Connect with us

Voltage regulation

Discussion in 'Electronic Basics' started by Wilhelm Lehmann, Jun 14, 2004.

Scroll to continue with content
  1. Hi,

    I have an application where my input voltage range from 28V-32V with the
    voltage changing in those ranges a few times per second.

    Using a 7812 regulator with 100uf on the input, and 10uf + 100nf on the
    output works, but I still see the fluctuations in the output.

    What do I use to create an absolute stable 12.00V output ? Voltage
    comparator ? FET's ? Any pointers in the right direction would be greatly
    appreciated. (maybe some example schematics ?)

    Regards

    Wilhelm
     
  2. John Larkin

    John Larkin Guest

    How do you define "absolutely stable"? How much is it changing now?

    What's the load current?

    Are you using a heat sink?

    John
     
  3. Dbowey

    Dbowey Guest

    wilhelm posted:

    << I have an application where my input voltage range from 28V-32V with the
    voltage changing in those ranges a few times per second.

    Using a 7812 regulator with 100uf on the input, and 10uf + 100nf on the
    output works, but I still see the fluctuations in the output.

    What do I use to create an absolute stable 12.00V output ? Voltage
    comparator ? FET's ? Any pointers in the right dir
    You don't say what the load current is, but the quick answer is put much larger
    capacitors on both the input and output. With those input conditions I'd think
    about 10,000 Mfd on both sides.

    What is causing the input voltage swing?

    Don
     
  4. John Fields

    John Fields Guest

    ---
    A 7812 is rated for a maximum input voltage of 35V, but its regulation
    is only guaranteed with input voltages not exceeding about 30V, so
    your 32V bumps may be more than it can handle. Take a look at the
    data sheet.

    If you've got a fairly stable load what you might be able to do is to
    put a resistor in series with the input of the 7812, and let it drop
    the 32V down to something the 7812 can handle. If you go that route,
    you might also be able to arrange for the resistor to be doing a good
    deal of the work and take some of the thermal load off of the 7812.

    Depending on your load current variations, if you do that you might
    have to increase the value of the input cap, and if you can post what
    your load looks like we ought to be able to come up with some numbers
    for you.
     
  5. You have enough excess voltage that you could easily use two
    regulators in cascade. Are you sure the output variations are
    getting through the regulator and are not based on variable resistive
    drop on the negative side of the supply as the input cap charges and
    discharges? Is the unregulated voltage fed to any load besides the
    regulator?
     
  6. John Fields

    John Fields Guest

     
  7. Wilhelm,

    You'd better have a look on the datasheet of that regulator, so you may find
    out the capacitors you need for a given load. Besides, coming down from
    28-32V to 12V at once is quite a jump. To get more stability you can use an
    7824 stabilizer followed by a 7812 one. For myself I'd look for a switching
    regulator but that's not the easiest way.

    petrus bitbyter
     
  8. Hi,

    Load is max 300mA

    The DC comes from a Transformer, Then Rectified using a 1.5A diode brigde.

    The instabilities comes from the source AC.

    I made the Caps Bigger and is seems to help, although there are still some
    minor fluctuations. (within Range)

    I now put a series resisror in on the input as reccomended, (1W to handle a
    bit of the heat)

    my output is now 11.96 to 12.85 quite acceptable for what I want to do, but
    are still interested in learning about switching regulators. Any pointers
    there ?

    Thank you for all the help.
     
  9. Hi,

    Any pointers on starting to learn about switching regulators ?

    Wilhelm
     
  10. Jerry G.

    Jerry G. Guest

    You are exceeding the maximum input Voltage specification for the regulator
    IC that you are using.

    First regulate your Voltage source down to about 18 Volts. Then add the
    second regulator to lower it down to 12 Volts. Make sure that the Voltage
    regulator you choose can handle at least 35 Volts on its input to be safe.

    Make sure your regulators are heat sinked to have best stability. Even if
    you are drawing less than the rating for a necessary heat sink, it is best
    to still use some type of heat sink.

    --

    Jerry G.
    =====


    Hi,

    I have an application where my input voltage range from 28V-32V with the
    voltage changing in those ranges a few times per second.

    Using a 7812 regulator with 100uf on the input, and 10uf + 100nf on the
    output works, but I still see the fluctuations in the output.

    What do I use to create an absolute stable 12.00V output ? Voltage
    comparator ? FET's ? Any pointers in the right direction would be greatly
    appreciated. (maybe some example schematics ?)

    Regards

    Wilhelm
     
  11. Gabe Garza

    Gabe Garza Guest

    I'm a "beginner" (I have a degree in EECS, but I focused more on the
    software side and am just now starting to play with electronics) that
    just built a circuit with a couple switching regulators.

    I used two regulators (an LM2575-5.0 5V at 1A and an LM2576 adjustable
    3A) from National's "Simple Switcher" line. They have tools on their
    web site that automatically generate a circuit for you that you may
    want to look at. Personally, I thought they were overkill--the
    circuits aren't that complicated and their datasheets are well-written
    and cover component selection very well.

    The only component that's non-trivial to find is the inductor: several
    manufacturers make a line of inductors specifically for this family of
    regulators. Digikey carries a line by "Alfa Mag"--if you do a parts
    search on their web site, note that they classify these as "Specialty"
    inductors. To look them up, do a "Part Search", go to "Inductors,"
    then "Specialty." The "For Use With" column identifies which
    regulator(s) the inductor is designed for.

    The only downside to the switchers is that they're significantly more
    expensive then the linears, especially when you factor in the
    specialty inductor (for small quantities, figure about $5-6 per
    regulator circuit). However, the efficiency gain is very nice,
    especially when you take heat into consideration...

    Gabe Garza
     
  12. John Larkin

    John Larkin Guest

    Wow. A 7812 should *never* put out 12.85. Something else must be
    wrong. Aside from the top-posting, I mean.

    John
     
  13. Kevin R

    Kevin R Guest

    Go to Linear Technology's site at http://www.linear.com/software/
    and get their Spice III based Switcher Cad program. It's not bad.
    It's an SMPS simulator for LTs devices

    If you can't get an off the shelf inductor to suit, then
    Switcher Cad will give you all the information you need to
    plug in to Magnetics Inc's Inductor design software, which will
    tell you which core to use, what wire thickness and how many turns.
    http://www.mag-inc.com/software/inductor.asp

    Of course, that won't teach you about how to design an SMPS,
    but unless you intend to make a serious passtime out of
    designing SPMSs, I wonder if its worth the effort when
    it is so much easier to follow manufacturer's reference
    designs. Why re-invent the wheel ?
     
  14. Rich Grise

    Rich Grise Guest

    Yes. Start with the basics. Ohm's law, Voltage, current, circuits,
    that sort of thing, then start learning about active devices, and
    work your way up - and build a linear regulator along the way.

    Good Luck!
    Rich
     
  15. Rich Grise

    Rich Grise Guest

    I was working on a switcher once, and the transformer and output inductor
    actually practically got designed for me by the ferrite core rep. I told
    him the chip I was looking at, and the target freq. and volts/amps, and
    he says, "oh, that'd be core no. blahblahblah, with xx turns of #y for
    the primary, and so on and such and such." I still had to figure it out
    for myself, I have no idea what kind of code that gibberish was supposed
    to be. Maybe he thought he was in a commercial. %-)

    Cheers!
    Rich
     
  16. Soeren

    Soeren Guest

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

-