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LM317 tripping out at 0.33 Amps Why

Discussion in 'Power Electronics' started by Eddie478, Jan 24, 2020.

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  1. Eddie478

    Eddie478

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    Jan 24, 2020
    Hi, have just built a small variable power supply using an lm317BTG, Its keeps tripping the thermal overload. just a basic circuit as per many on google, supply is a laptop supply at 24v @2.5a
    setting regulation at 11.5 to 13.5 volts.
    Its supplying a curtain, auto closer motor at 12v need adjustable for speed control all this works ok
    but the lm317 is over heating yet the motor is only drawing .33 amps
    r1 is a 120ohm 1/2 w resistor, and vr1 a 10k pot, c2 on output a 1uf cap, c1 on input a 100nf cap
    and a protection diode 1n4001 across the input and output on the reg,
    I cannot understand why the reg is getting so hot with this small amperage draw?, what am I missing?,
    Tested the motor on a variable bench supply built from a computer power supply set at 12v on variable supply and it will run all day no issues, and its basically the same circuit.
     
  2. Harald Kapp

    Harald Kapp Moderator Moderator

    9,544
    1,970
    Nov 17, 2011
    I might think it were very helpful to see the schematic you are referring to. Otherwise, what the heck is R1, VR1 etc? You can upload an image by simply dropping it into the edit box (note: max. ~ 100kB size).

    Chances are that the regulator overheats. An input voltage of 24 V and an output voltage of 12 V (a mean value from your numbers) results in a voltage drop of 12 V across the rgulator. 12 V × 0.3 A = 3.6 W. The device in a TO220 case (whic I assume you are using) has a thermal resistance Rth(junction-ambient) of 65 °C/W. At 3.6 W this means the junction temperature is at 3.6 × 65 °C = 234 °C above ambient temperature. No wonder it goes into current limit mode.
    You will have to add a heatsink.

    Or use a switch mode regulator module (cheap on the internet) which will drastically reduce your losses.
     
    Tha fios agaibh likes this.
  3. Eddie478

    Eddie478

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    Jan 24, 2020
    thanks Harad, I added heatsink and all is well, testing right now, been running for 20 minutes tested temp at 67c is that ok?
     
  4. Eddie478

    Eddie478

    4
    0
    Jan 24, 2020
    ps ambient temp is 32c
     
  5. Harald Kapp

    Harald Kapp Moderator Moderator

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    1,970
    Nov 17, 2011
    67 °C ? Measured where?
     
  6. Eddie478

    Eddie478

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    0
    Jan 24, 2020
    on the heatsink where it is screwed to the reg
    been running for 1 1/2 hours no problems now
    thanks for the help
     
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