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Basic Voltage Regulator Help

I have a circuit that contains some logic ICs and will sometimes be
needed to drive a strip of LEDs.

The logic is powered at 3.3V and the LEDs (strip) need 12V.

I want to power all this with a 12V brick.

Should I just use a buffered voltage divider for the logic? It is
pretty low power, so that seems like it should work. And if so should I
buffer with npn,mosfet,opamp or what?

Would a specialized IC work better?

Thanks for any and all help.
 
J

John Popelish

Jan 1, 1970
0
I have a circuit that contains some logic ICs and will sometimes be
needed to drive a strip of LEDs.

The logic is powered at 3.3V and the LEDs (strip) need 12V.

I want to power all this with a 12V brick.

Should I just use a buffered voltage divider for the logic? It is
pretty low power, so that seems like it should work. And if so should I
buffer with npn,mosfet,opamp or what?

Would a specialized IC work better?

A 3 terminal linear voltage regulator and a couple capacitors (all for
less than a dollar) would probably be the simplest. Something like
the 3.3 volt version of:
http://rocky.digikey.com/WebLib/Texas Instruments/Web data/UA78M00 Series.pdf
Keep in mind that the regulator will produce an amount of heat (in
watts) equal to your logic load amperes times (12-3.3) volts.
 
Thanks a lot! That datasheet doesn't mention anything about caps. I
take it that they would smooth out the DC. What values and types would
you recommend? Are they very necessary?

Thanks again!
 
J

John Popelish

Jan 1, 1970
0
Thanks a lot! That datasheet doesn't mention anything about caps. I
take it that they would smooth out the DC. What values and types would
you recommend? Are they very necessary?

Thanks again!
When applying such regulators, it is common to include a .1uF or
larger capacitor between input and common, to help stabilize the
regulator, since wiring inductance in the raw supply lead can
destabilize the regulator, and to also add a .1 uF or more between
output and common to improve the high frequency noise rejection of the
regulator, since its feedback voltage control has an upper frequency
limit of less than 1 MHz. But if the regulator is mounted very close
to the logic it feeds, the bypass capacitors normally used between 3.3
and common at the chips will perform this second function.

I have no idea what current your logic needs, but if the regulator
gets warm enough to need a heat sink, you can also transfer some of
that heat to a resistor in series with the input side (to waste some
of the excess voltage, and also help filter noise spikes from the 12
volt supply with the help of that input capacitor), as long as there
is at least 5.3 volts left at the input of the regulator.
 
Thanks a lot! That datasheet doesn't mention anything about caps. I
take it that they would smooth out the DC. What values and types would
you recommend? Are they very necessary?

Thanks again!

Dear "Acoustic Bass" (or is it "Electric Bass But Temporarily Without
An Amp"?),

You should probably take a look at something like
the LT1084 datasheet, at http://www.linear.com, just
as an example of an explanation of how and why
the capacitors should be used,
with a three-terminal voltage regulator.

I'm a bit surprised
that you have a three-terminal voltage regulator datasheet
that DOESN'T mention using the capacitors! They are usually
considered to be quite necessary, for stability, at the very least, as
well as for smoothing.

You could also download the free "LT-Spice" (aka "SwitcherCAD III")
software, from the linear.com site. It includes a relatively-large
library of Linear's "power supply" types of ICs and regulators,
et al, and ready-made circuit simulations for various switchmode
power supply configurations (and lots of other stuff), and a
pretty-handy built-in power-supply-IC selection tool
("File"-->"Switch Selector Guide"), along with suggested circuits and
simulations that are based on your specs, that you can run,
and modify and run.

And there's a thriving and very valuable discussion group and
file libraries that are devoted to LT-Spice, at
http://www.yahoogroups.com .

(Of course, LT-Spice is also an extremely-wonderful
GENERAL-purpose circuits (and more) simulator, which I think
everyone here should at least "have on hand", especially since
it's a free download, and is, for those still using dialup, mercifully,
the last time I checked, at least, less than 1 MB.

It also doesn't screw-around with WIndows' system settings, etc.,
as far as I know.) [And, for those blessed-enough to be running
Linux, LT-Spice runs great under WINE ("WINE Is Not an Emulator").
[Wait a minute! That acronym is recursive!] (Actually, I think that
I remember hearing that it runs faster with WINE than under Windows,
which doesn't surprise me. Everything probably does.)

Good luck.

Regards,

Tom


Tom Gootee

tomg at fullnet.com

http://www.fullnet.com/u/tomg

--------------------
 
J

John Popelish

Jan 1, 1970
0
I'm a bit surprised
that you have a three-terminal voltage regulator datasheet
that DOESN'T mention using the capacitors!

The data sheet link I provided mentions the capacitors in a note under
the specifications table. For instance, the table of specs on page 5
is followed by a note that says, "All characteristics are measured
with a 0.33-µF capacitor across the input and a 0.1-µF capacitor
across the output." It doesn't say they are required for stability,
but I would take it as a minimum recommendation.
 
K

Ken Smith

Jan 1, 1970
0
Linux, LT-Spice runs great under WINE ("WINE Is Not an Emulator").
[Wait a minute! That acronym is recursive!] (Actually, I think that
I remember hearing that it runs faster with WINE than under Windows,
which doesn't surprise me. Everything probably does.)

The latest I heard was a "no measurable difference" rating.

LT-Spice also is good for doing quick documentation of a circuit. It will
save the current window into a graphics file that you can then include in
a document.
 
John said:
The data sheet link I provided mentions the capacitors in a note under
the specifications table. For instance, the table of specs on page 5
is followed by a note that says, "All characteristics are measured
with a 0.33-µF capacitor across the input and a 0.1-µF capacitor
across the output." It doesn't say they are required for stability,
but I would take it as a minimum recommendation.

If I put a 10uF electrolytic across each side will that be good? Or is
that somehow too much?

Thanks for all the help.
 
R

Rich Grise

Jan 1, 1970
0
If I put a 10uF electrolytic across each side will that be good? Or is
that somehow too much?

I'd stick with the recommended value - the bulk filtering should already
have taken place back at the power supply, and the decoupling/bypass caps
should be near the chips. And FWIW, I've seen data sheets where they _do_
say that they're for stability. :)

Good Luck!
Rich
 
J

John Popelish

Jan 1, 1970
0
If I put a 10uF electrolytic across each side will that be good? Or is
that somehow too much?

Thanks for all the help.

Should work okay. It is not very critical at all. Getting them close
to the regulator is more important, so that they can't form a resonant
circuit with the wiring inductance.
 
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