Converting voltages using LM337

Discussion in 'General Electronics Chat' started by Oussama Zaidi, May 2, 2016.

  1. Oussama Zaidi

    Thread Starter Member

    Mar 1, 2016
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    0
    Hi
    I have 2 questions:
    Is it possible to generate -35V from LM337 regulator with -12V in input??
    The voltage output from LM337 is supposed to follow this equation, -Vout=-1.25(1+R2/R1)+Iadj.R2 but in ISIS simulation the output value is always the same as the input, I don't understand why, for example in the picture above output value is supposed to be -32V according to the resistances values but it is -39V because the input is -40V, the same for any other voltage.
    LM337.PNG
     
  2. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    5,776
    1,103
    You have R2 in the wrong place. It should connect ADJ to ground.
     
  3. AnalogKid

    Distinguished Member

    Aug 1, 2013
    4,518
    1,247
    No.

    Besides the schematic error, that is not how a linear regulator functions. Basically, it acts like an automatic attenuator. So the equation is correct as long as the output is less than the input. With a -12 V input, the maximum output is around -10 V. This is because the device needs a minimum of 2 volts or so (the exact number is on the datasheet) for it to function.

    Some switching regulator circuits can do what you want, but linear regulators sannot.

    ak
     
  4. Oussama Zaidi

    Thread Starter Member

    Mar 1, 2016
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    0
    @AnalogKid Can you give me some names of switching regulators can do the job, I do my research but I didn't found any, just anything can give lower than -30V.
     
  5. absf

    Senior Member

    Dec 29, 2010
    1,492
    371
    The chip used is LM2577 and the module is here.

    s-l400.jpg
    Allen
     
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