LM317 Votage Regulator Instability?

[John Robertson]

On 07/14/2016 11:47 AM, Cursitor Doom wrote:

On Thu, 14 Jul 2016 10:35:32 -0700, Jeff Liebermann wrote:

Maybe you blew up the
LM317?

Don't see how that's possible, Jeff, since it's still happy to supply
passive loads that are much heavier, and at a rock solid stable output
voltage.

Jeff's point is you are at the maximum recommended input voltage,
anything changing there and the device will go south,( east, west, or
north - ) which may prove catastrophic.

John :-#(#

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[M Philbrook]

In article <nm8bok$rig$1@dont-email.me>, curd@notformail.com says...

Hi all,

I'm having serious difficulty trying to get this issue resolved and was
wondering if anyone here has come across the same problem.

I've got a linear PSU that uses a LM317 (TO-220 case) to drop 37V down to
12V for a small 555 timer board that's set up as a pulse generator (at
about 0.1Hz). The thing is, as soon as I connect the board, the 12V from
the reg drops to between 6V and 7V and wanders around this level (like as
if I'd put a really heavy load on it). But the 555 board's resistance is
about 22k ohms so it only needs a very small current. When I swap the
board for a 10W 50 ohm power resistor drawing 0.25A, the reg output is
totally fine, staying rock steady at 12V and the resistor and reg get
warm as you would expect.
I can only think there may be something about the timer board that
'upsets' the LM317 and causes it to incorrectly partially shutdown,
because nothing on the board is getting warm and trying spare identical
timer boards causes the voltage to drop by the same amount also.

I can't recall this ever happening before! Any suggestions?
TIA.

You may have two problems..

One, the timer is pulsing and thus causing a load variation.

two, the 317 is a monolithic device and is sensitive to RF, meaning
it can actually act as a detector in a sense..

of course, oscillatiion is common with these things, hence the need
for caps.

You should have a good storage cap on the output to handle riples from
the load and also use some low inductive small caps across the
electrolytic, input side and ref pin.

~50 uf for the load cap and maybe .1uf ceramic caps or the like for the
RF snub. make sure you put a .1uf in parallel with the 50 or more uf
cap.

Jamie

 

[Jeff Liebermann]

On Thu, 14 Jul 2016 18:47:03 -0000 (UTC), Cursitor Doom
<curd@notformail.com> wrote:

On Thu, 14 Jul 2016 10:35:32 -0700, Jeff Liebermann wrote:

Maybe you blew up the
LM317?

Don't see how that's possible, Jeff, since it's still happy to supply
passive loads that are much heavier, and at a rock solid stable output
voltage.

Here's another way to blow up an LM317 using nearly maximum voltage
ratings:
<http://electronics.stackexchange.com/questions/148340/how-to-protect-lm317-from-output-short>
Just short the input or output to ground and the filter caps will
discharge through the LM317.


--
Jeff Liebermann jeffl@cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

 

[Jeff Liebermann]

On Thu, 14 Jul 2016 18:45:05 -0000 (UTC), Cursitor Doom
<curd@notformail.com> wrote:

When connected to an external bench power supply at 12V., the board draws
9.3mA. The bench PSU doesn't have any issues powering this board.

When connected to PSU that uses the LM317, however, it draws 3mA and the
voltage across it drops to around 6V.

Average or peak currents? Better yet, put a resistor in series and
look at the current waveform on a differential scope (using 2 probes).
I would not be surprised to see the LM317 oscillating at some fairly
high frequency. If so, you need a BFC (big fat capacitor).

--
Jeff Liebermann jeffl@cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

 

[Jeff Liebermann]

On Thu, 14 Jul 2016 18:47:03 -0000 (UTC), Cursitor Doom
<curd@notformail.com> wrote:

On Thu, 14 Jul 2016 10:35:32 -0700, Jeff Liebermann wrote:
Maybe you blew up the
LM317?

