Need circuit of LM317T for out-put of 1.5V

Discussion in 'General Electronics Chat' started by jaides, Jul 25, 2009.

  1. jaides

    Thread Starter New Member

    Jul 24, 2009
    1
    0
    Hi

    I bought LM317T from Radioshack and thought it will have some kind of regulator to control voltage but :(

    Anyway if somebody can help me in making a circuit which will give output of 1.5v. input is 5v.
     
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    This will get you close:

    [​IMG]

    The COM terminal on the LM317 symbol really should be labeled the ADJ terminal.

    C1 is necessary if the leads to the power source are over 6" long.
    R1 feeds current from the OUT to the ADJ terminal. The LM317 regulator attempts to keep the voltage from OUT to ADJ at a nominal 1.25v (known as Vref); but it can be as low as 1.2v or as high as 1.3v and still be within specifications.

    If Vref is the nominal 1.25v, then the current through R1 will be 1.25v/100 Ohms = 12.5mA, and if the ADJ terminal were connected to ground, then the regulator would put out 1.25v.

    However, you want 1.5v output.
    So, 1.5v-1.25v = 0.25v. You need a value for R2 that will drop 0.25v when 12.5mA current is flowing through it.
    Since R=E/I (Resistance = Voltage/Current), we can calculate:
    R2=0.25v/12.5mA
    R2 = 0.25v/0.0125A = 20 Ohms.

    In my schematic, R2 is shown as 15 Ohms. This is because the simulation model has a Vref of 1.29v (between OUT and ADJ). This gave a current of 12.9mA through R1, and roughly the same amount through R2.
    In that case, I needed 1.5v-1.29v = 0.21v with a 12.9mA current.
    So, R2 = 0.21v/0.0129A= 16.28 Ohms. 15 was a standard value of resistance; I just chose the closest one.
     
  3. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    Take care to connect the IC pins in the right way (according to the datasheet). Don't get confused with the LM78xx series pin configuration, LM317 has a different pin configuration.
     
  4. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    16 ohms is a standard 5% resistance.
     
  5. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    Here are the ranges I've seen, and have in my resistor kit.
     
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