Wiring a LM317 Voltage Regulator

Discussion in 'The Projects Forum' started by Seaker, Mar 2, 2013.

  1. Seaker

    Thread Starter Member

    Feb 3, 2013
    I am confused as to wiring a project that I am working on.
    The schematic and instructions are at the following site:


    The issue is concerning the LM317 voltage regulator in the diagram, it shows from left to right Pin #1 Input, Pin#2 Adj. and Pin #3 Output.
    However when I look at a LM317 specification sheet it shows that from left to right Pin #1 Adjustment, Pin #2 Output and Pin #3 Input.

    I have to assume that the specification sheet is right which means that the (+) rail must bypass the LM317 to connect to the input then the middle pin and this will complete the (+) power rail.

    I hope this makes sense.

    One last thing, at the bottom of the description it says that I could tie in two IN4001 Diodes and this would then allow the DC voltage controller to be set to zero volts, what are your thoughts?
  2. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
    No it doesn't. It does NOT show pin numbers at all. Then if you actually scroll down and look on the right you can see another diagram showing the proper pins of the LM317 and they match the datasheet.

    Just like reading english (read left to right) its "common" in a schematic to show the input of a circuit on the left and everything flows to the right towards the output.
    Also + at the top and - at the bottom.. typically.
  3. crutschow


    Mar 14, 2008
    The Fairchild spec sheet shows the same pin-out for the TO-220 case as your reference. Where do you see a picture showing Input, Adj., Output.

    The pin-out may be different for other packages.

    Don't see the circuit with the diodes. What data sheet are you looking at?
  4. Seaker

    Thread Starter Member

    Feb 3, 2013
    As you may have guessed I am a Newbie which I hate using as an excuse!

    I will take another run at this in trying to explain where my confusion lies.

    If I was to attach the LM317T to a heat sink then according to the documentation the pin configuration of that chip and looking from the front and from left to right is:
    (Pin #1 Adjustment, Pin #2 Output and Pin #3 Input)

    When I look at he the wiring diagram/schematic (at the top of the page)
    it shows the LM317T's pin configuration as (
    Pin #1 Input, Pin#2 Adj. and Pin #3 Output).

    This makes the drawing of this diagram simple to understand however when using a through hole PCB and inserting the LM317T with heat sink and facing from the front the (+) rail has to in my mind go around the voltage regulator with a trace to pick up the input, then one connects the 1N400X diode to the output as per the diagram and this then becomes the extension of the (+) rail.

    I am not trying to be difficult in understanding how best to do this I just don't want to get it wrong. It would have been simpler to just buy a kit but one learns less this way!

    Thanks again for the quick response!

  5. tracecom

    AAC Fanatic!

    Apr 16, 2010
    A schematic symbol is different from a pictograph. Many wiring diagrams today are drawn as pictographs, i.e., the components in a pictograph are drawn as the component actually looks. However, schematics don't use pictures of the components; they use symbols to represent the components, and those symbols don't look like the components. However, the schematic symbol and the pictograph of a component always have the same number of connection points, but the connection point (pins) are not necessarily shown in the same order on the symbol as on the pictograph.

    Pictographs make wiring simpler for simple circuits, but they become confusing on larger circuits. And, as you have seen, mixing schematic symbols and pictographs adds to the confusion.

    But the bottom line in regard to the diagram you posted is that there is no conflict between the symbol shown in the schematic for the LM317 and the pictograph of the LM317. Just go by the names of the pins and forget about numbering the pins.
  6. Conductanc3

    New Member

    Feb 28, 2013
    I had similar issues with the pin layout on these style of regulators vs. how they are depicted as I learned to use them. You will find that the layout varies from one part number of regulator to the next as well (they are not standardized at all). So what I do is keep the label from the packaging when I order voltage regulators, and draw one of the regulators on the label, marking which pin is Input, Adjust, and Output. I keep the label in the drawer with the regulators. That way when I pick one from my parts cabinet I can instantly see its pin layout.

    Unfortunately, just as with plumbing, some specs in electronics defy logic. :D
  7. #12


    Nov 30, 2010
    I just happen to have a few examples.
    edit: Some of the plastic drawers in my shop have the pinout drawn on the label. It saves me having to look up the datasheet for every transistor I plug into a protoboard for a quick experiment.
  8. kubeek


    Sep 20, 2005
    The schematic nowhere says that input is pin #1, it just shows which wires go to pin with the function "input". You need to refer to the datasheet to find out which pin in the particular package you're using is input.
  9. Seaker

    Thread Starter Member

    Feb 3, 2013
    I want to thank every one for the information and feedback, it was all very much appreciated. Thank you #12 for posting the PinOuts.PDF a very interesting concept
    and I will incorporate that into my parts storage boxes.

    A special thanks to those who understood where I was at and tried to assure me that all would be fine.