Connect with us

LM317 Power Supply: Two quick questions

Discussion in 'Power Electronics' started by TheLaw, Nov 24, 2010.

Scroll to continue with content
  1. TheLaw

    TheLaw

    119
    0
    Sep 27, 2010
    Hi guys,

    ---BACKGROUND---

    Well as you may or may not know, I am pretty new to electronics. I have some equipment but I still lack other important aspects of a good electronics lab. Well some of my previous experiments have been disasters. Maybe I was trying for something too extravagant. But I thought I'd try something more useful that will actually help me and won't kill my wallet or time in the process.

    [​IMG]

    This is currently what my setup looks like. Multimeter (Amprobe AM-220), soldering iron (Weller WLC-100), solder wire (Kester 44: 63/37), and a third hand.

    ---QUESTION---

    So I want to build a small low current variable voltage power supply for some projects I have a head of me.

    I am essentially following the schematic from the LM317 datasheet but I happen to find a nice picture filled tutorial so I said "why not". (http://www.ladyada.net/library/equipt/diypsupp.html)

    My question is to do with the source voltage. I would like to use a 15V unregulated power adapter instead of a 9V battery.

    1.) I would like to get a full spectrum of voltages. Does the value of the potentiometer affect the min and max output voltages?

    2.) In regards to the LED resistor, is 1K still appropriate even with an increased supply voltage?

    I am thinking about putting a fuse in between the transformer and the power supply circuit. I've bumped up the caps to about 100uF each, hopefully for better filtering. Any other things that can get me even cleaner power?


    Notice I am still not grasping a couple of concepts. I am trying but I am so eager to get working.

    Thanks.
     
    Last edited: Nov 24, 2010
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,497
    2,839
    Jan 21, 2010
    What current can your transformer supply?

    What current do you require for your load?

    Your input capacitance (at a rough calculation) should be at least 1000uF for each amp you wish to draw from it, but higher is better. Don't go way overboard as the inrush current will start to be a factor in heating your transformer.

    Keep the 0.uF capacitor on the input too.

    The circuit is fine, and if you check the specs for the LM317 (which was the first thing you did, right?) you will see that the max input voltage is around 35 volts (from memory).

    In this circuit the same current passes through R1 and R2. There will be 1.25V across R1, so the range of resistance required for R2 can be calculated with a couple of invocations of ohms law (or you can cheat -- I'll cover that later if you get the ohms law stuff right).

    Min voltage is always 1.25V. Max should be at least 2 volts (I think) less than the inpu voltage -- but remember the input voltage sags under load.

    1K resistor for a LED is fine. As a rule of thumb, it gives you 1mA per volt of supply voltage greater than Vf of the LED.

    A fuse is a good thing. Start with something perhaps two to three times the max current you're going to draw, but if your input capacitors are huge, you may require a higher value, or possibly a slow blow fuse (I rather doubt it though).

    You should also calculate the max power the LM317 will dissipate and determine if a heatsink is required. Max power dissipated is at min output voltage (or short) and max current.
     
  3. TheLaw

    TheLaw

    119
    0
    Sep 27, 2010
    Steve, I really wish the whole world was like you. Not to sound corny, but in all honesty. I appreciate it.

    My transformer is actually 12V. I'm not sure where I got the number 15 from...Anyway. It can supply up to 2A (or so it says).

    Right now, I have no load to speak of, but under most circumstances probably under 500mA.

    Input capacitance is referring to the input capacitors? As in the C2? Or C3 cap? I'm having a hard time reading this schematic. Is Ladyada's design, she has C2 and C3 as the same value (10uF), while in the datasheet, C2 = 10 uF while C3 = 1uF. So apparently the value isn't detrimental to the project? Probably the thing confusing me the most. I seriously have to start watching those 20 year old videos on how to read schematics....

    So if in theory I wanted to draw 2A, each cap should be ~2000uF? Hmm...that seems like a lot.

    I suppose the reason only 10uf and 1uF are used in the example scematic is because the current output from a battery isn't very much, but still...

    I am on the fence about input protection. PTC, MOV, or Fuse...Well, that's for me to decide I guess. I also have to do my research on PTCsand MOVs

    I also planned on heatsinking it regardless. I'll try to calculate what kind of dissipation I will actually need. But for now, I guess I'll focus on more important things.

