Variable Voltage

Discussion in 'The Projects Forum' started by mapleman555, Mar 19, 2015.

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  1. mapleman555

    Thread Starter Member

    Dec 15, 2014
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    Hello,
    I just bought a used CSi/Speco regulated power supply, 13.8 DC v. @4 amps, and I want to make it into and adjustable voltage power supply. I was thinking of hooking a potentiometer to it to adjust it. I was looking on the internet about this but all I have found so far are "build your own" versions on a breadboard or pcs.
    Are there any diy projects for this? Or what do I need?
     
  2. RayInMS

    Member

    Dec 12, 2012
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    The LM317 is a good option, but you're limited to less than 1.5A output without adding some external components.

    See here:

    http://www.ti.com/lit/ds/symlink/lm117.pdf
     
  3. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    Getting inside the supply to modify it to be adjustable from 0V to 12V opens a huge can of worms.

    Guessing this is a linear supply. The unregulated voltage across the filter capacitor is likely ~16V. The regulator is designed to dissipate about (16-12)*4 = 16W of heat when the supply output is at 12V and it is delivering 4A.

    Now ask yourself how much power must be dissipated in the regulator when you connect a 4A load if the supply could be adjusted down to 3V. (17-3)*4 = 56W, for which the heatsinking is no where near adequate enough!
     
    Last edited: Mar 19, 2015
  4. Dodgydave

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    Jun 22, 2012
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  5. mapleman555

    Thread Starter Member

    Dec 15, 2014
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    I wasn't planning on modifying the inside. Here is a picture of the outside. [​IMG]
     
    Last edited: Mar 19, 2015
  6. Dodgydave

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    Jun 22, 2012
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    So if your not planning on modifying it ,whats the problem then as per post #1???
     
  7. wayneh

    Expert

    Sep 9, 2010
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    Just use a LM317 and stay below the current limit for that chip. That will cover 90+% of anything you would do on a lab bench with DIY circuits. If you're using a breadboard, it's probably not rated to more than 1A anyway.

    You can make your own regulator or you can buy pre-made voltage regulators preassembled on a board with a pot on e-bay. Either way, you might consider a "low drop out" LDO regulator to get a higher top-end voltage. The LM317 will limit you to 9 0r 10V max. That's plenty for most projects unless you specifically need 12V.
     
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  8. BobTPH

    Active Member

    Jun 5, 2013
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    I wonder why you bought a fixed supply when what you wanted was a variable supply?

    Bob
     
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  9. mapleman555

    Thread Starter Member

    Dec 15, 2014
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    I bought a fixed supply because I wanted a fixed supply.......that could also be used as a variable.
    Right now I am using LEDs a lot and using resistors and say a L7805 but if I want some thing to change the speed of a motor I want a variable.
     
    Last edited: Mar 19, 2015
  10. Dodgydave

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    Jun 22, 2012
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    you will probably find it has a 7812 regulator inside, with a power transistor on an heatsink, the regulator can be replaced with a Lm317, to make it variable.
     
    Last edited: Mar 19, 2015
  11. mapleman555

    Thread Starter Member

    Dec 15, 2014
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    OK, thanks. I'm not really familiar with the Lm317 but going through my inventory I see I bought a few of them. I must have been thinking ahead. I'll download the data sheet on it and take it from there.
     
  12. RayInMS

    Member

    Dec 12, 2012
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    From the data sheet...very easy to do:
     
  13. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    Much easier to modify,take pictures of the inside so we can help you.
     
  14. mapleman555

    Thread Starter Member

    Dec 15, 2014
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    I get an inside image soon.
    Speaking of images, see the one I put here of the outside of the power supply? Does any one know the actual name of the power connector knobs? Notice there is only one on the negative side. I want to order some replacements.
     
  15. RayInMS

    Member

    Dec 12, 2012
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    You could probably remove them and replace with regular banana plugs. You'll need to remove the old jacks anyway to measure the size of the holes (if you want to try and find identical or similar replacements).
     
  16. Dodgydave

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    Jun 22, 2012
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  17. mapleman555

    Thread Starter Member

    Dec 15, 2014
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    Here is the inside.
     
  18. Dodgydave

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    Jun 22, 2012
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    looks like a transistor regulator with a power transistor on the casing ( not shown) on the blue,red,black wires,and a preset to alter the voltage,or current limit,can you show the power transistor.
     
  19. mapleman555

    Thread Starter Member

    Dec 15, 2014
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    This what your looking for?
     
  20. mapleman555

    Thread Starter Member

    Dec 15, 2014
    56
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    Great, Thanks.


    And thanks for the link to the banana sockets.
     
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