Limiting input voltage to an LM317?

Discussion in 'General Electronics Chat' started by spinnaker, Feb 21, 2010.

  1. spinnaker

    Thread Starter AAC Fanatic!

    Oct 29, 2009
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    I am building a bench power supply (may first project in about 25 years :)).

    The supply will be variable from 1.2V to 30V (or so). I am using an LM317 because it is so easy to use. It can provide 1 amp.


    My bridge rectifier is putting out around 32 VDC.

    The max input on the LM317 is 40V. Is this enough of a buffer? What about any surges?

    If I wanted to clamp the input to the LM317 at 32 V. How would I do it? Would I use a zener? If so, how do I calculate the wattage?
     
  2. mik3

    Senior Member

    Feb 4, 2008
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    The output of the rectifier is 32V DC when it is unloaded. When a load is applied its output voltage will drop (ripple voltage) depending on the current drawn by the load and the filter capacitor. The bigger the filter capacitor the less the ripple voltage. However, large capacitor will cause a current surge when the power supply is turned on because it will be uncharged. This can damage your rectifier diodes. Thus, you have to take care of it. Because of this voltage ripple at the output of the rectifier, the output voltage of the regulator will be limited to about 25V DC because it requires at least 2V on its input above the output voltage to work properly. Also, it required at least 10mA of output current to keep the regulation as specified. What is more, a heatsink will be required to dissipate heat fast, especially at low output voltages, because of the great amount of power will be dissipated in the regulator.
    The zener diode should be able to conduct roughly 1.5 times the rated current of the transformers secondary. It's purpose is only to clamp brief voltage surges and not for continous conduction. If a continous overvoltage exists the fuses on the transformer will blow. Another, method for overvoltage protection is the crowbar method.
     
  3. Audioguru

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    Dec 20, 2007
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    When it is not hot, the LM317 provides a guaranteed max output current of 1.5A only when the voltage from its input to its output is 15V or less. When the voltage differential is 40V then the guaranteed max output current might be only 150mA but is typically 400mA.

    Since your input is 32V then if the output is set to 5V the guaranteed max output current might be only 200mA.
     
  4. spinnaker

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    I have 2 separate 2000Uf caps. Other than limiting the size of the caps. How would I protect from a surge?




    Yep the LM317s have heat sinks.

    I have the supply perf boarded.

    I have two lm317s in parallel. One is to provide a variable voltage from 1.2 to 37 and the other a selectable of 4 pre set voltages.

    I just measured it again and I am getting 37V on the input to the regulators. Do they provide enough of a load? Am I bumping into potential problem?

    If I used a zener, I would need a 1amp - 1.5 amp? I am not finding any on Mouser above 1 amp.
     
  5. mik3

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    If the maximum output current is 1 Amp then your capacitors are fine if you want a 30V max output voltage. To protect the diodes from the surge current use a resistor in series with the capacitor to limit the inrush current and then bypass the resistor with a switch. Try another supplier, not only Mouser.
     
  6. Audioguru

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    The LM317 does not have a ground pin so it does not have a max input voltage spec. Its max allowed voltage from its input to its output is 40V which might occur in your circuit if its output is shorted to ground and its input voltage is 40V or more.
     
  7. spinnaker

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    Thanks AC. I think I am going to go out and get a 12V transformer and dial back my expectations a bit.
     
  8. spinnaker

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    What value would you recommend? What wattage?
     
  9. Audioguru

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    Rectifier diodes have a very high surge current rating because they normally feed big filter capacitors. The 1N400x rectifiers have a max average current rating of 1A but a max surge current rating for half a cycle of 30A.
     
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