Don't see how that's possible, Jeff, since it's still happy to supply
passive loads that are much heavier, and at a rock solid stable output
voltage.

I can't argue with that, except that several people and the data sheet
have mentioned that the LM317T doesn't like light loads and has a
minimum load current specification. Since this is your problem, not
mine, I can only offer suggestions and things to try. Since your
regulator doesn't like a light load, kindly put a load resistor across
the 12V line and see if the situation improves. (Hint: When
troubleshooting, I like to do easy things first. A resistor is easy).

Incidentally, it is possible to damage an IC to the point where it
still functions, but no in the manner commonly expected. I've done
this numerous times with RF power transistors, where there are a bunch
of transistor in parallel inside the packages. The LM317T probably
doesn't have this exact problem, but might not appreciate having some
holes blown in the IC's insulating layers. It's not easy to test for
this, so just replace the LM317T with one that has a good chance of
working.

I'm also wondering if there's something odd about your construction
methods. I've seen 3 terminal regulators work well on a PCB
breadboard with good grounding, and then go nuts when laid out on a
PCB, where the ground follows a long and twisted path to the
regulator. A big electrolytic on the output helps, but only if the
leads and traces are fairly short. If you built your regulator on one
of those solderless breadboard abominations,
<https://www.google.com/search?q=solderless+breadboard&tbm=isch>
then I suggest you find a piece of PCB material and build it better.
Then give the solderless breadboard to your worst enemy.

As for the 555, it allegedly draws very little current, but it doesn't
hurt to check. Insert a 1 ohm resistor in the PS line and measure the
drain with a differential input scope (i.e. using 2 scope probes).
Look for giant current spikes and high freq oscillations, that don't
belong.


--
Jeff Liebermann jeffl@cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

 

[Cursitor Doom]

On Thu, 14 Jul 2016 18:53:25 -0700, Jeff Liebermann wrote:

I can't argue with that, except that several people and the data sheet
have mentioned that the LM317T doesn't like light loads and has a
minimum load current specification. Since this is your problem, not
mine, I can only offer suggestions and things to try. Since your
regulator doesn't like a light load, kindly put a load resistor across
the 12V line and see if the situation improves. (Hint: When
troubleshooting, I like to do easy things first. A resistor is easy).

I did try that resistor idea before, but it didn't make any difference.
But it was 1500 ohms, so maybe too light to produce the desired effect.

I've now installed a 100uF electroylic in parallel with the existing
100nF ceramic on the output, but it's made no difference I'm sorry to say.

Incidentally, it is possible to damage an IC to the point where it still
functions, but no in the manner commonly expected. I've done this
numerous times with RF power transistors, where there are a bunch of
transistor in parallel inside the packages. The LM317T probably doesn't
have this exact problem, but might not appreciate having some holes
blown in the IC's insulating layers. It's not easy to test for this, so
just replace the LM317T with one that has a good chance of working.

OK, your 'partially-blown theory' is noted. I'll try replacing it if all
else fails as it's a bit tricky to get at.

I'm also wondering if there's something odd about your construction
methods.

Yeah, I'm aware of the importance of good layout so we can't attribute
the issue to that in this instance.

As for the 555, it allegedly draws very little current, but it doesn't
hurt to check. Insert a 1 ohm resistor in the PS line and measure the
drain with a differential input scope (i.e. using 2 scope probes). Look
for giant current spikes and high freq oscillations, that don't belong.

I do actually have a current probe somewhere. Anyway, before I try any of
that, I'm going to give these earlier suggestions from you and others a
chance:

a) Load it down with a lower-value resistor in parallel with the
electrolytic.

b)Insert bypass cap to the 'adjustment' pin

There is also the more remote poss that the regs I have a fakes. There
are fake regs out there and I've had some myself off Ebay, but I'd be
surprised if that were the case here since all the fakes I've heard about
can barely provide a few tens of miliamps. I shall order some fresh ones
from Farnell in a moment or two, though, just so I have *known* good
reference regs for comparison purposes.

