Variable power supply

The LM317T

lm317The heart of this circuit is, of course, the LM317, a variable voltage regulator. A voltage regulator takes the output of a transformer (which usually turns AC voltage into DC and steps down the mains voltage of 120V to something more useful, like 5–18V) and provides consistent power upon which integrated circuits and transistors depend. For instance, the transformer I’m using, an old RadioShack transformer, is rated at 13.5V output but actually puts out 18V.

The LM317 is a popular adjustable voltage regulator and the output voltage can be controlled by placing a resistor across pins 1 and 2 and a variable resistor (potentiometer) connected between R1 and ground. The output from pin 2 is our output power.

A couple of things to note here is that the more amps your circuits draws and the greater the difference between the input voltage (the transformer) and the output voltage (from the LM317), the more heat that’s generated, which is why it’s a good idea to pair a voltage regulator with a heat sink. (Choose a heat sink designed for a TOS-220 type chip.) Another consideration is that the input voltage to the LM317 has to be a few volts above the desired output voltage and that value is determined by the amperage of the circuit and the temperature of the regulator. Call it 3V to be safe. That’s one of the reasons why my voltage regulator can only produce about 16V despite the 18V supplied by the transformer.

The heatsink

I hope my heatsink will prove sufficient for my projects. It matches the TO-220 design of the regulator. Again my parts bins supplied the screw to attach the regulator to the heatsink. (I should probably add some thermal compound to ensure heat from the regulator is efficiently transferred to the heatsink.) They are two metal pins projecting from the base of the heatsink that help anchor it to the circuit board, but the pins are larger than holes on the board and don’t quite line up, so I used a pin vise and drill bit to enlarge and slightly offset two holes.

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Jennifer Petkus divides her time between writing Jane Austen-themed mysteries, creating websites for the dead, woodworking, aikido and building model starships. She has few credentials, having failed to graduate from the University of Texas with a journalism degree, but did manage to find employment at the Colorado Springs Sun newspaper as a cop reporter, copy editor and night city editor before the paper died in 1986. She has done little since. Her husband is the night editor at The Denver Post. Her best friend is a cop. She watched Neil Armstrong walk on the Moon live. She is the author of, Good Cop, Dead Cop: A Novel about the AfterNet. She’s writing the sequel, The Background Noise of Souls, and another AfterNet related story, Jane Austen’s Book Tour.

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