Problem on battery charger Voltage drop

Discussion in 'General Electronics Chat' started by cheegii, Nov 7, 2010.

  1. cheegii

    Thread Starter Member

    Nov 24, 2008
    17
    1
    Hello all. I had a problem PSU circuit that have LM317 regulator and Tip35C transistors. I used 24AC transformer and rectified. Then used Capacitor 470uF LM317. I am making regulated power supply 1.2v-24V one transistor's base connected to LM317's output. And 4 piece of parallel connected Tip35 transistor's bases connect to first transistors emitter. making more currents. But one problem is When i connect a load to circuit Voltage drops. How to run exact output voltage. I know output transistor could drop voltage app 0.6V. Transformer have 240watt. Load have only 2Watt. I set 1kOhm resistor on output and set voltage to 12.05 volts. Then connect the load. It's voltage drops to 11.40V. Load is 12volts 0.6A CPU cooler fan. The fan is just test load. How to run exactly fixed voltage? I searched some circuits. But no luck.
     
  2. bertus

    Administrator

    Apr 5, 2008
    15,647
    2,346
    Hello,

    Show us your schematic.
    That way we can see where the trouble is.

    Bertus
     
  3. cheegii

    Thread Starter Member

    Nov 24, 2008
    17
    1
  4. bertus

    Administrator

    Apr 5, 2008
    15,647
    2,346
    Hello,

    Where does the collector voltage for the powertransistors come from?

    [​IMG]

    Bertus
     
  5. cheegii

    Thread Starter Member

    Nov 24, 2008
    17
    1
  6. bertus

    Administrator

    Apr 5, 2008
    15,647
    2,346
    Hello,

    I merged your last posts, you can use the edit button to add or change a post.

    You have a large voltage drop due to the emittor follower configuration.
    The schematic shown from the reuk site is comparable to the one in the datasheet and will have no voltage drop.

    Bertus
     
  7. tom66

    Senior Member

    May 9, 2009
    2,613
    214
    You'll want to include a 100n capacitor on the output to reduce the chance of oscillations from the LM317 ruining your day.
     
  8. cheegii

    Thread Starter Member

    Nov 24, 2008
    17
    1
    [​IMG]

    PNP darlington transistor could control voltage? Collecter is output right?
     
  9. bertus

    Administrator

    Apr 5, 2008
    15,647
    2,346
    Hello,

    Yes, the collector of this PNP transistor is the output.
    It must be connected to the output of the LM317 too, as you can see.

    Here are some more LM317 circuits at Bill Bowdens site:
    http://www.bowdenshobbycircuits.info/page12.htm

    An other regulator with a higher output current in the LM338.
    When cooled enough it is capable of 5 A.
    (see the datasheet for more info).

    Bertus
     
  10. cheegii

    Thread Starter Member

    Nov 24, 2008
    17
    1
    Thank you very much. I got it! Third drawing circuit is mostly sensetive to output voltage. I've been searching this for 2 days. :D
     
  11. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    The big problem is that the only place the output voltage is regulated is at the output of the LM317. After that, you have the equivalent of three diode drops; D2, the base-emitter junction of Q1, and then the base-emitter junctions of Q2-Q5.

    Look at National Semiconductor's datasheet for the LM117/LM317, "High Current Adjustable Regulator", top of page 17. Download it from their website.
     
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