 

[Bruce Esquibel]

Cursitor Doom <curd@notformail.com> wrote:

When connected to an external bench power supply at 12V., the board draws
9.3mA. The bench PSU doesn't have any issues powering this board.

When connected to PSU that uses the LM317, however, it draws 3mA and the
voltage across it drops to around 6V.

If the current requirements are that low, why don't you use something like
the LM7812 instead of the 317?

They are far simpler to use, aren't as flakey as the 317 and about as cheap.

Unless the supply the 317 is in is needed to be voltage adjustable or
high current (or higher than a LM78xx can produce), you'll save a lot of
hair pulling using the LM7812.

-bruce
bje@ripco.com

 

[Cursitor Doom]

On Fri, 15 Jul 2016 10:35:52 +0000, Bruce Esquibel wrote:

If the current requirements are that low, why don't you use something
like the LM7812 instead of the 317?

They are far simpler to use, aren't as flakey as the 317 and about as
cheap.

Unless the supply the 317 is in is needed to be voltage adjustable or
high current (or higher than a LM78xx can produce), you'll save a lot of
hair pulling using the LM7812.

-bruce bje@ripco.com

It really doesn't need to be adjustable, but if I'm going to swap to a
different device I'd really rather get something with a bit more
'headroom' than these 40V max regs, since I'm inputting ~37V. I wasn't
aware that the 317 was regarded as flaky but bow to your superior
knowledge of the subject!

 

[Cursitor Doom]

On Thu, 14 Jul 2016 18:53:25 -0700, Jeff Liebermann wrote:

Incidentally, it is possible to damage an IC to the point where it still
functions, but no in the manner commonly expected.

This is now the front-running suggestion from tests I've just carried
out. Confirmation one way or the other in a few hours.

 

[Cursitor Doom]

Well, 'er indoors came home and set me a load of tasks around the house
so I've had to put off further experimentation until tomorrow. However,
the partially-blown theory looks good, because I've hooked one of the 555
boards up to a fresh 317 which is just inserted in breadboard with NO
'supporting' caps at all and it was like *totally* fine. In fact I was so
sure it wouldn't work that I didn't bother to set the 12V up first, so
ended up with 22V for about 10 seconds across the 555. AIUI, the max for
this device is 15V, so I was convinced I'd destroyed it, but amazingly it
survived! Reset the Vout to 12 and reconnected and everything was
completely normal. UNLESS of course I've done some *partial* damage to
the 555 in the same way as I appear to have done with the first 317, of
course.
Definitive results tomorrow when I'll swap out the known good 317 for the
duff one.

 

[Phil Hobbs]

On 07/15/2016 06:35 AM, Bruce Esquibel wrote:

Cursitor Doom <curd@notformail.com> wrote:

When connected to an external bench power supply at 12V., the board draws
9.3mA. The bench PSU doesn't have any issues powering this board.

When connected to PSU that uses the LM317, however, it draws 3mA and the
voltage across it drops to around 6V.

If the current requirements are that low, why don't you use something like
the LM7812 instead of the 317?

They are far simpler to use, aren't as flakey as the 317 and about as cheap.

Unless the supply the 317 is in is needed to be voltage adjustable or
high current (or higher than a LM78xx can produce), you'll save a lot of
hair pulling using the LM7812.

-bruce
bje@ripco.com

What's flaky about the 317? You can blow them up by overvoltage, or by
pushing a large current backwards into the ADJ or output terminals, but
otherwise they're pretty well-behaved. Well, they will oscillate if
you're running at low gain and you put 10 nF on the output. However,
it's a 1-A class regulator, and just about any circuit needing anything
like that much juice will have a lot more capacitance than that.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

160 North State Road #203
Briarcliff Manor NY 10510

hobbs at electrooptical dot net
http://electrooptical.net

 

[whit3rd]

On Thursday, July 14, 2016 at 11:45:07 AM UTC-7, Cursitor Doom wrote:

When connected to an external bench power supply at 12V., the board draws
9.3mA. The bench PSU doesn't have any issues powering this board.

