Help with my power supply project!

Discussion in 'The Projects Forum' started by Bryan47, Mar 4, 2011.

  1. Bryan47

    Thread Starter New Member

    Dec 1, 2010
    6
    0
    I designed a power supply in high school and am giving a go at designing another one. My basic requirements are that it will have multiple voltages available. It will have fixed 12V, 5V, and variable voltage 0-24V. I'll only be pulling about 2amps maximum as well. I have attached my schematic in the hopes that you could tell me what I need to change and what could be improved. Thank you so much!

    I have a multisim file and it said it was an invalid file type. How do I get the file up here for you guys? Thanks!
     
  2. Wendy

    Moderator

    Mar 24, 2008
    20,764
    2,534
    Zip it for the short term. However, only a few will see it, I don't use Multism for example. You are better using a graphic image for the schematic, such as .gif or .png.
     
  3. tgotwalt1158

    Member

    Feb 28, 2011
    111
    18
    hi Bryan47! In what ways do you require improvements, give some idea and upload the diagram.
     
  4. Bryan47

    Thread Starter New Member

    Dec 1, 2010
    6
    0
    Here is a picture and a zipped version of the file. I just want to make sure I am using all the correct parts and everything is connected properly, my electronics brain is a bit fuzzy!

    Thank you!
     
  5. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    2A is too much for a 7812. Even 1A is pushing it; LARGE heat sinks are required.

    Consider converting an ATX form factor computer power supply to a bench supply.
    Google: "ATX Bench Supply" for lots of ideas. It's easy, cheap, lots of current, and efficient.

    That leaves the variable supply - but for most purposes, an ATX's +/-5v, +/-12v, and +3.3v outputs will be quite adequate.
     
  6. Bryan47

    Thread Starter New Member

    Dec 1, 2010
    6
    0
    I've already considered converting an atx supply and even started doing it but it was too bulky for the case I'm using and I want to add things like fans and things to make it my own. I want it to be designed and built %100 by me. Thanks for the suggestion however.

    Is 2A too much even in a TO-3 case? What should I use instead
     
  7. wayneh

    Expert

    Sep 9, 2010
    12,087
    3,027
    Whatever case, the power still has to be sinked and dissipated. The datasheets for these devices include several techniques for controlling higher current levels, either by placing them in parallel or by using power transistors.

    But again, no matter what, there's going to be a lot of heat if you drop 2 amps from over 20V to under 10 volts.
     
  8. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    Look at the datasheet for the LM317 adjustable regulator. You are using the 240 ohm resistor and 5k pot that are used for the more expensive LM117. The LM317 uses a 120 ohm resistor and a 2.5k pot to prevent the output voltage from rising when the load current is low. It is explained on the datasheet.
     
  9. DickCappels

    Moderator

    Aug 21, 2008
    2,647
    632
    You can probably get by with the 240 ohm resistor, though regulation will suffer some. Maybe drift as well. I used 470 Ohms on a bench supply I use now, for both the LM317 and LM337. Similar range 1.25V to 26V. That's because I had to make it work with available pots.

    Per Audioguru, using the recommended value will give the overall best performance.
     
  10. tgotwalt1158

    Member

    Feb 28, 2011
    111
    18
    hi Bryan47! since you need 2A output current, use NTE1934 or Fairchild's KA278R for 5V, NTE1936 or KA278R for 12V and LM350 for 1.2 to 33V variable output with max up to 3A. Find/calculate appropriate R1 & R2 for LM350 from its data.
     
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