7805 vs LM317

Discussion in 'General Electronics Chat' started by Konstabel, Jan 31, 2008.

  1. Konstabel

    Thread Starter Active Member

    Jan 31, 2008
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    If I want a regulated 5V supply, is there any reason why I would rather use a LM317 circuit rather than the 7805?
     
  2. scubasteve_911

    Senior Member

    Dec 27, 2007
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    Less components need for a fixed voltage regulator. This simplifies design and layout. Having an adjustable regulator allows you to get a more precise voltage at the output, but I believe it may be more likely to drift with temperature because of the resistors.

    Steve
     
  3. SgtWookie

    Expert

    Jul 17, 2007
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    What are you planning on powering with this 5V supply?

    If it's for digital, consider a switching power supply; they're far more efficient.

    If it's for linear, you really need a linear supply.

    The 78xx and LMx17 ICs have been around for a long time, and they still work. However, they have a minimum Vdrop of about 1.7V across them, which means they consume power and generate heat.

    There are quite a few "low dropout" regulators on the market now by various vendors. You might do some research to see if there's something better for your particular application.

    Or, say what you want to do, and we'll try to give you some pointers ;)
     
  4. Konstabel

    Thread Starter Active Member

    Jan 31, 2008
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    I would like to power an ATMega88 at 5V from a 9V supply.

    I have heard of a switching supply, but do not know how it works. Is it on the basis of a pwm?

    What is meant by 'low dropout voltage'?
     
  5. SgtWookie

    Expert

    Jul 17, 2007
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    Yes, switching power supplies are based on a form of PWM. There is a capacitor on the output of the supply, and the charging source is turned on when the capacitor reaches a low voltage limit, and turned back off when the capacitor is fully charged.

    By "low dropout voltage" it means that the difference between the input and output voltage is lower than the typical 1.7V. Those regulators use a more efficient scheme to pass current through than traditional linear regulators.
     
  6. Konstabel

    Thread Starter Active Member

    Jan 31, 2008
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    Thanks, that's what I thought.

    Are there any discrete packages available for switching supplies?
     
  7. SgtWookie

    Expert

    Jul 17, 2007
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    By "discrete packages", do you mean individual IC's, or a complete supply?

    "Buck regulators" are step-down switching regulators available in small packages. You feed them an unregulated DC voltage above their output rating, and they do the rest. Very efficient.

    Google is your friend here.
     
  8. scubasteve_911

    Senior Member

    Dec 27, 2007
    1,202
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    I wouldn't recommend a newbie to buck conversion, just stick to linear supplies. Your microprocessor won't be taking much current, so your losses won't be ridiculous. Without careful routing and placement consideration, you might do more harm than good. ie. Routing a switching trace along an analog input or a digital trace, may cause a considerable amount of cross talk.

    Switching converters definitely have their place though, don't get me wrong. I'm running two ~20W switching converters in my current project, plus three linear regs.

    Steve
     
  9. Konstabel

    Thread Starter Active Member

    Jan 31, 2008
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    Scuba,

    If you had to give a rule of thumb as to when to use which type of regulator, what would it be?
     
  10. nanovate

    Distinguished Member

    May 7, 2007
    665
    1
    There are DC-DC bricks and modules but they are expensive.
    I agree with Steve about starting out on a switcher. The layout is critical. For your application assuming 9V in and 5V out and 100mA max average current you should be fine with the 7805 or LM317. If your currents will be higher and you are battery operated then you would enter the region where the complexity of the switcher becomes less of a factor.
     
  11. scubasteve_911

    Senior Member

    Dec 27, 2007
    1,202
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    hey,

    You just have to do a bit of simple math. It depends on three factors, what voltage do you want to regulate to and from?, what currents do you expect to source?, and how important is efficiency?

    The first two questions relate to the power lost by the linear regulator element. If you are going from 40V to 5V, then you have a 35V drop. Any small amount of current will cause a large power dissipation across the regulator. So, if you even need as little as 250mA, then you'll be wasting 8.75W of power as heat! This is a huge amount to get away from nearly any semiconductor package. Not only is it inefficient, it is a thermal challenge to get that much heat away without destroying the IC. Following this same line, going from 8V to 5V with the same amount of current is feasible, since that is a 0.75W drop and can be handled easily.

    If you are powering battery operated equipment or are trying to design for energy efficient criteria, almost any wasted power is too much. In these cases, switching regulators are a must!

    The downside to switching regulators is more noise and more to go wrong when laying it out. I'd steer clear of them for your project.

    Steve
     
  12. beenthere

    Retired Moderator

    Apr 20, 2004
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    If cost enters into the calculation, the $.64 per 7805 can make up for minor shortcomings.
     
  13. kender

    Senior Member

    Jan 17, 2007
    263
    1
    Take a look at this app note, if has a good introduction to switch-mode power supplies (SMPS): http://www.maxim-ic.com/appnotes.cfm/an_pk/4087
     
  14. John Luciani

    AAC Fanatic!

    Apr 3, 2007
    477
    0
    Using the 7805 saves you two resistors. If you think you may want other
    voltages besides 5V (3.3V, 12V, 15V) buying a batch of 317's could save you
    some money and space in your part drawers.

    If you are new to electronics I would not recommend a SMPS. Sometimes
    the layout can be tricky. I believe Linear Technology and TI sell some small
    DC-DC converters in the 1W range. They wil be a lot more money than
    the 7805 or 317.

    I have a couple of pictures of a DC-DC converter board I did at http://www.luciani.org/works-in-progress/works-in-progress-index.html

    (* jcl *)
     
  15. Gadget

    Distinguished Member

    Jan 10, 2006
    613
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    I'd be using the 7805 simply due to component count, simplicity and cost. It's a tried and true little beastie.
    I tend to use the 317 if I need odd voltages, or a variable regulator.
     
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