QKits Electronic Kits: QK68, Adjustable Power Supply Kit

This is a basic, text-book, adjustable voltage regulator circuit using an LM317, 3-terminal regulator in a TO-220 package. The 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 kit via a socket or terminal block. 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. However, one has been provided in the kit.

NOTE: the input voltage must NOT be mains voltage. The purpose of the kit is to make a regulated and variable power supply from an unregulated input.

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 maximum 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 you could 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 dischargin back into the regulator in the event of abnormal operating conditions, like a sudden short circuit on the input or the ouput, 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 (Texas Instruments data book.) However, circuits from most other sources settle on using either 220R or 240R. We have used 240R. 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 ouput voltage. 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 240R 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.

Construction. Check all the components supplied against the components listing above. it is usually best to solder the lowest height components onto the PCB first and then do to the taller components. Note the polarity of the diodes and capacitors. The back of the LM317T is marked on the overlay.

	~ Celebrating 14 Years of QualityKits in 2010 ~		02-Sep-2010