A DC variable power supply is one that includes some means for the user to easily adjust the output voltage and sometimes the current. Adjustment is most often accomplished with a potentiometer, but may also be done with an analog control voltage, a digital input, an autotransformer, etc. Generally, DC power supplies have the following scheme: An input AC source sends an AC signal to a power transformer which then sends the step-down signal to a rectifier circuit, after which a regulator tunes the DC signal to the desired voltage level. So, in this project, we are going to design a Variable Power Supply (100mA) Using the LM317L Voltage Regulator.
LM317 is a three-terminal voltage regulator IC, with a high output current value of 1.5A. The LM317 IC has many features such as current limiting, thermal protection, and safe operating area protection. It can also provide a float function for high voltage uses. If we disconnect the adjustable terminal still, LM317 will be helpful in overload protection.
Hardware Components
The following components are required to make a Variable Power Supply Circuit
| S.no | Component | Value | Qy |
|---|---|---|---|
| 1. | Step down Transformer | 230V/28V, 200mA, 50Hz | 1 |
| 2. | Voltage regulator IC | LM317L | 1 |
| 3. | Electrolytic Capacitors | (2200μF, 100μF, 10μF)/50V | 1, 1, 1 |
| 4. | Diode | 1N4007 | 6 |
| 5. | Ceramic Capacitor | 0.33μF | 1 |
| 6. | Resistor | 240Ω | 1 |
| 7. | Potentiometer | 5.1KΩ | 1 |
| 8. | Breadboard | – | 1 |
| 9. | Connecting Wires | – |
LM317L Pinout
For a detailed description of pinout, dimension features, and specifications download the datasheet of LM317L
Variable Power Supply Circuit
Working Explanation
An input voltage of 230V/220V is applied to the primary of a transformer which steps it down to 28V 200mA through mutual induction of the primary & secondary windings while maintaining the frequency at 50Hz. After which the 28V AC signal goes through a bridge rectifier that converts the AC signal into a rippling DC signal.
The rectified voltage then goes into the input of an adjustable voltage regulator LM317L. This three-terminal voltage regulator has an output voltage ranging from 1.2V to 37V DC with a maximum load current of up to 1.5A. The diodes D1 & D2 are used to protect the regulator from excess -ve feedback flow, in case a battery or any other voltage source is connected across the output terminals of the regulator. The output voltage range is controlled by connecting a 5.1KΩ pot at the ADJ pin of the regulator. The DC signal then passes through a smoothing capacitor of 100μF before going on towards the output.
Applications
- DC Variable power supply can be used in many applications such as applying variable voltages to High voltage DC-DC converters to adjust the gain, etc.
