LM2940 drives me crazy

Erlend

I just needed to hook up a voltage regulator - real simple - I thought. I've done this a hundred times before. But - what I get is not the 5V, but around 9V with a small load resistor, or around 11V without. What is this? I turns me crazy!
I did try with two 47uF capacitors over the input and output (in case of instability issues) but with no change. Input voltage is 13.6VDC.
This is just too simple. I feel stupid.

Erlend

RDL2004

You probably need the correct cap on the output. It must be a low ESR capacitor. It's very finicky about this.

Erlend

The Forum rules!
I did first try 47uF's over input and output, but I must have had a lousy contact or something, because when I now did as RDL said, and put on a Cout (still 47uF), the voltage dropped down to nice 5V : )
The good old 7805 were never that finicky, were they?

Erlend

btw, I really need something beefier - 5A or even 10A - and I found some automitve modules on €bay, but does anyoune have a good recommendation?

RDL2004

Be advised that it may still be flaky. The output capacitor really needs to be a low ESR type. The datasheet diagram wording is a bit odd. What they mean is that the regulator may actually work even if the ESR of C2 is as high as 3 ohms. In general, electrolytic caps are bad for having high ESR and tend to change a lot as they age. You can always parallel capacitors to reduce the ESR of electrolytics or increase the capacitance of better quality types such as polyester or ceramic.

Leon

The 7805 and 7812 could be fussy. A friend of mine copied my circuit for a floppy drive PS, and it didn't work. I found that the regulators were oscillating because he'd left off the output capacitors.

Erlend

I'll dig out a polyester capacitor to parallel with the electrolytic one. Guess I have been lucky all these years - or the circuits on the receiving end have had the required capacitors.

Erlend

Phil Pilgrim (PhiPi)

Multilayer ceramic caps are a very good choice where high capacitance, low ESR, and low price are desired. But even they can be troublesome with some regulators. For example, the LM2937 that's ubiquitous in Prop circuits specs an ESR no lower than 0.01 ohm.

So you really do have pay close attention the the regulator's datasheet.

-Phil

Tracy Allen

Ceramic capacitors in the X and Z class can have stability problems too, even to the point of emitting a high pitch squeal as the regulator output oscillates. I recall there were squeals like that from some older Parallax boards that used the '2940. High µ ceramics have a high voltage coefficient, capacitance decreases as voltage increases, and that effect contributes to instability. Electrolytic and tantalum capacitors do not have such a pronounced voltage dependence. Modern regulators may include as a feature a sentence like, "stable even with small ceramic capacitors."

For the requirement, to bring a 12V nominal battery voltage down to 5V at 5A or 10A, a switcher would be the way to go, a module. I don't have a specific suggestion.

kwinn

As per Tracy's post a switching regulator is much better for that kind of current and voltage drop but if you want a higher current linear regulator put a high power PNP transistor in parallel with the 2940. The transistor emitter goes to the + supply, the base goes to the input pin of the 2940, and the transistor collector connects to the 2940 output.

RDL2004

I just wanted to point out for the beginners, or anyone else really that may run across this thread, a very good introduction to capacitors is EEVBlog #33. It's a two part episode on YouTube. Be ready to take notes. You will learn a lot.

LoopyByteloose

If you want 5 to 10 amps at +5 volts, you might as well use a pre-made switcher. There are no linear regulators that go above 3 amps and wallwarts generally don't go about 2 amps.

Of course, if you want to regulate a battery source for 10amps, a different approach is needed. But it is still a switching regulator of some sort -- not a linear device.

I see you mention 13.6 VDC input. If that is from an automotive source, it will be higher during charging, about 14.2 VDC. And you more than likely want some cleanup of your power input to avoid spikes and noise from the automotive power source, before you actually regulate the voltage with any precision.

Generally, blocking anything that gets over about 20 volts prevents potential damage of your regulation circuit.

The LM2940 actually was designed to provide power in an automotive environment and has a lot of features that the LM7805 lacked. LDO is important for automotive instabilites, and so is reverse polarity protection. Thermal shutdown is handy too.


http://forums.parallax.com/showthread.php/151631-Steady-12V-from-car-cig-lighter-socket-how-to

Erlend

The power source is a 12v lead-acid battery, but not automotive. It will be charged by solar panels and should be free from spikes as such. Back in the old days a fat transistor would do the job, but would need a huge heatsink. Certainly a switchmode module is more efficient - and obviously not too expensive - so I opted for this one from the bay. I'll find some nice capacitors to put on the small regulator I've built - in this thread - and use it for some other project where 1A is sufficient.
Mainly I need the 5V heavy current for driving an igniter coil, which means it does not need to be clean, nor continous load capable. Maybe I will tap the same to step down further to 3.3V for the propeller and stuff too, but maybe not. I suspect that coil will generate a lot of EM noise, so it may be better to keep it separate.

Erlend

davejames

Loopy Byteloose wrote: »

There are no linear regulators that go above 3 amps and wallwarts generally don't go about 2 amps.

Loopy - both TI (TPS7-series) and Microsemi (LX8-series) offer linear regulators in excess of 3A with Microsemi's going as high as 10A.

RDL2004

The LM338K will do 5 amps and the LT1038 from Linear can do 10 amps. In the early days of personal compers there were many 5 volt regulators in the 8 to 10 amp range. They will all require massive heatsinks though.

LoopyByteloose

davejames wrote: »

Loopy - both TI (TPS7-series) and Microsemi (LX8-series) offer linear regulators in excess of 3A with Microsemi's going as high as 10A.

Hmmm.. Can one actually buy these in small quantity? Or are these just part of a catalog of device that one must order 1000 or more to buy?

RDL2004

Can one actually buy these in small quantity?

Sure. I didn't check Digi-Key, but Mouser had a bunch of the TPS7 series (a couple were 7.5 amp versions), and several types of the LM338. Hundreds and hundreds in stock, all available in singles if that's what you need.