LM317 - a multipurpose voltage / current regulator

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Well, my favorite discrete component of the week has to be the lowly LM317 voltage/current regulator. As one of my previous posts showed, I'm using it to current regulate some high power LEDs and I also use it as a voltage regulator.

Here's the quick and easy way to wire up an LM317 as a voltage regulator:


where the values of R2 and R1 are calculated as follows to give Vout:


R2 is usually set to 240 ohms and you can ignore Iadj to a point. Rearranging the equation gives you:

R2 = 192 * V - 240

So if you want a 5 volt output, then R2 = 720 ohm (and R1 = 240 ohm). The TO-220 form factor of the LM317 that I'm using can provide up to 1.5 amps of output current and can be paralleled if I need more.

Late update: or you can just use an online calculator: http://www.jlab.org/~hansknec/index.html

Note 1: Although the schematic from the datasheet shows Vin > 28 V that's only if you want 25 volts out. In reality, as long as Vin + 2~3 volts > Vout, you should be good. So for a 5 volt output, Vin should be more than 7 volts.

Note 2: Note that the excess voltage is bled as waste heat since this is a linear regulator, so you shouldn't make Vin too high or the output current too much or you'll heat up the chip. If you want to really step-down voltage a lot, you should use a buck converter.

Note 3: The actual value of Iadj in the equation above is 100 μA which is 0.0001 amps. Thus in the equation above, including the Iadj factor would be 720 ohms * .0001 amps = 0.07 volts, resulting in a total of 5.07 volts or around a 1.4% higher voltage than expected.

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This page contains a single entry by Karen Nakamura published on February 10, 2010 8:36 PM.

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