Connect with us

78xx output cap

Discussion in 'Electronic Design' started by Pimpom, Jan 14, 2011.

Scroll to continue with content
  1. Pimpom

    Pimpom Guest

    I know that, to ensure stability when using a standard 3-terminal
    regulator, the output capacitor should be placed as close as
    possible to the regulator. Usually this is no problem. But I'm in
    the middle of a PCB design in which it's more convenient to have
    slightly over an inch of track between the two. This is on both
    the hot and the ground sides. The tracks are shared by load
    currents of a few mAs. Part of the load is a CMOS counter IC
    operating at a few kHz while another section has further RC
    filtering on the supply. Is this likely to cause problems?

    I'm asking here instead of simply trying it out because I have to
    make a dozen of this circuit. Even if the test unit works, I
    won't know if it's only marginally stable and I don't want to
    risk having some units or future replacements go unstable.
     
  2. Pimpom

    Pimpom Guest


    Thanks. I have the NatSemi Linear Application Handbook but
    haven't seen where they cited the 3-inch figure (haven't looked).
    Yes, it's TH.
     
  3. Jeff Johnson

    Jeff Johnson Guest


    Is it possible to add a smaller cap that near the regulator? I doubt you'll
    have any serious problems but make sure you bypass that counter. Increase
    track thickness if you can and properly bypass(low ESR caps). The track
    length isn't going to be a huge issue if done right.

    If your not sure and are worried how bout including some SMT caps near the
    regulator and near the bypassing just in case? These can be quite small,
    have low ESR, and you don't have to add them until there is an issue.
     
  4. Nemo

    Nemo Guest

    From National's 78XX data sheet at:
    I tried that with a NatSemi 7805 last year.

    Went unstable. The next respin included a 22uF OSCON and some 1uF
    ceramics, and it's fine (a bit ripply perhaps but usable).

    I also found an old Analog Devices app note suggesting that to minimise
    noise, put a ferrite bead between the output of the 7805 and its
    decoupler. That might have been valid many years ago but it made the
    regulator oscillate until I replaced the ferrite with a zero ohm resistor.

    Nemo
     
  5. I've never been able to make a 78x05 oscillate under sensible
    conditions. We shipped >10K units with zero output bypass cap (benign
    load of a few mA fairly steady).

    Putting an inductor on the output without a bypass right at the
    regulator would not be in my "sensible" category, though.


    Best regards,
    Spehro Pefhany
     

  6. Since you are using TH mounted components you could consider making some
    room on the side opposite the 78XX part to place a 0805 or even 0603
    sized set of SM pads between the OUT and GND pins of the regulator.
    These would disrupt the layout in a minimal way and then if it proved
    necessary to have some cap installed there it can be added at the last
    minute. For example if the cap that you plan for being ~1 inch away was
    a 22uF cap the one placed on the SM pads could be maybe 0.1uF. In the
    0805 and 0603 sizes there are a whole range of values of capacitance
    that you can choose.
     
  7. Providing a bit of forward feed?

    Making it respond to regulatory stimulus faster.
     
  8. Put it ON the lead of the regulator where it mounts.
     
  9. Rich Grise

    Rich Grise Guest

    I concur here, if your F above was meant to be microF. (uF, µF ;-)

    The last time I used a 7812, it was on a hobbyist prototype, and
    I hung a 0.1 uF cap on both the input and the output, tightly snugged
    up against the regulator, with a lead length of about 0.1". But that
    was when I was young and afraid of everything. :-D

    It _did_ serve its purpose, however, of providing 12 VDC to the 9"
    monitor I used for my "TV Typewriter," distortion-free. :)

    Cheers!
    Rich
     
  10. Pimpom

    Pimpom Guest

    Thanks for all the replies. I always had the impression that
    weekends were slow times in technical NGs, but I was pleasantly
    surprised to see that the two threads I started received quite a
    bit of attention.To reply to the various posts, some with similar
    contents:

    No problem adding an SMD or a ceramic disc close to the regulator
    on the copper side. I just don't like 'afterthought' additions if
    they can be avoided. And yes, there's plenty of room to increase
    the track sizes which are already 30 and 40 mils wide.

    The output cap is a 0.1uF ceramic disc which I've always used as
    a standard output cap for 78XXes except where the nature of the
    load makes heavier buffering desireable. Here's the design as it
    is now:
    http://img148.imageshack.us/i/outputcap.png/
     
  11. I don't think you'll have any trouble with that, although personally I
    would have tried to fit it between the 78xx and the chip, maybe by
    sliding those power input pads up and down, or by moving the chip
    left a bit.

    It's been years since I've seen a PCB design done from the "bottom
    view"-- used to do it all the time that way for single-layer boards.


    Best regards,
    Spehro Pefhany
     
  12. Pimpom

    Pimpom Guest

    I do most of my designs as single-layer TH. It's only in recent
    years that we've had online PCB services here in India and the
    nearest one is literally more than 2000 km away. To avoid weeks
    of delay and 20x the expense, I still make most of my own PCBs
    using iron-on transfers. I'm reluctant to use smaller than 15 or
    20 mil tracks and I haven't yet found a source for carbide drill
    bits. So I use single-sided phenolic boards with TH components
    most of the time.
     
  13. Jeff Johnson

    Jeff Johnson Guest

    Why not attempt to do SMT? I have found that it's much easier and faster.
     
  14. Sjouke Burry

    Sjouke Burry Guest

    Should the output cap not be as close as possible to the
    regulator?
     
  15. ehsjr

    ehsjr Guest

    Pimpom wrote:

    Dental burs work well for drillng PCBs. Maybe your dentist will
    give you some used burs to try.

    Ed
     
  16. Pimpom

    Pimpom Guest

    That's the whole point of the thread. Please read the first few
    sentences of my opening post again.
     
  17. Pimpom

    Pimpom Guest

    Thanks for the suggestion. I work a lot with members of the
    medical profession including dental surgeons, but the idea of
    using a dental bit never occurred to me. If it's good enough to
    drill teeth, it should be good enough to drill FR4.
     
  18. Sjouke Burry

    Sjouke Burry Guest

    Well, it sounds a bit like gambling to me, and maybe risking
    the occasional oscillator.
    But, if it works for you....
     
  19. Pimpom

    Pimpom Guest

    I've been thinking about it for quite some time now. Maybe this
    is a good time to start.

    I've accumulated a wide range of parts over the years, most of
    them bought on visits to the major cities and courtesy of
    friends. It's almost impossible to deal with Indian retailers
    from a distance and the bigger distributors ignore orders smaller
    than production quantities. The biggest obstacle to switching to
    SMT will be buying the parts.
     
  20. Sjouke Burry

    Sjouke Burry Guest

    In the past I have seen some suppliers offer boxes with assorted
    values surface mount components.
    Try and find that.
     
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

-