So I'm looking at the datasheet for the LM317:
and I realize I don't have a 240-ohm resistor. However, I've got lots of 100-ohm resistors, 10% tolerance I think.
How critical is this resistor? Can I simply put two 100-ohm resistors in series? I suppose I could even put two more 100-ohm resistors in parallel, and put this blob in series with the two 100-ohm resistors, but at 10%, I'm looking at all kinds of junk.
Thanks,
Michael
R1 and R2 form a potential divider between Vout and GND. The regulator works to maintain a 1.25V drop across R1 at all times. The ratio R1 / (R1+R2) determines the approximate output voltage. Unfortunately, a small temperature-dependent current (
There's a minimum load current that it "likes" or else the regulation drifts up a little. See the datasheet for the applicable graph and the last sentence in the first paragraph under Application Hints. 240 is a "failsafe" R1 and it could be larger for a given application.
Using 100 ohms for R1 will work and should regulate well (assuming you solve for R2 correctly), you'll just be pulling about 13 mA across it which you'll need to account for in the total load.
-- Rich Webb Norfolk, VA
The whole purpose of percentages is that they are independent of absolute magnitude(under addition).
100 at 10% is 90 to 110.200 at 10% is 180 to 220.
two in series both at 10% is
90 to 110 + 90 to 110 = 180 to 220It also works for parallel because of the nature of the formula.
What it means is that you do not have to worry about what part tolerences play when dealing with components all having the same tolerance.
It's been suggested that one use a 120ohm resistor,instead of 220ohms,to ensure that the minimum load requirement is met,even with no load. (the '317 seems to like ~10ma minimum load.) So 100ohms will be fine,assuming you adjust R2 accordingly.
IIRC the regulation tolerance is *guaranteed* with the 10mA loading but it's "usually" within tolerance at 5mA, hence the common 240R. You can go to about 1k if there's always some >5mA loading (like an LED indicator) though you'll need to take the error term into consideration.
-- Yorkshire rules, OK
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