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Dallas
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I'm trying to use an LM317 to regulate an unregulated 30V power supply down to 27V. The power supply puts out about 36V no-load. For some reason, my circuit is just not working.


I have it hooked up just like the schematic. R1 is 1kOhm and R2 is a 500Ohm pot. The way I see it that should be adjustable from 36V down to around 8V. But I can only get the thing to output around 4V, and when I adjust the pot, the reference voltage changes. I was under the impression that the reference voltage is supposed to stay at the same ~1.2V value no matter what; that's kind of how the thing is supposed to work. Instead, when I adjust the pot, the reference voltage actually changes from like .8 to 1.2V. There's definitely something up but I don't know what it is. I have tried two LM317s. Any ideas how you could hook up an LM317 wrong to where the reference voltage would change with the pot adjustment?
« Last Edit: June 28, 2011, 11:45:00 pm by BetterSense » Logged

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Yes, all the datasheet examples show the pot R2 being about 10X R1.
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The IC regulator attempts to keep the reference voltage (between the Vout and ADJ pins) at 1.25V.

This is different than what I thought. I thought the IC regulator attempted to keep the reference pin voltage AT 1.2V (from ground). In that case, to regulate to 27.5V you would want a ratio of R1/R2 of about 26.3/1.2=22. So my bigger R1 makes sense.

If the regulator attempts to keep the reference voltage at Vout-1.2, then you would want a ratio R1/R2 = 1.2/26.3=.045. So it looks like I need to switch my pot and resistor around (and maybe use a bigger resistor as well).
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The actual value of R1 is also important, although I can't remember off the top of my head why (stability? current consumption of the regulator? dunno, RTFM). I'd stick with something around 240.

There are calculators online; google lm317 calculator.

-j
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This is different than what I thought. I thought the IC regulator attempted to keep the reference pin voltage AT 1.2V (from ground). In that case, to regulate to 27.5V you would want a ratio of R1/R2 of about 26.3/1.2=22. So my bigger R1 makes sense.

Always read the datasheet!  Its done this way to make the closed loop gain fairly independent of the output voltage and thus easier to keep stable.  For the same reason they recommend a decoupler across the Radjust.
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