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Generate -+5V from 12V

Discussion in 'Electronic Basics' started by vic, Jun 5, 2009.

  1. vic

    vic Guest

    Hi,

    I need to generate a symmetric +/-5V supply from a single +12V supply. I
    need about 150mA from the +5V side, and less than 50mA from the -5V side.

    I've thought about using two 7805 in the following configuration :


    Supply + ----+--------[7805]---- +5 V
    | |
    `-[7805]----+------ 0v
    |
    Supply - ---------+------------- -5V


    does it make sense ? Would it be OK for the 7805 to sink current instead
    of sourcing current, in the case of the 7805 in charge of the 0V supply ?

    If this idea is garbage, what would be the recommended solution to this
    problem ?

    Thanks.
     
  2. Guest

  3. Baron

    Baron Guest

    Supply + ----+--------[7805]---- +5 V
    cap |
    |------+------+------ 0v
    cap |
    Supply - ------[7905]------------ -5V

    Whats wrong with doing it this way. Use a pair of 1000uf caps. Bearing
    in mind that if the supply -ve is grounded the 0v will be +5 volts
    above it.
     
  4. Baron

    Baron Guest

    Doh ! Dropout voltage hadn't occurred to me. Those things need two
    volts of headroom.
    Thanks for pointing that out. Its something I should have known. :-(
     
  5. vic

    vic Guest

    Thanks for this idea, I'll give it a try.
     
  6. Baron

    Baron Guest

    Yes you are quite right ! I just didn't realise it until it was pointed
    out to me.
     
  7. Baron

    Baron Guest

    Giving this a bit more thought...
    Take the regulator tails to the opposite rail. That would ensure that
    each regulator had sufficient overhead.
    Replace the caps with a pair of equal value resistors in series passing
    say the max expected current. Using the centre point as the 0v.
    Put the caps across the output.

    Would that work ?
     
  8. Baron

    Baron Guest

    Yes I see how that works. One to stick in the notebook for reference.
    Thankyou for taking the time to explain things.
     
  9. Jon Kirwan

    Jon Kirwan Guest

    Sadly, Verizon (and other major service providers) have completely
    eliminated all alt.* groups from their servers. This affects me, as
    well. It may yet force me to get an extra-cost service added.

    Jon
     
  10. Jon Kirwan

    Jon Kirwan Guest

    Thanks, Bob. I got both postings just fine. Apparently, Verizon lets
    through binaries on non-alt.* groups they support. Perhaps giganews
    blocks binaries on groups marked as text? If so, that's yet another
    consideration when I'm out looking for a service to pay for.

    Things sure have evolved on internet.

    Jon
     
  11. Jon Kirwan

    Jon Kirwan Guest

    Okay. I just ran a .op and a .trans on it, using venerable 2n2222 and
    2n2907 to set the 6V mid-point. I'm not sure how they stack up with
    the 1N914's and I'm uncomfortable with the choice. The -rail isn't
    correct either on the .op or the .trans.

    I'm thinking about that pair of diodes and replacing them with an
    adjustable BJT diode-thingy. Something like:

    |
    ,---+
    | |
    \ |
    / |
    \ |
    / |
    | |
    | /c
    +-|
    | >e
    | |
    \ |
    / |
    \ |
    / |
    | |
    '---+
    |

    Do R8 and R7 need to be sized so that they greatly exceed the 26mV of
    the BJT emitter resistor by 20X or 30X to yield better sharing?

    Jon
     
  12. Baron

    Baron Guest

    Unfortunately most if not all "Free" news servers don't carry binary
    groups. Also I wont pay to suffer all the spam and other rubbish that
    comes along with with it !
     
  13. Baron

    Baron Guest

    Thankyou Bob. Thats very useful !
     
  14. Jon Kirwan

    Jon Kirwan Guest

    First time it got my attention. I'll give it a shot.

    Thanks,
    Jon
     
  15. Jon Kirwan

    Jon Kirwan Guest

    Okay. It got my attention long enough to register and check for the
    group, alt.binaries.schematics.electronic. It doesn't appear on their
    list. Besides, they don't support NNTP and would be quite difficult
    to make use of, if I understand what this means.

    In short, it doesn't look usable for this purpose because it doesn't
    include the group. And it doesn't look usable, more generally,
    because the mechanism (emailing every message, or every message of a
    thread) isn't a means I would willingly choose.

    Jon
     
  16. Jon Kirwan

    Jon Kirwan Guest

    Thanks. What I did before was to go to the main page at which you
    referred me and then clicked on "get the USENET messages" link. That
    took me to:

    http://www.usenet-replayer.com/sign_up_feed.html

    I then went through the process of signing up. They sent me an email,
    which I saved and edited into an HTML page. Loaded that, then clicked
    on the "Feed On" button. (I gather they allow me to sign up for 10
    different groups.) When I clicked that JAVA link, it set me up on a
    page where I could ask it to display the ALT groups I might want to
    include in my 10. alt.binaries.schematics.electronic wasn't in the
    list.

    What you've shown me is something different. It appears to be a bit
    strange. I'm struggling with the format, which appears to only be the
    pictures and no text bodies when messages don't include pictures. If
    so, that means I may be able to access pictures this way but not
    comments sent to the group about them. Threading seems... well, I'm
    not seeing how, yet. It looks as though I may be able to pick up on
    some pictures this way, though. If I'm quick about it... otherwise I
    may have to weed through pages and hope that my eye can pick out what
    I'm looking for in the thumbnails.

    Perhaps they don't include alt.binaries.schematics.electronic in their
    emailing service, but do offer a web page with pictures-only component
    from the group?

    Jon
     
  17. Jon Kirwan

    Jon Kirwan Guest

    I did, some time ago. Finding a good NNTP server that includes a
    binary newsgroup like alt.binaries.schematics.electronic was a "no
    show." I spent the spare time of a few days on it. Not looking
    forward to that, again. Especially, if someone has a specific idea on
    hand that does NNTP for that group in particular.

    Of course, when I get a strong enough interest to push the time
    envelop out some more in the future, I'll take another shot at it.

    But for now, yeah. Been there, done that, came up zero.

    Jon
     
  18. Baron

    Baron Guest

    Ian. Would you like to correct that ip 202.177.16.??? I guessing its
    132. Since that responds with "Operation in Progress"
     
  19. Jon Kirwan

    Jon Kirwan Guest

    Hehe. 1321 is indeed impossible -- at least with the traditional
    system which allows only up to 255.

    Jon
     
  20. Baron

    Baron Guest

    Thanks. That one works ! Its a bit slow but its got the
    alt.binaries.schematics.electronic group. I also got the attached pdf.
    Good one.
     
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