psu problems

Discussion in 'General Electronics Chat' started by adamclark, Oct 30, 2013.

  1. adamclark

    Thread Starter Member

    Oct 4, 2013
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    hello again, my old psu quit and instead of trying to fix it, I decided to build a new one. heres what I need it to do.. Im trying to keep the old carbon fiber case I built for the old board. it has 5 switches and leds so Im stuck at keeping it that way, I need it to have 3.5v,5v,9,12v,15v, and 24v. It has to fit in a 2.5" by 7" area with 3 inches of height.. The old one has shut down with less than 300ma of draw, I would like the new one to be at least 2a.. I want to try to use a lot of the stuff I have laying around to build it and I have a trunk full of salvaged parts. I see a lot of schematics around that use the lm317 as the voltage regulator but the data sheet says its only good up to 1.5a. I have a few of these so can I use 2 of them to get the current up or is there a better solution? I also have a bunch of transformers from small to pretty big. however most of them have no writing at all on them. So how do I find the primary and secondary coils in them and find out what the voltages, amphere of them are? So this is where im at with my project. my skill level is somewhere between beginner and moderate so the easier the better.. thanks

    is it the size of the transformer or the capacitors or the diodes on the bridge rectifier that controls the current the most?
     
    Last edited by a moderator: Nov 15, 2013
  2. wayneh

    Expert

    Sep 9, 2010
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    I'd reconsider. Old computer PSUs are essentially free, you might even get someone to pay you to take them away. Just go to any electronics recycling center and help yourself. Why spend hours building something?
     
  3. ScottWang

    Moderator

    Aug 23, 2012
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    You can using the LM317 and in series with a 0.2Ω/5W resistor of the output, and then connecting all the resistors together to be the last output, it will be double or triple the current, I = 1.5A*n.

    It will be like these ways:
    Vin─┬─LM317─0.2Ω/5W─┬─○ Vout
    ......└─LM317─0.2Ω/5W─┘

    Vin─┬─LM317─0.2Ω/5W─┬─○ Vout
    ......├─LM317─0.2Ω/5W─┤
    ......└─LM317─0.2Ω/5W─┘
     
  4. ronv

    AAC Fanatic!

    Nov 12, 2008
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  5. adamclark

    Thread Starter Member

    Oct 4, 2013
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    that's why I asked if there was a better regulator to use, the only schematics I could find had the lm317. I don't necessarily need one.. and I could just use a psu from a pc but I want to build one of my own just because I want to learn...
     
  6. adamclark

    Thread Starter Member

    Oct 4, 2013
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    so I can double up the lm317's and double the current? im still new to this and im having a hard time making sense of this...
     
  7. wayneh

    Expert

    Sep 9, 2010
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    Fair enough. I've built LM317 based power supplies and it can be satisfying.

    But these days the action is in DC-DC converters. Just look on e-bay and you'll find tons of cheap options. They're more efficient, cheaper, smaller and more powerful than anything you can do with the old wall-wart, LM317 designs. So to my mind, it would be more useful to learn about the newer technology. Maybe build your own DC-DC converter using kine of the modern ICs.

    Of course these converters need DC, so you still need a DC supply.
     
  8. adamclark

    Thread Starter Member

    Oct 4, 2013
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    so could I double up the lm317 like scotwang said in the post above yours? would tha keep things cooler? I have a handful of them if adding more would be beneficial..
     
  9. adamclark

    Thread Starter Member

    Oct 4, 2013
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    here is the one of many schematics ive found that I think mine will be based from, how would I fit the other lm317's in it? and it says its variable but I see no way of adjusting it.
     
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  10. ScottWang

    Moderator

    Aug 23, 2012
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    You can see the LM317 datasheet from TI, you will find the answer on page 25.

    Here are some other ways to do.
     
  11. adamclark

    Thread Starter Member

    Oct 4, 2013
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    thanks for the links,, the adjustable 4a regulator on page 25 is exactly what I want to use,, the lm308 is an op amp so am I going to need -v as well as +v? and how would I make that in my circuit?
     
  12. ronv

    AAC Fanatic!

    Nov 12, 2008
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    Have a look at the link in post 5.
     
  13. adamclark

    Thread Starter Member

    Oct 4, 2013
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    that's a cool piece and would do what I need done,, however I could jus but a psu that has all the bells and whistles.. or I could just repair the on ive got that served me well up till now.. the point in all of this is to build it myself and have it just a or more reliable than one I purchased... thanks though.. that would save a ton of headaches,, but by the time im done we will have full schematics on here of a fully functioning variable power supply that just about anyone could build. and its going to be designed by everybody here that adds to the build with their assistance..
     
  14. ronv

    AAC Fanatic!

    Nov 12, 2008
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    Sure I understand. But make sure you decide on the specs before you start building. Right now there is a conflict between the size of your box and the heatsink and circuitry required for an analog supply. Nothing worse than thinking you have a 4 amp 3 to 24 volt supply and finding out you really only have a .5 amp supply.
     
  15. adamclark

    Thread Starter Member

    Oct 4, 2013
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    yea that would suck..
     
  16. ScottWang

    Moderator

    Aug 23, 2012
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    They just comparing at the positive, so you don't need the -V,
    the below is an easy way to in parallel with two LM317, so you can try in.

    [​IMG]

    Some people may using 2 3A diodes to replace the R2,R3.
     
  17. adamclark

    Thread Starter Member

    Oct 4, 2013
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    that's almost like the one im building but I believe im using 3 317's to make 4a.. I think ive finally found the right transformer for my psu. Its an old tube amp power transformer. it has a H.V. sec with a center tapped wire that measures 44.5vac/62.47vdc when multiplied by 1.404... its has another sec. that measures 38.5vac/54.05vdc, and a coil that measures 8.1vac/11.37vdc. The bridge rectifier I chose is an mb354-w, 40-100v 35a.. So I believe I have solid basics to put together a decent psu.. however, I am going to have to build another case for it, it will never fit into the one I was trying to save.. So I gues my next step is figuring out how I want to adjust the voltage. I am thinking about using a 10k 15 turn pot as the main and a 2k 10 turn for fine tuning. I also got some man74 7 segment displays that I want to use to make a digital voltage/amp display, so I am going to need as much help as you can give to figure that out. I know I can buy digital display meters for it, but ive built everything from parts ive had laying around so far, I want to keep it that way. The knowledge ive gained so far in this project vast and I fell theres more to be gained from it..thanks
     
  18. adamclark

    Thread Starter Member

    Oct 4, 2013
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    im thinking 0-40vdc 4a psu with digital volt/amp meters and maybe a few other features that haven't come to fruition yet.. One more big concern of mine is a protection circuit that protects the output from over current or short circuit.. any ideas there?
     
  19. PackratKing

    Well-Known Member

    Jul 13, 2008
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    Simple solution here, is to run 3 or 4 LM317's in parallel, on a respectable heat sink.

    I have that setup on a 0-12v rectified psu I built from scratch, and it has served me well for 25+ years as a "battery replacement" for consumer-level electronic strobe flash units,, the larger of which can draw upwards of 8 - 10 amps briefly while charging the main capacitor...

    Black square, is the sink for the rectifier, of " perfboard " aluminum, the alum. channel on up-rt., is the LM317 sink. with a pic as best as possible of the pcb for the vregs... psu cabinet is stainless steel.
     
    Last edited: Nov 14, 2013
  20. ScottWang

    Moderator

    Aug 23, 2012
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