lm317 inaccurate Output

Discussion in 'General Electronics Chat' started by CaptainPrice, Jul 12, 2011.

  1. CaptainPrice

    Thread Starter Active Member

    Aug 12, 2009
    49
    0
    I can't figure whats wrong.

    I connect 2 18650 batteries (8.4V) up to the circuit.
    I have 1kΩ from Output to Adjust and 3kΩ from Adjust to ground.

    The output pin should have 5 volts. but I measure 5.5 volts
    I tried putting 3MΩ and 1MΩ in place of the resistors and the voltage is 7.5 out
    also tried 2k and 1 k to get calculated voltage out of 3.75 and i get 4.4 out??
    also the resistance from ouitput to ground seems to affect the output voltage...

    Datasheet from supplier
    http://www.taydaelectronics.com/datasheets/A-177.pdf
    (could I have wrong pinout?)
     
  2. SgtWookie

    Expert

    Jul 17, 2007
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    For starters, use a 120 Ohm resistor from OUT to ADJ. That ensures the minimum current flow of 10mA to guarantee regulation will be met.

    Then if you want 5v out, you'll need 360 Ohms from ADJ to GND.
     
  3. Jaguarjoe

    Active Member

    Apr 7, 2010
    770
    90
    The middle of page 3/11 of the datasheet lists the minimum current requirements.
     
  4. Wendy

    Moderator

    Mar 24, 2008
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    [​IMG]

    Something else that needs mentioned, capacitors are not optional.
     
  5. CaptainPrice

    Thread Starter Active Member

    Aug 12, 2009
    49
    0
    Ok. will do so, I know I have under 10ma draw
    Ill have to go dig through my pile of resistors...


    thanks


    So you are saying add a .1μf Cap 'from out to ground' and 'in the ground' and a bigger 200uf cap from in to ground?
    I thought the data sheet said it was optional to improve performance?
     
  6. Wendy

    Moderator

    Mar 24, 2008
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    Not really optional, the odds of the circuit oscillating is very high without them. You might get by without C1, but the other caps are pretty well required.
     
  7. SgtWookie

    Expert

    Jul 17, 2007
    22,182
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    Where you run into problems with these types of regulators is when there is just a small value of capacitance on the output; from a few pF up to 50nF or so. That capacitance coupled with the inductance of the interconnecting wiring creates a resonant circuit. Having a 0.1uF/100nF cap to ground from the output terminal swamps this tendency to oscillate.

    Adding a larger cap (say, 10uF) in parallel with the 0.1uF cap will help with output transient response.
     
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