LM series regulators

Discussion in 'Homework Help' started by priyaa, Jul 25, 2013.

  1. priyaa

    Thread Starter New Member

    Jul 25, 2013
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    can any one tell me the usage of LM series regulators,their types and suggest me which series is applicable if my rectified input voltage is 18v to get 12v at the output.:confused:
     
  2. pujulde

    Member

    Jul 24, 2013
    95
    1
    The usage is that you can transfer DC to DC without any difficulty, the difference between the input and output voltage is advisable to be not grater than 2-3 volts because its getting hot and you need to sink that hot. There are LM78xx and LM79xx, LM78xx means that the output voltage is positive and the LM79xx means that the output will be negative. Now lets to describe the second part(LM78xx) xx may be 05 and it means that LM7805 --- has its output voltage 5volt. Input voltage may be 7-36 volts range.
     
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  3. tshuck

    Well-Known Member

    Oct 18, 2012
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    You will need to provide your current needs before a suggestion can be made.
     
  4. WBahn

    Moderator

    Mar 31, 2012
    17,743
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    As opposed to regulators made by other companies?

    All the LM tells you is that it was manufactured by National Semiconductor (or one of the series of owners since then) or someone that has licenses the right to use that designator for parts they produce that meet the same specs.

    LM stands for Linear Monolithic and is nothing more than part of a part numbering system used by National Semiconductor.
     
  5. priyaa

    Thread Starter New Member

    Jul 25, 2013
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    thank you...but i got a problem with this and if possible please suggest me.now let me explain.i have connected 63v capacitor across a rectifier whose o/p is 18v which is the i/p of my regulator..then i used LM317 regulator,its 3rd pin given to i/p(via capacitor),2nd to o/p and 1st pin is grounded through an 1k ohm pot.i have connected a 100 ohm resistor across 2nd and 1st pin and again i have connected a 25v capacitor across 2nd pin and ground and finally i measured the o/p across 2nd pin and the ground.it supposed to be 12v but i got nearly 15v.then i replaced my 63v capacitor with a 35v capacitor,but it got burst. :( i m trying to get 12v exactly.
     
  6. WBahn

    Moderator

    Mar 31, 2012
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    Have you tried following the directions in the LM317 datasheet?

    The voltage ratings on your caps are immaterial as long as they are comfotably greater than the voltages they will be subjected to.

    It would be a LOT more helpful if you were to provide a schematic, even a rough sketch in Paint.

    How have you connected the three terminals on the pot?

    What do you mean by the capacitor got burst?
     
  7. priyaa

    Thread Starter New Member

    Jul 25, 2013
    3
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    my circuit looks something like this (i have attached it)
    my capacitor was not able to withstand the over voltage and therefore it got blown away,but i m unable to understand why this happened so..:confused:
     
  8. WBahn

    Moderator

    Mar 31, 2012
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    First off, the equation in your figure is incorrect relative to your schematic. R1 and R2 are swapped.

    What kind of supply voltage do you have? You say it is from a rectifier that has an output of 18V. What kind of output? Is it regulated DC, or is it a wall-wart, or what. If it's an unregulated rectifier that is putting out 18Vrms, then the peak voltages could be 25V and it may go well above that under no load conditions.

    A 100Ω feedback resistor and 1kΩ ground reference pot should give you and output range of about 1.25V to 13.75V. What range do you see as you adjust the pot from one end to the other?

    What is the voltage difference between the Output pin and the Adjustment pin that you are seeing?
     
  9. donpetru

    Active Member

    Nov 14, 2008
    186
    25
    What is the required current at a voltage of 12V?
     
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