Help ;lm317 in parallel ?

Discussion in 'General Electronics Chat' started by Roudha, Apr 11, 2014.

  1. Roudha

    Thread Starter New Member

    Apr 11, 2014
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    Hey guys , could someone draw for me how to connect lm317t in parallel ?
    For 2 lm317t or just answer me about this ; do I have to put for each lm317t resistor ?:confused::confused::confused:

    Thanks..
     
  2. KMoffett

    AAC Fanatic!

    Dec 19, 2007
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    It's normally not done. You didn't say what your intended input voltage or output current are. That's critical to the power handling requirements of the regulator. If you're limited to LM317's then check page 14: http://www.mouser.com/ds/2/405/slvs044v-261132.pdf

    Ken
     
  3. Roudha

    Thread Starter New Member

    Apr 11, 2014
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    My input would be 24volt, and am using 240ohm resistor and potentiometer of value 5k, am trying to rise the output current.
    U mean page 13 i didn't get the draw sorry I'm just a beginner in these stuff
     
  4. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    Desired output voltage?
    Required current?
    can you ONLY use a LM317?
     
  5. #12

    Expert

    Nov 30, 2010
    16,252
    6,751
    Start by reading the minimum load values. A 317 chip needs to waste about 10 ma and the circuit (Fig. 13) says it needs a 30 ma load.

    Adjusting the 5k resistor will not, "rise the output current". It will change the output voltage.
     
  6. KMoffett

    AAC Fanatic!

    Dec 19, 2007
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    Sorry, that should have been Figure 14 (page 12)
    If you can you might switch to an LM350K (on a big heatsink). It will handle 3A.

    Ken
     
  7. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    You can parallel two LM317 using "emitter resistors" if you don't mind a slight reduction in voltage regulation.

    Just put a resistor after the Vout pin, one resistor on each LM317, then parallel the Vin pins, the Adj pins, and the other ends of the two resistors.

    I think LM317 are rated for about 1.5A, so drop 0.15v at 1.5A;
    R = E/I = 0.15 / 1.5 = 0.1 ohm
    P = E*I = 0.15*1.5 = 0.225W (use 1/2W or better still 1W resistors).
    :)
     
  8. PackratKing

    Well-Known Member

    Jul 13, 2008
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    I did this on a bench supply 25+ years ago, 3 LM317's parallel, with no added components, just slightly upgraded input / output capacitors, and extended traces on the printcircuit to accomodate the spacing for the 317's...and used a 10-turn wirewound Clarostat for the adjusting element... see pic..

    Ohms law states, that current divides equally between parallel components, and being a novice,
    [ still one ] gave no consideration to "inconsistencies " in individual components, and neither ever blew, so I figured the regs would continue to perform OK.
    They are on the aluminum channel on the right... the double perforated alum. sink on the left, is for the rectifiers...

    Wiring and pcb schematics drawn retro is still available...
     
    Last edited: Apr 12, 2014
  9. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    Nice one! It's always good to hear from someone who did the un-doable (and got away with it). :)

    They do have a small voltage sag at higher loads due to internal resistance so that might have been enough to balance them.

    And if they were badly balanced, there is an internal current limit at about 1.8A so the one passing the most current would top out at 1.8A and the others would pass more current.

    It would be awesome if you could connect a 1 ohm resistor on each device between the main rect cap and the Vin terminal of each device. That won't affect the balancing, which is entirely dependent on Adj and Vout pins and each device's variance in its 1.25v.

    Then load the PSU up to 3A and you can measure the voltage on each 1 ohm resistor to see how well the current sharing works.

    In a pinch you could just put an ammeter in series each device's Vin pin and do 3 readings.
     
  10. PackratKing

    Well-Known Member

    Jul 13, 2008
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    I have a bad habit ? of bending physical law to see what the workable limits are...

    I think it would be very interesting / educational to do as you suggest this far down the road, just to find out how my inexperience of the day, and faith in Ohms' Law fared...:D

    At the time, I figured since the regs came off the same production line, the inconsistancy, if any , couldn't be all that severe...

    I wanted to do the pass-trans. circuit, tho' at the time did not have an adequate t'sistor on hand... Now I do...

    I long since snagged a flat of 100 2N5302 TO-3 units from a former employer R&D obsolete stock... as well as a salvage 1000 watt UPS tranny [ picture - 6" scale on top ] I am designing a kick-ass high-amp AC / DC supply around...
     
    Last edited: Apr 13, 2014
    THE_RB likes this.
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