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I want to get 1.8v to power a gps unit, from the 5 volt pin of an Arduino.

What do I need to change on this schematic to achieve that voltage?

enter image description here

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  • \$\begingroup\$ Have you read the data sheet? \$\endgroup\$
    – starblue
    Nov 30, 2011 at 13:35
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    \$\begingroup\$ Why not just use a fixed 1.8V regulator? \$\endgroup\$ Nov 30, 2011 at 13:49
  • \$\begingroup\$ @OlinLathrop -- likely because he doesn't have any 1.8V fixed regs handy (I certainly don't!) \$\endgroup\$ Nov 27, 2016 at 15:02

3 Answers 3

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Have R1 as a 500Ω resistor and R2 as a 220Ω resistor.

That should give you 1.7999999999999998 volts.

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    \$\begingroup\$ +1 for the calc. But it'd be more useful if it had a list of standard value resistors, you put in a voltage and a percentage tolerance and it gave you all pairs of standard resistance within a given tolerance of your voltage. \$\endgroup\$
    – rfusca
    Nov 30, 2011 at 2:27
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    \$\begingroup\$ Your calculator is some kind of faulty: 1.25*(1+(500/220)) is 1.8, not 1.799999.... Adding the Iref error: around 1.822V \$\endgroup\$ Nov 27, 2016 at 11:20
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    \$\begingroup\$ 220Ω (or 240Ω) across Vout/Adj is not okay with LM317. It's just a very well estabilished misconception. Minimun output current to mantein regulation is 10mA for LM317 and this calls for R1=120Ω. That picture with 240Ω is an LM117 application, which is okay with load above 5mA. \$\endgroup\$
    – carloc
    Nov 27, 2016 at 11:43
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    \$\begingroup\$ Oh and here come the pedants crawling out of the woodwork. Come on, boys, you're only 5 years late. \$\endgroup\$
    – Majenko
    Nov 27, 2016 at 16:58
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    \$\begingroup\$ Significant digits matter. In science and engineering they are expected to be used properly. By using so many fraction digits, you are saying the voltage is way more accurate than it could ever be. That's just plain wrong. \$\endgroup\$ Nov 27, 2016 at 17:12
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For the LM317, Vout = 1.25(1 + R2/R1)

Using R1 = 220 ohms and R2 = 100 ohms, Vout will be 1.82 volts

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  • \$\begingroup\$ What is the difference between the two capacitors in the schematic? \$\endgroup\$ Nov 30, 2011 at 1:13
  • \$\begingroup\$ Well the capacitors aren't exactly needed, the output capacitor is optional and it helps improve the transient response. the datasheet always helps :) national.com/ds/LM/LM117.pdf \$\endgroup\$
    – Shungun
    Nov 30, 2011 at 4:21
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    \$\begingroup\$ The 0.1 uF cap C1 is a bypass for the input, and needed if the LM317 is more than 6 inches from the filter caps on the DC supply used as the input source. The 1 uF cap C2 filters transients on the output. You can use any value from 1 uF up to 1000 uF for additional filtering. \$\endgroup\$
    – tcrosley
    Nov 30, 2011 at 6:29
  • \$\begingroup\$ +1 for not abusing significant digits. \$\endgroup\$ Nov 27, 2016 at 15:23
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Right values:

R1 = 390 Ω
R2 = 172 Ω
Vout = 1.25 V (1 + 172 Ω / 390 Ω) = 1.8012 V
Vout = ~1.8 V

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