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LM317L Voltage drop with load

Discussion in 'General Electronics Discussion' started by jgauthier, Apr 21, 2013.

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  1. jgauthier

    jgauthier

    33
    1
    Mar 22, 2013
    Hey Everyone,

    I am struggling with an LM317L (TO-92) which seems to be having a significant voltage drop.
    Let me set the stage:

    12V coming into the circuit, that is then going through a 5V regulator. I have a steady 5V.
    After that, the rest of the circuit is turned on with a mosfet.

    At this point I have about 4.75 volts. Then into the LM317L, where I need 3.3V. I do have 3.3V (or 3.4V), until I apply a load, even a simple LED. Then voltage drops to an unusable 2.8V or lower. I am not sure why.

    I have attached the circuit diagram of the LM317L part. I appreciate any pointers.

    Thank you.
     

    Attached Files:

  2. Harald Kapp

    Harald Kapp Moderator Moderator

    11,532
    2,656
    Nov 17, 2011
    What is your load current? Figure 4 of the datasheet will tell you that at 500mA and 25°C you need an input voltage that is at least 2V higher than the output voltage. In your case that would be 5.3V. But you only have 4.75V. The regulator will not be able to operate in a stable manner.
    A low drop regulator, e.g. the LT1086 with only 1V votage drop will be better suited.
     
  3. jgauthier

    jgauthier

    33
    1
    Mar 22, 2013
    Thanks! I see that even with a 20mA load the dropout voltage is 1.5V, so yeah. I will try another part!
     
  4. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

    4,960
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    May 8, 2012
    Along with what Harald said you should be aware that your caps don't appear to be grounded. In your schematic, that is. Your Cad program uses dots where connections are made. I know this because I see them in your schematic but both negative terminals of both caps appear to just rout over the ground node with no dot to indicate a connection.

    Chris.
     
  5. jgauthier

    jgauthier

    33
    1
    Mar 22, 2013
    Wow, nice catch! I just threw that together to demonstrate what I was working on, and the caps are connected to ground on the physical circuit. :)
     
  6. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

    4,960
    652
    May 8, 2012
    Yeah, I figured that it was just the schematic. ;)

    Chris.
     
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