# Voltage regulation

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

1. ### Wilhelm LehmannGuest

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 LarkinGuest

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

Are you using a heat sink?

John

3. ### DboweyGuest

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 FieldsGuest

---
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. ### John PopelishGuest

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?

7. ### petrus bitbyterGuest

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. ### Wilhelm LehmannGuest

Hi,

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. ### Wilhelm LehmannGuest

Hi,

Any pointers on starting to learn about switching regulators ?

Wilhelm

10. ### Jerry G.Guest

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

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

12. ### Gabe GarzaGuest

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

13. ### John LarkinGuest

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

John

14. ### Kevin RGuest

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 ?

15. ### Rich GriseGuest

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

16. ### Rich GriseGuest

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