LM317 Calculations

[petemoore]

  First, the data sheet shows the IC facing or facing away, let's call it print facing you, legs down:
  ADJ       OUT         V-In
  Second, I'm using a 15.6v supply [measured with Theremax circuit loading it], and a 12vdc for the tremolo circuit is a good target for the 317.
  All the the LM317 calculators show 28v supply at Vin, I'm trying to figure out how to use it with the '16v' I have...I'm going back to booksworth again now to try to cypher it out.
  I'm back, I think with the 240 as load resistor and 4k7 as the adjust resistor, It'll bring me to about 25.73 / 28ths of the supply voltage...
  Or does the ratio of voltage drop signifigantly 'curve' as voltage gets lower, causing my ballpark calculations [scaling the 28v/25.73v down to ...just under say 14vdc [ballpark.
  So I'll go with 3k9 as my answer for now, tremolo circuit has a pretty good sized ballpark for supply under ~14v.
  ..according to the data sheet rant:
  I'm surprized something as useful and common as a Vreg IC would have such concerns about capacitor materials, they spend a couple paragraphs describing the advantages of tantalum, but the picki-preferential-ness didn't extent to Silver Mica, CC or even Mylars...go figure.
 

[Jered]

  I just use a Helipot ten turn 5k precision pot, get the desired voltage, measure pot resistance and drop in a resistor of the same value. Make sure you have a good heatsink on there or it will burn up.
  Jered

[pjwhite]

The input voltage to an LM317 is not relevant to the resistor calculations.  (If the output voltage was dependant on the input voltage, it wouldn't be very useful as a regulator, eh?)

For a 12 volt output, you would want a 240 ohm resistor between the output pin and the adj pin, and a 2K resistor from the adj pin to ground.  The thing to remember is that the voltage between the output and adj pin is a constant 1.25 volts.  Putting a 240 ohm resistor here produces a current of 5.2 milliamps (V / R).  The current drawn by the adj pin of the LM317 is negligible.  This means that the same current is going to go through the adj-to-ground resistor.  To compute this resistor, subtract 1.25 from the desired regulator output voltage (12 - 1.25) and divide by the current.  10.75 / 0.0052 = 2067 ~= 2k.

The only restrictions on the input voltage are the maximum and minimum voltages, and the power dissapation (Vin - Vout) * output current.

[petemoore]

  Thank You PJ White. Your explanation, to me, makes some sense, to others it must be well written and an accurate description of what I asked for: "LM317 calculations".
  Thank you.
  I tried working the figures and got about half way, or all the way to .0052 on the calculator and got the feeling that the decimal did fall in the right place with the right numbers in the right positions around it.
  Negligible sounds correct.
  Maybe I'll try again to type a question about the answer, which looks to be well thought out, complete, concise, and accurate...denied...my understanding may grow if I try harder or something snaps!