high current power supply help

Discussion in 'The Projects Forum' started by bengi, Apr 8, 2010.

  1. bengi

    Thread Starter New Member

    Feb 24, 2010
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    0
    i am building a 20 amp power supply using 6-338's for
    current, and 2-317k's for voltage control. i need a
    circuit diagram showing just where to tie them together.

    TKS bengi
     
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Are you using LM338K (TO3) or LM338T (TO220) regulators?

    What will be your input voltage, and expected output range? Note that power dissipation in the regulators will be very high if you are expecting much of an output voltage range.

    Do you have very large heat sinks to mount the regulators on?
     
    Last edited: Apr 8, 2010
  3. bengi

    Thread Starter New Member

    Feb 24, 2010
    5
    0
    sgt. thanks for your responce to my post. 338 are 5 amp.
    will be using 2 317k, 3 amp ea. input voltage is 33 volts.
    output, 1.2-30 v. 20 amps. (well heatsunk) i have several
    diagrams of the circuits. however, some diagrams show
    the voltage reg. output going to the adj. of the current
    reg. i don't think this is what i need. what is the best way
    to hook the two reg. together

    tks
     
  4. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    I know that the LM138/LM338 regulator family is rated for 5 Amperes output current.
    My question was, are you planning on using the LM338T (TO220 case) or LM338K (TO3 case) regulators?

    I am afraid that you are mistaken, as the maximum rating for the LM317K (TO3 package) is 1.5 Amperes. Please look at the datasheet. (Note that National Semiconductor's datasheet was recently updated, please visit their website and download a copy of the LM117/LM317 datasheet)

    You will have a very hard time with power dissipation in the regulators.

    Show an example of the heat sinks that you are planning on using.
    The first question you need to ponder is why do you believe that such high current over such a wide voltage range is necessary?

    If your input is 30v and your output is 15v, half of the power dissipation will be in the load, and the other half in the voltage regulators.

    With the same input voltage, if your output is 5v, then 1/6 of the power dissipation will be in the load, and 5/6 of the power dissipation will be in the regulators as waste heat.

    If your output is 1.2v, then 24/25 of the power is dissipated in the regulators, and 1/25 in the load.

    So, before you get really excited about building a super-duper 20A 1.2v-30v power supply, start going through some of the math involved.

    Note that for the TO3 package, thermal resistance is 1°C per 1 Watt of power dissipation, and the maximum temp is 150°C
    For a TO220 package, thermal resistance is 4°C per 1 Watt of power dissipation, and the maximum temp is 125°C.
     
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