LM317 power variable supply.

Build an Adjustable 2-30 volt power supply with the LM317

by Lewis Loflin

The LM317 is an adjustable three-terminal positive-voltage regulator capable of supplying more than 1.5 A over an output-voltage range of 1.25 V to 32 V. It is exceptionally easy to use and requires only two external resistors to set the output voltage. Furthermore, both line and load regulation are better than standard fixed regulators.

By using a heat-sinked pass transistor such as a 2N3055 (Q1) we can produce several amps of current far above the 1.5 amps of the LM317. Please note that at low output voltages at high current Q1 can get very hot.

Circuit Operation

L1 is a 120 to 24 volt transformer. The one I used was rated for 25.2 volts RMS at 3 amps from Radio Shack. Diode bridge D1 should also be rated at 3 amps or greater and at least 50 PIV or greater. D1 could also be four 3 amp diodes. C2 is a 2200 uF 50 volt electrolytic that will charge up to almost 40 volts with a 25.2 volts. (1.414 * 24.5 volts RMS)

R1 is a 180 ohm half watt and R2 is a 5K ohm potentiometer. This is used to set the output voltage of the LM317. C1 is a 10-47 uF electrolytic while Q1 can be a TIP41 (TO-220 case) or 2N3055 (TO-3 case).

Pin diagram LM317T

This LM338T voltage regulator chip (aka LM338) works in exactly the same way as the LM317T voltage regulator, the only difference being it can deal with higher currents. The LM338T is rated at 5 Amps continuous current. Used with a suitable heat sink the LM338 will produce continuous currents of up to 8 Amps.

In all cases VOUT = 1.25 * ( 1 + R2/R1 )

LM317 current boost circuit.

Above we boost the current out of a LM317. See LM317 Adjustable Voltage current Boost Power Supply