LM317

[last updated: 2021-09-10]
go to: LM317 voltage regulator for fixed 9v output built on PerfPlus
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Constant Voltage Circuit

Constant Current Circuit

High-Current Voltage Regulator

• In voltage regulation mode:
  R₁ is nominal, though should be greater than 120ohm, but does not affect output voltage level. A constant "reference" voltage of 1.25v will be across R₁, regardless of R₁'s value.
  The value of R₂ determines the output voltage:
  R₂ = R₁ ( (V_o / 1.25 ) - 1 )

LM317 has built in current limiting and thermal shut down capabilities.

LM317 has good output voltage regulation as long as the load current is greater than about 10ma, so R1 should be no less than about 120 ohm.

About 2.5v ("dropout" voltage) is lost as heat in the regulator, and input voltage must be at least that much greater than desired output voltage for good regulation under load.

If less dropout is required, consider the LM2941T which has only 0.9v dropout, though it's only good for 1a output current, vs. the LM317 which can do 1.5a.

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