Basic Voltage Regulator Help

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:

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!

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.

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

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

.

(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

--------------------

snipped-for-privacy@fullnet.com wrote: (snip)

The data sheet link I provided mentions the capacitors in a note under=20 the specifications table. For instance, the table of specs on page 5=20 is followed by a note that says, "All characteristics are measured=20 with a 0.33-=B5F capacitor across the input and a 0.1-=B5F capacitor=20 across the output." It doesn't say they are required for stability,=20 but I would take it as a minimum recommendation.

In article , wrote: [...]

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.

--
--
kensmith@rahul.net   forging knowledge

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

Thanks for all the help.

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

Should work okay. It is not very critical at all. Getting them close=20 to the regulator is more important, so that they can't form a resonant=20 circuit with the wiring inductance.