Current limiting circuit?

Discussion in 'General Electronics Chat' started by aris, May 12, 2010.

  1. aris

    Thread Starter Member

    May 5, 2010
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    Hi all,
    I have a power supply of 12V-7.0A(max)-84W(max). I want to power a circuit which uses a CMOS 4000 IC and I want to limit the current up to 500mA.
    Could someone help me on how to do this.
    I have already plenty of BC547 npn transistors and resistors if those could be used....

    Thank you for help guys!!
     
  2. bertus

    Administrator

    Apr 5, 2008
    15,646
    2,344
    Hello,

    When a circuit is well configured, it will not take more current than it needs.
    If the 4000 circuit takes 10 mA, it will take just this 10 mA and not more,
    even when the powersupply is capable of much more.

    Bertus
     
  3. Bychon

    Member

    Mar 12, 2010
    469
    41
    I wrote this for a somewhat different situation, but it might help you. Please, you can laugh at me, but don't curse.
     
  4. studiot

    AAC Fanatic!

    Nov 9, 2007
    5,005
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    Now why would I believe a poodle that says

    'trust me'

    But a cool document all the same Bychon
     
  5. Bychon

    Member

    Mar 12, 2010
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    41
    Thank you. This is the first time I have allowed that document to go out in public and was quite afraid ppl would think me crazy.
     
  6. retched

    AAC Fanatic!

    Dec 5, 2009
    5,201
    312
    Oh, your definitely crazy. But it is a good document for the n00bz!.

    I would throw an LED sample formula worked out. For the "I have a 9v battery connected to a LED. Why do they always explode?"
    You can use that line if you want, its all yours. ;)
     
  7. Bychon

    Member

    Mar 12, 2010
    469
    41
    I have 2 pages on Watt's Law and 2 pages of examples. I'll see what I can do with "The exploding LED trick".
     
  8. retched

    AAC Fanatic!

    Dec 5, 2009
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    So aris, the answer to your question is, regardless of the AVAILABLE amperage of the power supply, you want a resistor to be sure nothing in the circuit pulls more amps throught the part.

    In these cases, you use a 'current limiting resistor'.
    Ohms law will tell you how many ohms you need to limit the amperage.

    So you want 10mA at 12v.

    resistance = voltage divided by amps


    12 / .01 = 1200 OHMS
    So you want a 1200 ohm resistor or a few resistors in series that equal 1200 ohms to limit the current to 10ma (.01A)
     
  9. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    Not if you still want 12V when the load is, say, 5mA. You will then need something more sophisticated than a resistor.
     
  10. retched

    AAC Fanatic!

    Dec 5, 2009
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    Ahh, very true, my friend.

    You will need a current regulator that will adapt to the changing load and "regulate" the current.

    Maybe thats why they named them that. ;)
     
  11. aris

    Thread Starter Member

    May 5, 2010
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    0
    thank you all for you response!!
    in my case a simple resistor as retched describes would do for me!!!

    Thank all of you once more for your time!!!!!
     
  12. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    You may be disappointed.:rolleyes:
     
  13. retched

    AAC Fanatic!

    Dec 5, 2009
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    Listen to Ron H. You will probably end up like most of my dates, dissapointed. ;)

    Remember, your voltage will sag more when you have a load connected to the resistor. All depending on what your circuit is, it mat be ok. But as voltage changes, so does the resulting current. Ohms law continues regarless of what you want.
     
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