The simple LM317 variable supply may not be so simple (by Brindsley Archer)

Component: LM317

Category: Linear voltage regulator

-These 3-terminal regulators allow any normal voltage to be generated so long as it's above 1.25 and about 2V below the input voltage; two resistors is seen as a small price to pay and the formula is simple. These regulators drop less than the 7805 etc. and their idle current is also lower. These considerations will appear to be the only thing going for them when you read below.

-The worst case ground current under load is significant meaning relatively low value resistors. Rule of thumb 10mA to keep accuracy. Designing analogue circuitry that needs oddball supply voltages went out in the 1960s. Hence the most common use of these devices is for variable supplies. The power dissipation of the earth resistor is significant if the supply runs from 1.5 - 30 Volts. In fact a standard carbon potentiometer will be unreliable and a wire wound unit kind of defeats the purpose of the solid state device. So we hang extra circuitry around the carbon pot to take the power loading off it. The stabilisation capacitors to ensure stability given different manufacturers and output volts or amps are substantial. Precautions must be taken when these low ESR tantalum caps discharge suddenly like under short circuit otherwise the product could fail. The simple variable power supply isn't so simple anymore.

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