Constructing a Universal Power Supply using LM317

This is a basic, text-book, Universal Power Supply voltage regulator circuit using an LM317, 3-terminal regulator in a TO-220 package. The Universal Power Supply output voltage can be set to anywhere in the range 1.5V to 30V by selecting two resistances. By using a potentiometer, R2, as one of the resistors you can dial up the output voltage wanted. Either AC or DC input can be supplied to the PCB via a socket or terminal block. Connection can be either way around. This is because we have provided a bridge rectifier on board. The input DC voltage to the regulator must be at least 2.5V above the required output voltage. An off/on switch is provided.

For many applications (say 12V at 60mA) a heat sink will not be necessary. The LM317 will provide slightly higher output voltages than 30 volts. However, for most hobbyists over 30V will not be needed. So to make a small PCB we have used some electrolytic capacitors rated to 35 volts. To be safe for continuous operation the maximun input DC voltage to the regulator should not be over 33V. With a 2.5V to 3.0V drop across the regulator this will give a regulated output of 30V. You can draw up to 1.5A from the LM317. If you need higher then use an LM338T rated to 5A.

When external capacitors are used with any IC regulator it is good practice to add protection diodes to prevent the capacitors discharging back into the regulator in the event of abnormal operating conditions, like a sudden short circuit on the input or the output, or a back emf from an inductive load. That is the function of D1 and D2.

The value of R1 can range anywhere from 120R to 1200R (see Data Sheet on www.ti.com) However, circuits from most other sources settle on using either 220R or 250R. We have used 240R or 250R. The voltage drop across R1 is 1.25V for all values, and this is the key to the design. 1.25V is the reference voltage of the regulator. Whatever current flows through R1 also flows through R2, and the sum of the voltage drops across R1 and R2 is the output voltage. (Additional current Id also flows in R2 but it is typically 50uA so is negligible.)

The design formula are:

VOUT = 1.25 (1 + R2/R1) volts, or alternatively

R2/R1 = (VOUT/1.25) - 1

So if you know VOUT and R1 is 250R then you can calculate R2. If you find that the 5K potentiometer used for R2 does not give you the degree of fine control over the voltage output range that you want then you can use these formulae to adjust R1 and R2 to better suited values.

Universal Power Supply Schematic Diagram

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