Newbie trying to understand the lm317

SgtWookie

Joined Jul 17, 2007
22,230
You can use the higher resistance with the LM317, as long as your circuit will draw the remaining required current to provide the guaranteed regulation.

Note that a Vref of 1.25v is nominal, for the LM317 it can be anywhere from 1.2v to 1.3v and still be within specifications. That's why 120 Ohms is used for R1, as 1.2v/120 Ohms = 10mA, which satisfies the minimum output current specification.

A caveat: Iadj, the current sourced from the ADJ pin, is usually between 30uA and 80uA, nominally around 50uA. When R1 is 120 to 240 Ohms, Iadj is so small as to be inconsequential. However, when you start increasing the value of R1, the value of R2 also increases to maintain the same voltage - and that 50uA starts to make it's presence known. If you don't plan for it, your output voltage could be a good bit higher than you thought it would be.

For example, if you wanted 5v out, using 120 for R1 and 360 for R2 would usually be close enough.
1.25/120=10.4166mA, 5-1.25=3.75, 3.75/10.4166mA = 360. 50uA adds 18mV to that, for a total of 5.018v.

But if you increased R1 to, say, 2.4k (because you were going to have an LED on the output that would always draw at least 10mA so you figured you could use less in the voltage divider portion); you'd then have 1.25/2400 = 520.833uA through R1; and 3.75/520.833uA = 7.2k. Now that 50uA Iadj makes a relatively large difference, as the error will now be 360mV.

So if you decide to increase R1/R2, don't forget Iadj.
 
Last edited:

Wendy

Joined Mar 24, 2008
23,415
One of my favorite uses of the LM317 is that of a current regulator. Within its range it is cheap, and can handle decent power levels. Something like power LEDs it is cheap and dirty, but works very well.
 

SgtWookie

Joined Jul 17, 2007
22,230
The current regulator configuration is also great for finding out the Vf of "unknown" LEDs, or for LED matching. One 62 Ohm resistor from OUT to ADJ, and you essentially have a 2-terminal (IN and ADJ) current limiter with a minimum dropout of ~3v that you can use to either sink or source current.
 
Top