LM 317 regulator.increase output

Discussion in 'General Electronics Chat' started by dannycn8@hotmail.com, Sep 19, 2012.

  1. dannycn8@hotmail.com

    Thread Starter New Member

    Sep 18, 2012
    I have built a DC Power Supply. Output Voltage is about 17.5 Volt.
    Current capacity is about 30 Amps.
    I intend to use this power supply to charge 12 V. Sealed Lead Acid batteries.
    On the side of the batteries there is printed two set of values,one for cycle charge and another for standby charge. generally I think the voltage should be around 14 volt for safe charging.
    The problem I have is how do I get my 17.5 V to a regulated 14 volt.
    I have experimented with a LM 317 K regulator. It's a fantastic IC,but not enough current.Any suggestions as to how I can boost the current output of the LM317K.?
    I'm a hobbyist with little technical knowledge,please help. Thank you.
  2. t06afre

    AAC Fanatic!

    May 11, 2009
    Why not use a charger IC while you are at it. What will be the capacity of the batteries you plan to charge. Also how did you calculate your power current capacity.
  3. Dodgydave

    Distinguished Member

    Jun 22, 2012
    Try the circuit on diagrams 13 or 14 of the manual,it uses a slave transistor follower, note the minimum in to out voltage is 3v, so it might not do for you, unless you can increase the supply from 17.5 up-to 20V say?

  4. cfh

    New Member

    Mar 20, 2012
    One small issue I have is that the 317 has a mimium dropout of 3vdc. with only 17.5 in and wanting 14 out you are rather close to that limit, and depending on your supply ripple may go into dropout each cycle. while a battery charger doesn't need the close regulation, my supplies always have the 'back of the envelope' 3000uF/A as thier filter. Lesse for 30A, that's 90000 uF at oh make it a minimum 25Vdc. For that current, I would go to a darlington for the pass. 2n3055's just are high enough gain (beta) above 5A ( I figure beta=2 -- so your need 15a base drive for the set of 2 or 3 30545s.. not nice.. (also the reason there were 2n3053 and 2n30t54s :) 2n3772 is a better fairly available choice among old line transistors. an lm195 or two would be handyier as well. And NEVER forget the reverse bias protection, I'd be making it 1n4002 or better.
  5. chrissyp

    Active Member

    Aug 25, 2008
    Use the circuit suggested but replace the LM317 with an LD1085 they are a low drop out regulator better suited for when the input voltage is close to the output voltage. You can get a suffix P version which is encapsulated in plastic,so no need to use isolators.