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Math Help

TCTC Posts: 1,019
edited 2006-12-09 02:01 in General Discussion
OK, I have no idea what size resitor I need for a LM337 adjustable negative voltage regulator.

www.st.com/stonline/products/literature/ds/2167/lm237.pdf

Page 8, Figure 17 is the one I am going to use.· I am looking for -13.6V output.

Please help
TC

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We all make mistakes when we are young………That’s why paste is edible!

Comments

  • Mike GreenMike Green Posts: 23,101
    edited 2006-12-09 01:43
    The formula given in the text and schematic is what you need. Usually R1 is 120 ohms. I think for 13.6V, R2 is 1120 ohms. You're limited by standard values. I would use a 1K resistor and a small adjustable resistance like 150 or 200 ohm and adjust it to get the exact voltage you want. Alternatively, you could get a 10 turn variable resistor with a value somewhere between 1.5K and 2K and adjust it to "fit".
  • PJAllenPJAllen Banned Posts: 5,065
    edited 2006-12-09 01:43
    The equation is on Page 8 of 14.·

    If you don't like wrestling with numbers, then rig up the circuit with a 240ohm resistor (from·OUT to ADJ) and a 5Kohm pot (from ADJ to GND.)· When you dial that to your desired voltage, take the pot out circuit and measure it.·

    [noparse][[/noparse] You're not going to find a resistor of/with this precise value. ]

    ** Post Edit **· Well, as you'll see, your Uncle Stamptrol took all of the thinking out of this for you.


    Post Edited (PJ Allen) : 12/9/2006 1:54:38 AM GMT
  • stamptrolstamptrol Posts: 1,731
    edited 2006-12-09 01:44
    TC,

    Looking at page 8, fig 17, the output voltage equation is given.

    You have to flip back to page 4 to get Iadj from the specs, but it is very low, about 65 uA. In practical terms, it will have little effect on Vout.

    Therefore the equation looks like:

    Vout= - 1.25(1 + R2/120)

    In your case:

    -13.6 = -1.25(1 + R2/120)

    which simplifies to:

    - 1482 = - 1.25 R2

    R2 = 1186 ohm

    Cheers,

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    Tom Sisk

    http://www.siskconsult.com
    ·
  • PJMontyPJMonty Posts: 983
    edited 2006-12-09 01:58
    TC,

    Check out this page:

    www.interq.or.jp/japan/se-inoue/e_ckt21.htm

    It has a dual sided supply using LM337 regulators. This next page:

    www.interq.or.jp/japan/se-inoue/e_ckt21_2.htm

    ...(which can be found by clicking on "Circuit Explanation" on the first page) explains the formula in much better detail than that datasheet.

    In addition, there is this page (on a different site):

    www.ski.org/rerc/sktf/vol04no2Spring1983.html#national

    ...which further explains the math.

    The thing that is really confusing about the datasheet is the "Iadj" term. Read the other pages and you'll see that it can be completely ignored. According to both of these sites, the formula to directly solve for R2 is:

    R2 = 120 * (( Vout/1.25) - 1 )

    I added some extra parentheses and an explicit multiplication sign to make sure the formula is understandable.

      Thanks, PeterM
  • TCTC Posts: 1,019
    edited 2006-12-09 01:59
    Thank you all for the fast replys. They all helped very much.

    Thanks, and have a happy holidays

    TC

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    We all make mistakes when we are young………That’s why paste is edible!
  • TCTC Posts: 1,019
    edited 2006-12-09 02:01
    Thank PeterM you helped me with the formula.

    TC

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    We all make mistakes when we are young………That’s why paste is edible!
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