Please all, I need some help in designing power lines for my project. I intend to use one LM117 (or LM317) to have at least three voltage lines on my breadboard, i.e 3.6V, 1.8V and 1.1V from a 12V supply (all DC).

I used the following formula to try and get a 1.1V DC line.

$$ R_2=R_1 \left( \frac{V}{1.25}-1 \right) $$

The tutorial I read suggested a standard value for R1 to be 240 ohms, which leaves me with a negative value for R2 (and for some reason i think this is wrong), for example:

$$ \begin{align} R_2 &= 240 \left( \frac{1.1}{1.25} - 1 \right)\\ R_2 &= -28.8 \end{align}$$

  1. Any ideas where I am going wrong? Should I be using a different value for R1? For a background, I am trying to have three power lines of 3.6V, 1.8V and 1.1V from a 12V source (all DC) using the LM117/337
  2. Can I have all three power lines off one LM117/317

1 Answer 1


That regulator will not provide output voltages below 1.25V.

The device uses an internal 1.25V reference voltage and the output voltage (via R1 and R2) is potted down to equal 1.25V. If the desired output voltage is less than 1.25V then it doesn't work.

AD8605 op-amp regulator for supplying 30mA at 1.1V: -

enter image description here

  • \$\begingroup\$ Any suggestions on regulators that can give the voltage I want or indeed how to step down a 1.25V line to 1.1? Also, can I have the other two voltages off the single regulator \$\endgroup\$
    – pekatete
    Jul 1, 2013 at 13:25
  • \$\begingroup\$ How much current do you need? \$\endgroup\$
    – Andy aka
    Jul 1, 2013 at 13:26
  • \$\begingroup\$ For the 1.1V i need 30mA \$\endgroup\$
    – pekatete
    Jul 1, 2013 at 13:30
  • \$\begingroup\$ PS. Could you suggest a regulator that can offer my desired voltage range without having to fidget with the power lines of the LM117/317s? \$\endgroup\$
    – pekatete
    Jul 1, 2013 at 13:35
  • 1
    \$\begingroup\$ I'd use an op-amp from the 3.6V rail - unity gain, non-inverting configuration with a potential divider from the 3.6V rail generating 1.1V at the non-inverting input. AD8605 springs to mind but there will be several that meet the spec i.e. will run from 2.7V to 5V (decent range) with rail-to-rail inputs and outputs. \$\endgroup\$
    – Andy aka
    Jul 1, 2013 at 13:48

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