Connect with us

Linear Voltage regulators

Discussion in 'General Electronics Discussion' started by krishna42099, Oct 2, 2012.

Scroll to continue with content
  1. krishna42099

    krishna42099

    20
    0
    Oct 1, 2012
    Hi, when I looked at the datasheets of various linear voltage regulators, I've seen they just given ' current - internally limited ', what does it mean, what are the factors limit the current in the voltage regulator?? and there isn't any proper data on its maximum current it can handle??

    thx in advance..
    krishna
     
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,480
    2,828
    Jan 21, 2010
    There is typically a rated current.

    The current limit (where one exists) is typically unstated and greater than the rated current. The current limit is to protect the device in cases of fault. It is not designed for normal use. Typically the device will current limit until it reaches a temperature where it shuts down.

    If you want more specific answers, please point me to a datasheet for a particular device and I'll try to interpret for you.
     
  3. krishna42099

    krishna42099

    20
    0
    Oct 1, 2012
    thx steve, its really appreciable...

    one more question, can you give an idea,...I'm doing a project involving a 12.7 volt lead acid battery, is there any chance that I can make circuit switched off automatically once the battery is at or below 12.1 volt???


    thx a lot in advance
    krishna
     
  4. davenn

    davenn Moderator

    13,786
    1,936
    Sep 5, 2009
    There's masses of circuits out there googling "battery voltage monitor schematics" .... ( Remember GOOGLE is your friend) :)

    brought up lots of hits .... like the one below which woudl be easily modified to drive a relay to switch power on and off to a bit of gear

    http://www.reuk.co.uk/12-Volt-Battery-Low-Indicator-LM741.htm

    Dave
     
  5. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

    4,960
    651
    May 8, 2012
    Besides the fact that the 741 isn't and never has been spec'd as single ended supply compatible, they're also old and have been superseded by far better choices. The circuit concept in that link is OK but a 741?? ...Nah. I'd start with an OpAmp that's specifically designed for single ended supplies. Even an old 324 would be a better choice. ;)

    Chris
     
  6. davenn

    davenn Moderator

    13,786
    1,936
    Sep 5, 2009
    agreed, Was just trying to get the OP 'er to go looking at the chjoices available :)


    D
     
  7. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,480
    2,828
    Jan 21, 2010
    Of course the question is... How does the op-amp know if the power supply is single or double ended?

    The answer is that it doesn't.

    It's mostly a question of how you determine a reference, and granted, that is less trivial with a single ended supply..
     
  8. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

    4,960
    651
    May 8, 2012
    I would have posted that link too. I'm just curious why the author and so many others choose 741's.

    Chris
     
  9. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,480
    2,828
    Jan 21, 2010
    Because it's simple, cheap, and well known. If I see something other than a bog standard op-amp used somewhere I wonder what special characteristics are required. Someone using a 741 is telling me "none".
     
  10. BobK

    BobK

    7,682
    1,688
    Jan 5, 2010
    I guess I have to say my usual about single vs dual supply op amps. The only difference is how close the inputs and outputs can be to the + and - rails. If they don't need to be any closer than the spec, a dual op amp can be used with a single supply. If you need to have the output go to ground, a '741 will not work becuase it cannot drive the output to the - rail.

    Bob
     
  11. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

    4,960
    651
    May 8, 2012

    OK guys, I've compiled an exhaustive argument to these quotes. See insightful retort below..

    Actually, as for using a 741 for the comparator circuit in question, I don't have a good argument, mediocre argument or even a feeble argument. That's different! :D

    Chris
     
  12. BobK

    BobK

    7,682
    1,688
    Jan 5, 2010
    Actually, if it works in that circuit it is by luck. According to a random '741 datasheet I looked at: http://www.ti.com/lit/ds/symlink/lm741.pdf

    When supplied with +- 15V, the output can only go +-12V. So I am going to guess that with 12V power, the output can do not much better than 3V to 9V. In which case, the LED will be lit dimly when the battery is good and brighter when the battery is low, assuming a red LED with a forward voltage of about 2.0V. If you use a white LED it might work better.

    Bob
     
  13. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,480
    2,828
    Jan 21, 2010
    When the output is high, how does it sink current to keep the LED lit?
     
  14. BobK

    BobK

    7,682
    1,688
    Jan 5, 2010
    Yes, of course. I was thinking of a stiff output at 9V, but that is not what is happening. So it would work okay.

    Bob
     
  15. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

    4,960
    651
    May 8, 2012
    It does need some feedback to non inverting input though. As it stands I don't think it has any hysteresis at all. If the LED is replaced with a relay this would cause serious contact chatter.

    Chris
     
  16. BobK

    BobK

    7,682
    1,688
    Jan 5, 2010
    True, if you are actually cutting off power when the battery is low, the voltage will undoubtedly rise as soon as you cut it off, causing oscillation.

    Bob
     
Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Electronics Point Logo
Continue to site
Quote of the day

-