LM317T Having Issues

Discussion in 'The Projects Forum' started by imruffsdad, Mar 23, 2014.

  1. imruffsdad

    Thread Starter Member

    Jul 18, 2008
    17
    0
    Hello,

    I'm trying to create a regulated 3 volts with an LM317T voltage regulator. In the attached photo I have the circuit and the three different output voltages ranging from 2.1 to 5.6 depending on the input voltage. Input voltage of 6 gets me the closest but shouldn't they all be "regulated" to somewhere in the neighborhood of 3 volts with this setup? Really appreciate any insight as I've been knocking around on and off with this for a week.

    Thanks a bunch,

    Dennis
     
  2. crutschow

    Expert

    Mar 14, 2008
    12,991
    3,227
    It looks like the adjust pin is floating and not connected to the resistors.
     
  3. #12

    Expert

    Nov 30, 2010
    16,257
    6,757
    It also doesn't have any capacitors to keep it from oscillating.
     
  4. imruffsdad

    Thread Starter Member

    Jul 18, 2008
    17
    0
    crutschow, I'm not sure what you mean by "floating." If you mean not connected they are solid into the breadboard, if you mean not connected correctly then I'd appreciate any help in knowing where I've hooked it up incorrectly.

    #12 I've tried it with and without capacitors with similar results. A wide shot of that breadboard would show a similar setup with capacitors and various resistor combinations. At 6 volts in I get close to 3 volts out but it still fluctuates +/- 1-3 volts depending on the input voltage. Decided to start over and strip it down to bare bones; should it vary that wildly without capacitance. The voltage coming in is pretty clean when checked on my multimeter.

    I've tried it with different LM317T's and for different formulas for the output voltage to know avail. If by looking at the photo it looks like it is wired correctly then I'm at a loss. I was hoping someone would look at it and go "Oh, you've got the resistance in the wrong pin", or "the LM317 should be on the other side of the resistors." Have I done something obviously wrong?

    Thanks again,

    Dennis
     
  5. odm4286

    Active Member

    Sep 20, 2009
    155
    5
    [​IMG]

    This might help, as far as values for your R1 and R2 go use this calculator
    http://www.electronics-lab.com/articles/LM317/

    By floating he means your adjustment pin is not grounded(correct me if im wrong crutschow)
    I've used this regulator a bunch of times, if you wire it how it is in this schematic you will be fine.
    Here's the resistor values you need for 3vdc output. R1 = 240Ω R2 = 336Ω
     
    Last edited: Mar 23, 2014
  6. imruffsdad

    Thread Starter Member

    Jul 18, 2008
    17
    0
    Thanks #odm4286. I think I am grounded. My "home made" 308 Ohm goes to ground off of the pin on the left which is the adjustment pin. Is my picture not wired the way the diagram you posted shows? Minus the capacitors of course, which didn't help my variance across the higher supply voltages so I tried to strip it down to the minimum. I used that calculator and 220 and 308 should give me 3 volts but it's giving me the voltage in the pictures 2.16, 4.12 and 5.67. Could the difference between a 6 volt supply giving me 2.16 and a 12 V giving me 5.6 with a difference of almost 3 volts be "normal"?
     
  7. crutschow

    Expert

    Mar 14, 2008
    12,991
    3,227
    I mean the adjustment pin does not seem to be connected to the adjustment resistors. "Floating" means no connection.
     
  8. odm4286

    Active Member

    Sep 20, 2009
    155
    5
    Actually I think you have your pins wrong...your adjustment pin is pin 1 and that's not grounded as far as I can tell.

    Update: I looked over your picture again and yes pin 1 is grounded. I'm not sure what is causing your problem
    [​IMG][​IMG]
     
    Last edited: Mar 23, 2014
  9. k7elp60

    Senior Member

    Nov 4, 2008
    478
    69
    The 220Ω resistor needs to be changed to 120Ω, put a resistor on the output that will be a 10mA load. The 120Ω resistor insures the minimum load is 10mA. The 240Ω is specified for the LM117 not the LM317.
     
  10. crutschow

    Expert

    Mar 14, 2008
    12,991
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    When I look at the picture it appears that the column of sockets with pin 1 is one column to the right of where the resistors are connected. Am I looking at it wrong? :confused:
     
  11. odm4286

    Active Member

    Sep 20, 2009
    155
    5
    I'm new but doesn't that contradict what this spec sheet says?
    http://www.fairchildsemi.com/ds/LM/LM317.pdf

    check page 5
     
  12. RichardO

    Well-Known Member

    May 4, 2013
    1,230
    382
    The spec. sheet is confusing. It contradicts itself. It states that the maximum Minimum load current is 12 mA. With the reference at 1.25 volts, 240 ohms would be about half that value. This assumes that the intent of their example is to insure that the voltage will be correct with no additional load as shown.
     
  13. imruffsdad

    Thread Starter Member

    Jul 18, 2008
    17
    0
    I swear I tried this with capacitors as outlined in the spec and it wasn't working. Today I started over with capacitance and whatever voltage I wanted worked based on the resistance formula calculation. Tolerance was +/- .4 points of what the calculation said. Thanks to everyone for their input!

    I likely need to ask this as a separate question but I'll give it a shot here. I've built a 6V and a 3V LM317T circuit on the same breadboard, which was the point all along. I was going to add a 9V as a 3rd LM317T on the same board. I'm powering all of this with the 12V wall wart. With just the 6v and the 3v under load the 6v is fine but the 3 volt LM317T is kicking up some heat. To hot to touch heat. I added a heat sink and that seems to have made it merely warm. I also dropped the source voltage to 9volt and that dropped the heat enough that I didn't need the heat sink on the 3V.
    - The LM317T is rated for this so am I going to be ok with the heat sink and 12V input?
    - How about when I add the 9V in to the mix?
    - The ultimate goal is to drive a bunch of electronic kits that I've built that require varying voltages off of one wall wart; is this a good approach for that purpose?

    Thanks everyone!
     
  14. #12

    Expert

    Nov 30, 2010
    16,257
    6,757
    The way I did this was to have the 9V output supply the 6 volt regulator which supplies the 3 volt regulator. That way they all share the heat.
     
  15. imruffsdad

    Thread Starter Member

    Jul 18, 2008
    17
    0
    I changed it to that method and it helped with the heat tremendously.

    Thanks to everyone for their help, I've got this resolved now.

    Dennis
     
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