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I am wondering if I should use an LM317 or LM1117 for a 5v circuit that needs 3.3v for a sub-circuit far (20ft) from the power supply.

Any help would be greatly appreciated. Thanks in advance.

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Use a LM1117 for the following reasons:

  1. LM1117 will require less components.
  2. It may be difficult to get exactly 3.3V using a LM317 and off the shelf resistor values.
  3. LM1117 has a lower drop out voltage. The LM317 requires 3V headroom so just off that it will not get the job done. The LM1117 just barely suffices with a dropout voltage of 1.2V. Your final voltage will be $$Vo = 5V - 1.2V - Vloss$$.

Where Vloss is the voltage drop caused by the resistance of the cable.

In terms of efficiency, they are both linear regulators so the wasted power in each will be roughly the same.

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  • \$\begingroup\$ LM317 is not guaranteed to regulate if Vi-Vo < 3V. Which is impossible in this case, without even considering 20ft of unknown cabling and unknown load current. \$\endgroup\$ – Wesley Lee Sep 16 '16 at 16:03
  • \$\begingroup\$ 1117 is also cheaper than 317 last time I checked. \$\endgroup\$ – winny Sep 16 '16 at 16:37
  • \$\begingroup\$ @winny I wanted to add this to my answer so I just checked digikey and the result is surprising. At 1000 quantity the cheapest lm317 is 4 times cheaper than the cheapest lm1117. Wasn't expecting that \$\endgroup\$ – bitshift Sep 16 '16 at 16:41
  • \$\begingroup\$ @bitshift Oh! Last time I checked several years ago, AP1117 undercut any 7805 or lm317 by a factor of three in 100k+ per year quantity. \$\endgroup\$ – winny Sep 16 '16 at 17:06
  • \$\begingroup\$ @bitshift I get 9 cent for AP1117 and 29 cent on LM317 on octopart. \$\endgroup\$ – winny Sep 16 '16 at 18:21
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From LM317 datasheet: enter image description here

And from LM1117 datasheet:

enter image description here

So, if you can guarantee at least 4.5V+safety margin at 800mA (and a tiny bit less if the load is lighter) after 20ft of cable from 5V, then LM1117. If not, none of the above.

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