3
\$\begingroup\$

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

\$\endgroup\$
  • \$\begingroup\$ Have you read the data sheet? \$\endgroup\$ – starblue Nov 30 '11 at 13:35
  • 1
    \$\begingroup\$ Why not just use a fixed 1.8V regulator? \$\endgroup\$ – Olin Lathrop Nov 30 '11 at 13:49
  • \$\begingroup\$ @OlinLathrop -- likely because he doesn't have any 1.8V fixed regs handy (I certainly don't!) \$\endgroup\$ – ThreePhaseEel Nov 27 '16 at 15:02
4
\$\begingroup\$

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

That should give you 1.7999999999999998 volts.

\$\endgroup\$
  • 2
    \$\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 '11 at 2:27
  • 1
    \$\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\$ – Adrian Maire Nov 27 '16 at 11:20
  • 1
    \$\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 '16 at 11:43
  • 2
    \$\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 '16 at 16:58
  • 1
    \$\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\$ – Olin Lathrop Nov 27 '16 at 17:12
5
\$\begingroup\$

For the LM317, Vout = 1.25(1 + R2/R1)

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

\$\endgroup\$
  • \$\begingroup\$ What is the difference between the two capacitors in the schematic? \$\endgroup\$ – AndrewFerrara Nov 30 '11 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 '11 at 4:21
  • 2
    \$\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 '11 at 6:29
  • \$\begingroup\$ +1 for not abusing significant digits. \$\endgroup\$ – Olin Lathrop Nov 27 '16 at 15:23

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.