    Thanks a lot everyone.
     
    Last edited: Nov 24, 2010
  4. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,497
    2,839
    Jan 21, 2010
    The peak voltage from your transformer is 1.4 times 12 (because 12 volts is the RMS value, the peak is that times the square root of 2 (for a sine wave).

    You have various issues to do with VA ratings and the fact that you won't be drawing power evenly throughout the AC cycle that means you should limit the current to around 1A DC.

    Also the LM317 isn't rated for 2A :)

    The 1000uF capacitor is not even really shown on your circuit diagram. It is part of the rectifier/filter. Check this. The circuit you have is the regulator, you need all the other bits.

    Does that transformer (I resume it's a plugpack -- or is it a bare transformer?) have DC output? If so then it has the rectifier in the same box (not unusual). Frequently though they have no, or minimal filtering.

    The input and output capacitors on the regulator are needed for reasons other than filtering (although they perform a little of that too). In general you should be guided by the specs. Often they will give minimum values for capacitance, so an increase is not an issue as long as it's not way over the top. The LM317 is quite stable, so these are not really critical.

    Don't plan on doing much more than testing and drawing very low current until you have fitted a heatsink. The LM317 will do an overtemp shutdown, but having it trigger is a sign of bad design (or failure), not a feature you ever want to see in a routine situation.
     
  5. TheLaw

    TheLaw

    119
    0
    Sep 27, 2010
    Alright,

    Yes it is a plugpack (wall adapter). I bought it off Jameco and it was classified as an unregulated transformer, which I assume is just a bare transformer with outputs...hmm...

    So I guess I should put in something like a bridge rectifier in. I am trying to look for some good guidelines for building a power supply. Does filtering simply just mean putting some capacitors inline with the outputs?

    Alright, I'll leave you alone. I think I understand what I have to do. I'll do my research and try to come up with a new schematic/design.

    Thanks for answering the two original questions and more.
     
  6. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,497
    2,839
    Jan 21, 2010
    You seem to be on the right track.

    BTW, your equipment seems perfectly acceptable for this sort of work.

    I tend to lay out a newspaper to work on so I don't burn the desk :) Cleaning up is then as easy as wrapping up the top couple of sheets and binning them. If you're working with tin/lead solder it helps keep the lead waste (bits of solder and stuff) fairly well contained.
     
  7. TheLaw

    TheLaw

    119
    0
    Sep 27, 2010
    Thanks as always.
     
  8. TheLaw

    TheLaw

    119
    0
    Sep 27, 2010
    Okay, one last question regarding power supplies. I mean quick this time. I understand that ceramic caps should be used around ICs. You don't have to split this into a new thread because I think it's pretty much on topic.

    For what reason is this true? On what side of the IC do I place them. Input or Output? Both? And does capacitance value make a difference? Like 0.1uF vs 1uF?

    Should I use polypropolene caps instead? Or solid polymer?

    Thanks.
     
    Last edited: Dec 7, 2010
  9. davenn

    davenn Moderator

    13,866
    1,958
    Sep 5, 2009
    both sides of the LM317 or other voltage regulator is the commonly done thing
    the different vlaues are for bupadding different freq's of ripple/other noise that may appear on the DC rails.
    The 1uF 's can be tantalum or electrolytic with a voltage rating double that of the DC voltage across them is the rule of thumb.

    The 0.1uF can be almost anything, eg. .... polyprop, disc ceramic, polymer

    cheers
    Dave
     
  10. TheLaw

    TheLaw

    119
    0
    Sep 27, 2010

    Thanks but should I use like a 1uF cermaic or a 0.1uF ceramic. Thanks for reply.
     
  11. davenn

    davenn Moderator

    13,866
    1,958
    Sep 5, 2009
    BOTH .... for the reasons I said in my previous post :)

    cheers
    Dave
     
  12. davelectronic

    davelectronic

    1,087
    12
    Dec 13, 2010
    Hi there you could use the LM338T or LM338K for a bit more current, the LM338T has the same pin out as the LM317T and is also a TO220 package if you draw heavy loads dont need to use large heat sinks keeps H sinks small and use a thermal switch and small axial fan all this fits in a small form case and makes a neat supply if using a plug pack yo could use chassis dc power connections in the case for quick connection disconnection.
     
Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Electronics Point Logo
Continue to site
Quote of the day

-