     |
Variable DC Power Supply Using LM317 Regulator
This project is a positive variable power supply that is compact and easy to build. It is ideal for powering any application requiring a DC supply at current levels up to 1.5A. This power supply project should be among the first project that all electronic hobbyist should embark on. With this power supply, one can use it to power up many electronic kits and projects instead of using batteries. The features of this circuit are: · Output reverse polarity and back-voltage protection
Diodes D1 to D4 form a bridge rectifier which converts the AC input voltage into a DC level. Capacitor C1 smooths the DC output of the bridge whilst C2 provides high frequency decoupling. The LM317T is an adjustable regulator IC providing the desired output voltage. Diode D5 is reversed biased during normal operation and is used to protect the regulator if the output is connected to a voltage of the same polarity. Diode D6 protects the regulator if a reverse polarity voltage is connected to the output. The regulator develops a nominal 1.25V reference voltage between the output and adjust terminals. This constant voltage is applied across R1, causing a constant current to flow. This constant current flows through trimpot VR1. By varying VR1, the voltage across it will vary and hence the output voltage can be set. The output voltage is calculated by: V(out) = 1.25(1 + VR1/R1)
Capacitor C3 improves the ripple rejection of the regulator while capacitorS C4 and C5 provide high and low frequency decoupling respectively. The LED indicates that power is present at the output. The current through the LED should be between 5 and 20mA and is set by resistor R2. The variable DC power supply using LM317T regulator is capable of supplying 1.5A over an input voltage range of 1.2V to 37V. It needs at least a 2.5V input-output voltage differential. In practice the limiting factor on the output voltage and current will be the power dissipated by the regulator. For example, if the input voltage is 30V & the output voltage is 10V and the output current is 1A then the power dissipated by the heatsink is (30-10)*1 or 20W. This would need a big heatsink. So it is desirable to keep the input voltage as low as possible to achieve the required output. This is done by using a right choice of stepdown transformer form the mains. A typical transformer value is 110/12 VAC or 110/18 VAC ratio is good enough depending on the variable output supply that is needed.
Variable DC Power Supply Parts List The Variable DC Power Supply project parts list is as shown below.
Variable DC Power Supply Home Page
|
