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I am using an LM337 to keep voltage stable at 3V from a battery supply (4x AAA). I've calculated R1 and R2 from the formula to be 100 Ohms and 140 Ohms respectively:

Vo = -1.25*(1+R2/R1)
Vo = -1.25*(1+140/100)
Vo = -1.25*(1+1.4)
Vo = -3

And I used 1uF solid tantulum and 10uF electrolytic capacitors.

Yet it gives me about 4.5V output. Did I screw up something?

Image from LM337 docs

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1 Answer 1

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The numbers seem correct, as do the capacitors. Check the regulator pinout that you are using (it is different from similar positive regulators) and double check the resistor values if the pinout is correct.


Edit: since it appears you would be better served by a positive regulator, I suggest you do a parametric search at a distributor or series of manufacturers. Many parameters are important and modern LDO regulators tend to be much more specialized in terms of a combination of max input voltage, current (quiescent, max), power dissipation, package type, max and min output current and so on.

Just for example, the LP3869 but there are thousands of possibilities so chances are it will be sub-optimal in some way.

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  • \$\begingroup\$ Well LM317 seems to be: 1. Adjust 2. Out 3. In However the docs for LM337 shows: 1. Adjust 2. In 3. Out Is there a way to test it? \$\endgroup\$
    – TomasB
    Commented Jun 17, 2019 at 18:44
  • \$\begingroup\$ You are testing it- measure the voltages especially the voltage from Vo to adj and the input voltage. \$\endgroup\$ Commented Jun 17, 2019 at 18:46
  • \$\begingroup\$ In the docs they are talking about negative voltage. I thought I would hook it up to a battery and get a steady voltage of 3V but it seems to take negative voltage and give stable negative voltage. Since I am providing positive voltage could it be the problem? I perhaps need to buy the LM317 which seems to operate on positive voltage. \$\endgroup\$
    – TomasB
    Commented Jun 17, 2019 at 18:56
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    \$\begingroup\$ Ah, you need to give it negative voltage for sure. Since you have a battery, just connect it appropriately. Assuming you have not damaged the regulator it should work. \$\endgroup\$ Commented Jun 17, 2019 at 18:57
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    \$\begingroup\$ Yes, the LM337 is a negative voltage regulator. So it works with an input that is negative with respect to circuit common, and puts out a voltage that is negative with respect to circuit common. If that's too much negativity, then yes, you need an LM317. \$\endgroup\$
    – TimWescott
    Commented Jun 17, 2019 at 19:01

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