one's choice ranging from 3V to 25V.

Key features :

  • Inexpensive
  • Easy and convenient to install.
  • Versatile

Step 2: List of Items Required.

  1. Step down Transformer 2A (220V to 24V)
  2. lm317 voltage regulator IC with heat sink
  3. Capacitors (polarized): 2200 micro farad 50V ,100 micro farad 50V, 1 micro farad 50V ( note: The voltage rating of capacitors must be higher than that applied across its terminals.)
  4. Capacitors (unpolarized) : 0.1 micro faradPotentiometer 10k
  5. Resistor 1k
  6. LED display Voltmeter
  7. Fuse 2.5 A
  8. Screw terminals
  9. Connecting wires with a plug.
  10. Diodes 1n5822.
  11. Printed Circuit Boards (PCB).

Step 3: Circuit Assembly

  • In the figure 1, the transformer is connected the AC supply.This steps down the voltage to 24 V but the output still has Alternating nature of frequency 50 Hz.
  • In figure 2, the four diodes have been connected as shown to form bridge rectifier. The 1n5822 diodes allow the current to pass in the forward bias condition. It blocks the current under reverse biased condition. hence the output voltage has a pulsating DC nature of frequency 100 Hz.

Step 4: Circuit Assembly Continued...

  • In the figure 3, a 2200 micro Farad Capacitor has been added , this filters out and provides a steady and stable 24 V of DC nature.
  • At this stage you may wish to add fuse in series to give protection to the circuit.
  • So upto now we have :
    1. stepped down the voltage to 24 v having AC nature
    2. converted AC to pulsating DC of 24 V.
    3. filtered out to get pure and stable 24 V.
  • This will be connected to the lm317 circuit explained later.

Step 5: Intro to Lm317

  • Now our aim is to have control over the voltage and change it as per our desire. For this we will use lm317 voltage regulator.
  • lm317 has 3 pins as shown in the figure. The description has been provided.
  • lm317 rejects heat upon application, hence heat sink is required.
  • Heat sink is metallic object placed in contact with the IC to dissipate its heat to the surrounding.

Step 6: Understanding Lm317 Circuit

  • This is the continuation of the earlier circuit diagram. For the sake of understanding the lm317 circuit has been shown exclusively.
  • The 0.1 micro farad capacitor is recommended for filtering purpose. It is highly recommended if it is not in close proximity with the filtering capacitor (in our case 2200 micro farad capacitor).
  • 100 micro farad capacitor is recommended to improve ripple rejection. It prevents amplification of the ripple as the output voltage is adjusted higher.
  • 1 micro farad capacitor improves transient response, but is not needed for stability.

  • Protection diodes D1 and D2 (both 1n5822) .The diode provides a low-impedance discharge path to prevent the capacitor from discharging into the output of the regulator.

  • R1 and R2 are required to set the output voltage.

  • The governing equation is shown in the figure. Here R1 is 1kohm and R2(potentiometer of 10k) is variable .Hence a variable voltage is obtained at the output as per the given approximated equation by changing R2.
  • If one wants additional information regarding the lm317 IC datasheets are available online for reference.
  • Now at the output one can connect the LED display voltmeter or may use their Multimeter to find the voltage.
  • Note: One can choose values of R1 and R2 as per their convenience. I mean there is no fixed rule that R1 should be 1k and R2 should be 10k variable . Also if one wants fixed output voltage he/she can fix R2 value instead of using variable one.Using the formula one can use R1 andR2 of their own choices!

Step 7: Final Circuit Assembly

  • The final circuit looks as per shown in the figure.
  • Hence now by controlling the potentiometer ie. R2 one can obtain variable voltage at the output.
  • At the output one can obtain pure, ripple free, steady and constant voltage required to feed to their load.

Step 8: Soldering on Circuit Board.

  • This is a self doable part.
  • Make sure that all components are connected strictly as per circuit diagram.
  • Apply screw terminals at input and at output .
  • Double check the circuit before plugging into the mains supply.
  • For safety wear insulated or rubber shoes before plugging into the mains.
  • There will be no possibility of any hazards if circuiting and connections are done rightly.However the responsibility is strictly yours!
  • My final circuit board is as shown above.(I have soldered the diodes on backside of board. forgive my poor soldering art!)

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