Help with LM317 power supply

Discussion in 'The Projects Forum' started by Ranthalion, Jan 8, 2012.

  1. Ranthalion

    Thread Starter New Member

    Oct 7, 2011
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    Hello,

    I'm just getting started in electronics and I'm trying to build a variable voltage power supply to use while prototyping and testing circuits. It seemed to be pretty straight forward - in fact the datasheet for the LM317 shows the circuit in the typical applications section. So I built the circuit and it works for a little while, but then it seems to stop working. It begins working again when I replace the chip, so I must be damaging it in some way.

    I got a 24 volt transformer and rectified it. Output is around 34v DC. C1 is .1uF ceramic, R1 is 330 ohm, R2 is a 6K pot, and C2 is 1000 uF electrolytic.

    Any ideas about what could be going wrong? I've even added the protection diodes as shown in figure 3 on page 10 of the data sheet, but I get the same results. I haven't really placed any significant load on the supply. So far, I've only connected LEDs with resistors to test it... The 317 is mounted to an aluminum block heatsink.

    -Ranthalion
     
  2. hgmjr

    Moderator

    Jan 28, 2005
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    The LM317 has an internal overheat shutdown mode. Is it possible that you are overloading the device?

    hgmjr
     
  3. Ranthalion

    Thread Starter New Member

    Oct 7, 2011
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    I've thought about that, but the heatsink is fairly large - about 2 cm X 3 cm X 6 cm and seems to stay quite cool.

    I also haven't pulled much current. Generally I've tried using an LED with a 1K resistor...
     
  4. hgmjr

    Moderator

    Jan 28, 2005
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    Keep in mind that the power being dissipated inside the LM317 is calculated by the difference between the input and output voltage times the current being delivered to the load. You need to put a fixed resistor in series with the pot so that you can avoid adjusting the output voltage below a predetermiend value.

    For example, if you adjust the output to 5V and the output current to the load is 100 milliamps. That gives you 31 volts times 0.1 amps. The LM317 is dissipating 3.1 watts.

    Keep in mind that the heatsink has a certain thermal inertia. If the load is applied too quick, there is a chance that the LM317's junction temp may exceed the thermal shutdown threshold before the heatsink can dissipate the heat.

    hgmjr
     
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  5. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    LM317 ??

    Gimme the complete no. first

    what comes after 7 ?
     
  6. Ranthalion

    Thread Starter New Member

    Oct 7, 2011
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    I'm using LM317T.
    You may be onto something with the thermal inertia since I was starting at 1.5 volts and slowly increasing the voltage in the tests. I'd like to have 1.5 - 24 volt range. Am I using this chip inappropriately? Would you recommend a different design, chip, or approach?
     
  7. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    Lower R1 to 240 Ohms as descibed in the datasheet.
    This will give a better stability.

    Bertus
     
  8. Adjuster

    Well-Known Member

    Dec 26, 2010
    2,147
    300
    If a large output capacitor like 1000μF is used, there might be problems with this discharging back through the IC when powering down. Diodes are sometimes added to regulator circuits to protect against this.

    This data-sheet for LM317 says that the circuit is relatively robust against this, but a protection diode circuit is explained on pages 9 & 10. http://www.ti.com/lit/ds/symlink/lm317-n.pdf
     
  9. Ranthalion

    Thread Starter New Member

    Oct 7, 2011
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    Thanks for the ideas. In fact, I've already added the diodes in case that was part of the problem. At least it doesn't sound like something incredibly obvious. I know I went a little over the recommended value for R1, but 330Ω didn't seem too far off and it paired very well with the pots I have to give the voltage range I was looking for. Combined with any circuit, it really should hit the 10 mA requirement.

    This was my first time soldering components to a proto board, and the layout really shows it... I think I'll try a completely new board and drop R1 down to about 240 Ω- including the protection diodes from the start. Maybe that will work better.

    All this work for a simple variable voltage bench supply... I'd really like to move on to some more interesting projects, but it seems like I should master this LM317T first...
     
  10. hgmjr

    Moderator

    Jan 28, 2005
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    The LM317T is a very simple part to use. There has to be some very simple explanation for why it is not working properly.

    hgmjr
     
  11. AlexR

    Well-Known Member

    Jan 16, 2008
    735
    54
    You need the large capacitor in the input side, not on the output. Shift the 1000uF to the LM317 input and add a 100uF plus a .1uF to the output side.
    With only a .1uf to filter the rectified DC your input drops below the regulator drop-out voltage on each 1/2 cycle of the rectified DC.
     
  12. Audioguru

    New Member

    Dec 20, 2007
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    No.
    240 ohms is used on nearly every schematic in the datasheet for the more expensive LM117. The cheaper LM317 needs 120 ohms and a pot that is half the value.

    The datasheet explains the problem of a huge 1000uF output capacitor and recommends a diode to discharge it to prevent if from blowing up the LM317.
    An output capacitor less than 25uF does not need a discharge diode.

    This is a voltage regulator. Why use a huge output capacitor? The datasheet shows that a 1uF output capacitor filters out a load current transient very well.

    EDIT: The rectifier filter capacitor is only 0.1uF? Then it doesn't filter so the input voltage to the regulator IC is too low most of the time.
     
  13. Adjuster

    Well-Known Member

    Dec 26, 2010
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    Good grief! I had assumed that the 0.1uF was just a little local decoupler, as is sometimes advisable, especially if the main reservoir is not mounted close by.

    If there is really no large reservoir following the rectifier you are bound to have problems, as the input ripple will be enormous. A few thousand microfarads per amp of load current is typical, preferably calculate how much you need to keep the voltage above the output voltage plus the regulator dropout.
     
  14. Wendy

    Moderator

    Mar 24, 2008
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    I had quite a bit of problem with one a long while back. The answer turned out to be simple, a straight forward wiring error. It is why all my schematics using the LM317 include a pinout.

    [​IMG]

    The other thing is loading, what is the input voltage when the chip is shut down?

    Project: Simple Power Supply
     
  15. bountyhunter

    Well-Known Member

    Sep 7, 2009
    2,498
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    Make sure the voltage peaks are not too high. The 317 is only a 40V part. At light loads, the transformer secondary voltage is a lot higher than rated value.
     
  16. sheldons

    Active Member

    Oct 26, 2011
    616
    101
    1000uf in place of c1,change your other parts as advised in the data sheet-silly question you are using a bridge rectifier(or 4 diodes)fed from your transformer to the ip of your ic?that particular reg is a very easy to use device
     
  17. Ranthalion

    Thread Starter New Member

    Oct 7, 2011
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    0
    I'm using 4 diodes, 1N4004.

    Would the small capacitor after the rectified output damage the chip? The chip no longer functions, even in the simple breadboard circuits with a filtered 9v DC supply. I think that's the part I'm having the most trouble understanding. What's causing the chip to be damaged?
     
  18. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    If you cannot show what you are doing. We cannot say for sure.
     
  19. Audioguru

    New Member

    Dec 20, 2007
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    A voltage regulator is supposed to have a fairly steady DC input. You did not have a high value main filter capacitor so the input to your regulator continuously pulsed from 0V to the peak over and over which is not normal. Your huge output capacitor also was not normal, especially when there is no input filter capacitor.
     
  20. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    This entire thread is a perfect example of why a complete schematic is needed in the first post.
     
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