Adjustable power supply

Discussion in 'The Projects Forum' started by Nathanrides, Aug 25, 2012.

  1. Nathanrides

    Thread Starter New Member

    Aug 25, 2012
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    Ok. I'm new here. I want to build a Variable input, adjustable power supply. I have some of the basics figured out, but the wiring and some other parts I'm not sure of. I have a project box, an input jack, an LM317 (for variable input of 12vdc to 30vdc), a potentiometer (volume control type with "click" off. If that shuts off the output voltage, great!), and a 12v digital display. I know I have to have another 12v regulator for the power side of the digital display, and the signal wire off the display would connect to the positive lead of the output coming from the box, but other than that, I have no idea how this thing will have to be wired. I have seen some diagrams online, but they show resistors, capacitors and whatnot. I wish I knew more about how the 317 and the pot works. does the pot control the signal lead of the 317? If so, isn't that all I need? What would I need the caps and resistors for? I'm fine with using them, just need more info.

    Bottom line. I need an adjustable (0 to 30v output) power supply that I can use any DC transformer from 12v to 30v as an input. The digital gauge is for the output voltage.

    I need a Parts list and a diagram. ANY help would be appreciated. THANKS!
     
    Last edited: Aug 25, 2012
  2. Ron H

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    Apr 14, 2005
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    What is the variable input for?
     
  3. Nathanrides

    Thread Starter New Member

    Aug 25, 2012
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    Well, It always seems like I have different power supplies laying around at different times for different things, so I want to be able to use different ones if I want to. I mean, I know that if I'm using a 12v transformer, I can't expect to get more than that out of it, but I can use it to power LEDs or test small equipment like cameras and the like. Then again, if I use a 30v transformer, I can do anything up to that. Just make it more versatile that's all.
     
  4. Ron H

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    LM317 basically has a maximum input voltage rating of 40V. With that, you can make a variable supply that goes from 1.3V to around 35V (max depends on input voltage ripple). This means that, for applications where you don't need high current at low voltage, you don't need a variable input voltage. Since the power dissipation of the regulator is
    Pdiss=(Vin-Vout)*Iout.,
    a high current load when the output voltage is low will result in a lot of heat being generated in the LM317. This is where a low voltage input option can be useful.
    Does this make sense? If so, can you clarify if and how and why you plan to create the variable input voltage?
     
    Last edited: Aug 26, 2012
  5. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    The LM317 is, by design, a variable input regulator. In addition, it can be configured to have a variable output. As such, it might be a good choice for your project. Your first step probably should be to read the datasheet for the LM317 and study its capabilities and the application examples. Then, you could decide if it's really the IC you need.
     
  6. SgtWookie

    Expert

    Jul 17, 2007
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    Actually, the formula is:
    Pdiss = (Vin - Vout) * Iout.
     
    Ron H likes this.
  7. Ron H

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    Doh!:(
    I guess I gave the formula for the transresistance. :D
    I'm gonna fix it.
     
  8. bountyhunter

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    Sep 7, 2009
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    Pdiss = (Vin - Vout) * Iout + ( Vin * IGnd )


    Where IGnd is the ground pin current of the linear regulator.
     
  9. Ron H

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    Yeah, IF the regulator has a ground pin. And even if it does, it ain't gonna be a deal breaker.:p
     
  10. bountyhunter

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    Depends. IN LDO regs, IGnd can be a very large power dissipation component at higher values of VIN.
     
  11. Ron H

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    I wasn't aware of that. Can you give some examples?
     
  12. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    Sure. Here's a 1A LDO good for up to 26V input. GND pin current can be up to 60 mA under "normal" (not in dropout) operation. However, in dropout it can go up to 150 mA or more (it's the effect called "carrot" current because the peak looks like a carrot).. Multiply the GND current times VIN and you can get several Watts. This is a good example of what we had to try to improve on the newer LDO's: higher gain PNP devices to reduce bias current but they all still have some degree of carrot current.

    http://www.ti.com/lit/ds/symlink/lm2940-n.pdf
     
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