LM317 help

Discussion in 'The Projects Forum' started by elechobby, May 31, 2009.

  1. elechobby

    Thread Starter New Member

    May 31, 2009
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    0
    Hello, thank you for taking the time to have a look at this post.
    I am having trouble getting the LM317 to work. I have arranged the components in the same configuration as the image below. The input voltage is 27v DC and I would like the output voltage to be a regulated 24v. R1 has a value of 270ohm and R2 is a pot that has been adjusted so the output voltage of the regulator is a measured 24V. The problem: When I connect a load across the output and ground the output voltage drops very low. For example, I connected a 56ohm resistor from the output to ground. I would expect to measure the full 24volts across this resistor but instead I measured 2.8V. I’m not understanding something, any help you could give me will be greatly appreciated.

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  2. SgtWookie

    Expert

    Jul 17, 2007
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    1,728
    What does the input to the regulator measure when the load resistor is connected?

    I'm guessing that it'll measure about 3v to 3.1v. If so, your 27v supply is only capable of supplying around 54mA.

    Do you have capacitors on both the inputs and outputs of the regulator?
     
  3. elechobby

    Thread Starter New Member

    May 31, 2009
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    I do have the input and output caps in place, I measured the input voltage with the load connected and It measures 27v. Thanks for your quick reply.
     
  4. SgtWookie

    Expert

    Jul 17, 2007
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    OK.
    Is the LM317 you're using in a TO220 package? (ie: an LM317T, not an LM317L in a TO92 package).

    Are you certain that you have good connections to the input and output terminals?

    Does the regulator get warm (hot) with the load connected?

    Do you have another LM317T regulator to substitute? (you may have inadvertently shorted the output previously, causing damage to it.)

    When there is no load attached, what voltage do you measure across the 270 Ohm resistor?

    When there is no load attached, what voltage do you measure between the ADJ and OUT terminals? (it should be the same voltage as the last question)
     
  5. elechobby

    Thread Starter New Member

    May 31, 2009
    3
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    Thank you for your help, the problem was on the input like you suspected! My breadboard has two adjacent holes shorted and not the ones that are supposed to be shorted J. Time to get a new one I guess.
     
  6. SgtWookie

    Expert

    Jul 17, 2007
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    Glad you found the problem :)

    The breadboard is probably still usable - just mark the damaged area with a Sharpie, and move your parts to a different location.
     
  7. Mike Mandaville

    Active Member

    May 27, 2009
    81
    1
    Hello Elechobby, and welcome to the forum. I use breadboards also, which I purchase at Radio Shack. When I have a problem with a breadboard, I peel the paper off of the back, and perform surgery. The metal parts come out, and can be rearranged. I don't recall ever having a short, although it seems to me that a piece of electrical tape where metal touches metal might solve the problem. The main problem which I have run into is an open along one of those long power rails which run up and down the left and right sides. When this happens, it's usually because I spilled some tea on the board earlier. Of course, when I smoke a component, the plastic melts, but the metal is still good.
     
  8. Wendy

    Moderator

    Mar 24, 2008
    20,766
    2,536
    While I haven't done this I have seen them cratered. Always a fun experience.

    So when your chippie welds itself to the protoboard, you have a choice. Leave it in place, or pry it off? :D
     
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