3.3V regulated circuit

Discussion in 'General Electronics Chat' started by impala454, Apr 4, 2012.

  1. impala454

    Thread Starter New Member

    Apr 4, 2012
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    Hello, I've got a small project using XBees. I've already gotten the meat of the project to work, the software is done, everything works great. So now my task is to take the arduino (which is more or less simply being used as a nice 3.3V power supply) out of the loop and just use a wall wart to directly power the XBee.

    What I have is a 9VDC/650mA wall wart, I have the barrel jack, a breadboard, and a 3.3V regulator (LD1117). My admittedly simplistic mind figured I could just plug everything in as per the datasheets and be good to go. Well a burned finger and dead XBee module (as well as some burned pride :p) later, I'm left figuring out what I need to do to supply the danged XBee with 3.3V from the wall (no batteries in this project). I have some 5V regulators as well (NCP7805) so if I need to use some combo of them, or throw in some caps/resisters, help me to understand this seemingly basic circuit!
     
  2. crutschow

    Expert

    Mar 14, 2008
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    The question is: Why did the LD1117 fail?

    You mentioned a burned finger. Was that from touching the LD1117? Was it on a heat sink? Did you have the recommended filter caps connected close to the LD1117's input and output pins?

    What current do you need for the 3.3V?
     
  3. impala454

    Thread Starter New Member

    Apr 4, 2012
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    I burned my finger on the LD1117, as I was checking to see if it was getting warm and it was blazing hot (didn't realize it could get that hot). I did not have any filter caps hooked up, I suppose that's the part I need to learn about. I don't think the LD1117 failed, as I tested it later with a known good 5V circuit and it output 3.3V as expected.

    The current needed for the 3.3V is ~50mA.
     
  4. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    When a linear reg cooks like that, here are some likely reasons:

    1) WIRED WRONG. Make sure the pinout is correct and the leads are connected to the right pins.

    2) TOO MUCH LOAD CURRENT. "The current needed for the 3.3V is ~50mA. " That can't make it cook, so there is current going somewhere else. Check circuit connections.


    3) " I did not have any filter caps hooked up, I suppose that's the part I need to learn about." If it is oscillating, it can cause problems. Should not fail, but it might.
     
  5. Audioguru

    New Member

    Dec 20, 2007
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    Your "9V" wall-wart might be 12V with a load of only 50mA. Then your 3.3V regulator heated with (12V - 3.3V) x 50mA= 0.44W.
     
  6. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    What kind of package is the 1117 IC in?
     
  7. impala454

    Thread Starter New Member

    Apr 4, 2012
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    Thanks for the replies,

    bountyhunter- I don't believe it was wired wrong, as the output voltage was indeed correct. The regulator is the "T220" packaging, sits upright and has the small heatsink with a hole through it.

    Audioguru- Excuse my ignorance, but is half a watt enough to burn the crap out of my finger?

    I guess I'd like to know if there's a good "standard" design for hooking up a 3.3V load to the wall. Filter caps were mentioned, how do I know what values for those and when it's appropriate to use them?
     
  8. Audioguru

    New Member

    Dec 20, 2007
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    The original poster does not say if he used a tiny surface-mount package or a normal through-hole TO-220 package. The tiny guy will overheat.
     
  9. BSomer

    Member

    Dec 28, 2011
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    RTDS!


    One of my instructors often used that. Meaning "read the data sheet". The data sheet says that a 10uF cap is needed for stability. On page 2, figure 4, it shows the standard wiring of the regulator which include a 100nF cap on the input and a 10uF cap on the output. Could the lack of either have caused your problem? I don't know for certain.
     
  10. impala454

    Thread Starter New Member

    Apr 4, 2012
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    Ahh ok well I guess I'm still new to reading data sheets, I'll give that a shot and see how it goes. Thanks!
     
  11. crutschow

    Expert

    Mar 14, 2008
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    It's truly amazing how many newbies try to build electronic circuits by blithely connecting the parts together without ever reading the data sheets for those parts. :rolleyes:
     
  12. impala454

    Thread Starter New Member

    Apr 4, 2012
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    Thanks "Zapper", what a constructive, helpful comment.

    I did look at the data sheets for the pin outs, I just didn't realize there would be some kind of suggested caps in there too. I came here to learn.
     
  13. BSomer

    Member

    Dec 28, 2011
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    I used to do the same thing until I realized what information was actually in there. You shouldn't stop at the data sheets either. Sometimes you can find app notes that deal with the device you are working with. App notes are little pieces of information wrote up by engineers that work for the company that made the IC. These engineers build a circuit based around a certain IC then document the circuit. Some are better than others at explaining the circuitry. The data sheet will sometimes reference these app notes.
     
  14. impala454

    Thread Starter New Member

    Apr 4, 2012
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    Thanks I'll have to look around for those as well
     
  15. crutschow

    Expert

    Mar 14, 2008
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    Sorry if I offended you. :p Don't take it personal. I was just venting a little about the large number of questions on this forum that are the result of application problems that could have been avoided by information in the data sheets of the components being used. It seems to be a common problem with many inexperienced designers who apparently feel that data sheets are too complicated to understand and so avoid reading them.

    Don't suppose there's any good solution to that. Perhaps we need a sticky on reading data sheets. :rolleyes:
     
  16. Barryg41

    New Member

    Jan 24, 2012
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    Being a newbie myself. Breaking down data sheets would be great!

    But they are prolly like MSDS documents, what data is put in there and how it is presented is dependent on the manufactor.
     
  17. impala454

    Thread Starter New Member

    Apr 4, 2012
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    Zapper, no worries I did not take offense. I just found your comment rather condescending. I did learn from this thread that I need to pay more attention to the data sheets. The data sheet for this voltage regulator is 18 pages long, and honestly I just thought all that mattered was voltage in voltage out.

    You obviously know a lot more than me about this subject, which is why I'm here. Just keep that in mind when you feel the need to vent. A lot of times, something that's obvious to you (such as reading the entire 18 data sheet for a part, or finding the app notes), may not be so obvious to a beginner.

    I'll put all this together again (after reading the entire datasheet this time :) ) and report back!
     
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