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my explaination

Discussion in 'Electronic Projects Design/Ideas/Reviews' started by bananasiong, May 21, 2006.

  1. audioguru

    audioguru Well-Known Member Most Helpful Member

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    An LMC555 or TLC555 is a low power 555 made with Cmos. It doesn't have enough output current to drive the loop of wire around your room.
     
  2. bananasiong

    bananasiong New Member

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    You mean, the cmos 555 doesn't have enough current to drive the loop of wire? I use power supply for my NE555, it works well.

    if i want to share the circuits with the same source (LM393 and microcontroller) with 9V battery, do i need to make any changes?
     
  3. audioguru

    audioguru Well-Known Member Most Helpful Member

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    The output high current of a Cmos 555 is only 10mA. A regular 555 has 200mA.

    The max recommended supply voltage for your Motorola u-controller is 5.5V. To use a 9V battery you should use a low-dropout 5V regulator IC for it and for the LM393. An ordinary 5V regulator begins to fail when the battery is 8V and is like new. A low-dropout regulator works fine until the battery drops below 5.5V.
     
  4. dave

    Dave New Member

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  5. bananasiong

    bananasiong New Member

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    But the microcontroller itself already has a voltage regulator (NOT 7805), so can i use a 9V battery to share them?

    *what is a low-dropout regulator??
     
    Last edited: Jun 4, 2006
  6. Hero999

    Hero999 Banned

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    Most regulators require the input voltage to be between 2V to 3V higher than the rated output voltage, this is called the drop-out voltage, for example for an LM7805 has a drop-out of 3V so to provide you with 5V at 1A you need an input voltage of at least 8V. Low dropout voltage regulators are as the name implies regulators with a very low drop-out voltage, typically less than 1V, but you can get fairly inexpensive regulators with drop-out voltages as low as 300mV.
     
  7. audioguru

    audioguru Well-Known Member Most Helpful Member

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    What is the part number of the voltage regulator used with your Motorola micro-controller?
     
  8. bananasiong

    bananasiong New Member

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    oh, i've forgotten. It looks like a power transistor, with 3 pins, is a 5v regulator, but it is not a 7805.
     
  9. audioguru

    audioguru Well-Known Member Most Helpful Member

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    Then we don't know if it still regulates the voltage when the battery voltage runs down. You could measure its output voltage when it is powered by a used battery.
     
  10. bananasiong

    bananasiong New Member

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    I think it is a 78M05CT.. (the microcontroller is not with me now.. :( i search it from the net..)
     
  11. audioguru

    audioguru Well-Known Member Most Helpful Member

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    Look at its datasheet!
    It is the same as a 7805 except its max current is only 500mA.
    It is guaranteed to regulate very well when its input voltage is 8V or more.
    It is shown to regulate poorly when its input voltage is 7V or less.

    Your 9V disposable battery voltage will quickly drop to about 7.2V then slowly continue dropping more as it runs down. A 9v rechargable battery voltage is only 7.2V.
     
  12. bananasiong

    bananasiong New Member

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    But my microcontroller works well with 4*AA's batteries, and it still works when they drop to 4.8v.
     
  13. audioguru

    audioguru Well-Known Member Most Helpful Member

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    Again, look at the datasheet!
    I think the micro-controller's minimum operating voltage is 3.0V.
    The 78M05 regulator won't be regulating with 4.8V to 6.0V as its input.
     
  14. bananasiong

    bananasiong New Member

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    Okay.. back to the question: If i connect the microcontroller with the LM393 circuit in parallel to share the 9v source, do i need to make any changes?
     
  15. audioguru

    audioguru Well-Known Member Most Helpful Member

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    Try it with the LM393 circuit powered by the output of the 78M05 regulator.
    If the LM393 goes crazy when the battery voltage drops below 8V, causing the regulator to drop-out, then replace it with a low-dropout regulator.
     
  16. bananasiong

    bananasiong New Member

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    Thanks, you've helped me a lot! Really very much!
    Where do u find from the datasheet that "the regulator won't be regulating with 4.8V to 6.0V as its input"? I found 4.8 and 5.2? It stops regulating 7.2v and lower.
    http://pdf1.alldatasheet.com/datasheet-pdf/view/8862/NSC/LM78M05CT.html
    In the condition of "the regulator won't be regulating with 4.8V to 6.0V as its input", when the battery drops to 6v, then everything back to normal, right?

    So if i use a 9v battery (without changing the regulatortype), it works well until drop-out. Then works well again when "the regulator won't be regulating with 4.8V to 6.0V as its input", right?

    And.. if i use 5v supply, then the LM393 circuit doesn't need to be connected to the output of the regulator, just simply parallel them, right?
     
  17. audioguru

    audioguru Well-Known Member Most Helpful Member

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    When you operate ICs at their limit, different manufacturers have different specs.
    Fairchild shows that with an input of 7V or less, their 78M05 regulates poorly. With an input of 8V or more the regulation is much better.

    These are minimum spec's. Each IC is different and maybe your IC is better than the minimum.
     

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  18. bananasiong

    bananasiong New Member

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    Wow.. so many brand.. i know the 7V to 20V, if input supply is between these range, the output will be fine, right?
    What about the Line Regulation? 7V to 25V---100mV---poor, 8V to 25V---50mV---not bad, what are they??

    *And.. if i use 5v supply, then the LM393 circuit doesn't need to be connected to the output of the regulator, just simply parallel it with the microcontroller, right? Thanks :)
     
  19. audioguru

    audioguru Well-Known Member Most Helpful Member

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    Line regulation is how much the output voltage changes when the input voltage is changed. It is a regulator and its output shouldn't change much. When the input changes from 7V to 25V then the output changes 100mV max which is poor. When the input changes from 8V to 25V then the output voltage changes only 50mV max which is better.

    The LM393 must have the same power supply voltage as your micro-controller so that it doesn't feed it a voltage that is too high. The max input voltage to a micro-controller is its power supply voltage.
     
  20. bananasiong

    bananasiong New Member

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    Last edited: Jun 5, 2006
  21. audioguru

    audioguru Well-Known Member Most Helpful Member

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    The LM2940 is perfect for a 5V regulator operating from a 9V battery. At fairly low load current it still gives a properly regulated 5V output when its input voltage drops to only 5.5V.
     

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