Fixed Power Supply to Variable Power Supply

Discussion in 'The Projects Forum' started by rcc1111, Apr 15, 2011.

  1. rcc1111

    Thread Starter New Member

    Apr 15, 2011
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    I currently have a Fixed 13.8V 25AMP Switching Power Supply that I want to change into a Variable Power Supply. I would also like to able to have variable current from 1.5 - 25AMPS. I have looked up a some stuff and haven't been able to find what I am looking for. Does anyone have any experience with doing something like this?
     
  2. Wendy

    Moderator

    Mar 24, 2008
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    Basic Bench Top Power Supplies

    This is actually routine. The exact regulator will be a problem, and from the sounds of it you will need two, constant current, and a voltage regulator. Each will drop some voltage, unless it is considerably more complex (and you will loose some current range).
     
  3. rcc1111

    Thread Starter New Member

    Apr 15, 2011
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    If I added a LM317 circuit to my supply and made it so I had a fixed voltage of 13.8V and a variable voltage. Do you think the LM317 circuit can handle a 25amp input?
     
  4. SgtWookie

    Expert

    Jul 17, 2007
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    The LM317 will handle a 13.8v input just fine. However, the maximum output voltage will be the input less ~1.7v to 2.5v depending on how much current is being drawn from the output of the LM317. You will not get more than about 12.1v out, even under light load.

    The LM317 is limited to 1.5 Amperes output current, and will get quite warm doing so; it will require a heat sink.
     
  5. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    25Amps .....caramba !! Are u serious.

    Are u sure that what u have can supply that amount of current.
    And why would you need that much current anyways?
     
  6. SgtWookie

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    Ham radio operators use supplies of that size and even more very frequently. It provides a way for them to operate their "portable" rigs (designed to be used in an automotive environment) inside a dwelling, operating from mains power.
     
  7. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    Ham ?? Don't you guys eat ham. :D

    We don't have those. but I remember those in the old days, they use like 14amps ones..big and heavy...
    I have never seen a 25Amp one.
     
  8. Wendy

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    Mar 24, 2008
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    There are ways to boost a LM317, but you'll need a different regulator for 25A, and there will be losses in voltage. All part of the price. I showed the link by way of example. 25 amps is a lot of current, you will need large gauges of wire, and electronics will have some trouble with currents that big. It can be done, but I suspect it is a larger job than you anticipated.
     
  9. SgtWookie

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    One way to control the output voltage is to change the resistive divider in the feedback path of the switching power supply. However, that's a bit of an advanced topic.

    You'll need to figure out which IC in the supply is the switching regulator by looking up the part numbers, then look at the datasheet for the switching regulator and determine where the feedback from the output voltage is obtained; and then figure out what resistance you'll need to change or vary.
     
  10. rcc1111

    Thread Starter New Member

    Apr 15, 2011
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    I am not 100% sure my supply puts outs 25amps. I am just going by what is says on it. Plus I don't have a way to measure that much current. I use the supply to run my my rc battery charger. Plus I got the power supply for free so I can't complain. I would just be nice to use it for some other stuff.
     
  11. Wendy

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    300W isn't bad. I suspect you need to define your needs a bit tighter. Are you after 12VDC?
     
  12. rcc1111

    Thread Starter New Member

    Apr 15, 2011
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    Yes 12VDC is the max that I will need.
     
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