When connected to PSU that uses the LM317, however, it draws 3mA and the
voltage across it drops to around 6V.

The LM317 requires an output current of 5 mA or more, to bias it. If you
really ARE only drawing 3 mA, that could be the problem.

 

[Cursitor Doom]

On Sat, 16 Jul 2016 01:07:53 -0700, whit3rd wrote:

The LM317 requires an output current of 5 mA or more, to bias it. If
you really ARE only drawing 3 mA, that could be the problem.

Nope. It was still malfunctioning even when loaded down with resistors in
parallel to the load to increase the current draw.
I expect to have a definitive answer to what was responsible for the
issue at some point today.

 

[Bruce Esquibel]

Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:

What's flaky about the 317? You can blow them up by overvoltage, or by
pushing a large current backwards into the ADJ or output terminals, but
otherwise they're pretty well-behaved. Well, they will oscillate if
you're running at low gain and you put 10 nF on the output. However,
it's a 1-A class regulator, and just about any circuit needing anything
like that much juice will have a lot more capacitance than that.

Well, for all the reasons you mentioned and the fact this thread is like
2 or 3 days old and the OP still hasn't made it work.

I know the 317 is probably the most common regulator used but just isn't
the best choice in some cases, like this one. Cost I'd guess is the best
reason to use them.

The LM78xx (and LM79xx for negative outputs) are far simplier to use at a
slightly higher cost. They have excellent ripple rejection, thermal
protected and short circuit protected. Usually except for a .001 cap they
don't even need a heat sink (current dependent of course) for the TO-220
package.

Even if they need a heat sink, the tab is ground so you can just nut/bolt
it to the chassis (provided it's at ground which is usually the case).

in-gnd-out is it, no fuss, no muss.

-bruce
bje@ripco.com

 

[Cursitor Doom]

On Sat, 16 Jul 2016 11:09:04 +0000, Bruce Esquibel wrote:

I know the 317 is probably the most common regulator used but just isn't
the best choice in some cases, like this one. Cost I'd guess is the best
reason to use them.

Yeah, I've replaced this one with the 60V variant just to be on the safe
side as my first choice was too near the supply rail for comfort. I'd
obviously partly damaged the old one with my prodding around under power
and the cap probably partially discharged through it.
So the 10 shilling postal order and box of After Eight mints goes to....
Jeff Liebermann!

 

[Jeff Liebermann]

On Sat, 16 Jul 2016 16:03:11 -0000 (UTC), Cursitor Doom
<curd@notformail.com> wrote:

Yeah, I've replaced this one with the 60V variant just to be on the safe
side as my first choice was too near the supply rail for comfort. I'd
obviously partly damaged the old one with my prodding around under power
and the cap probably partially discharged through it.
So the 10 shilling postal order and box of After Eight mints goes to....
Jeff Liebermann!

The logic was easy. You had already done most of the obvious
troubleshooting (increase load, add capacitors, check currents). There
was only one active device capable of causing a problem. All that was
necessary was to contrive a reasonable explanation and failure
mechanism. The near maximum voltage was an obvious problem and
blowing it up by shorting the input or output to ground was quite
likely. As Sherlock Holmes may have said:
Once you eliminate the impossible, ridiculous, absurd,
and disgusting, whatever remains, no matter how improbable,
must be what's wrong.

Thanks, but no gratuities needed. Besides, the body mechanic says
that I shouldn't eat chocolate. Sigh.

--
Jeff Liebermann jeffl@cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

 

[Cursitor Doom]

On Sat, 16 Jul 2016 19:45:41 -0700, Jeff Liebermann wrote:

The logic was easy. You had already done most of the obvious
troubleshooting (increase load, add capacitors, check currents). There
was only one active device capable of causing a problem.

False assumptions have always plagued me. In this instance I falsely
assumpted that because the reg was functioning fine under a heavy passive
load, there was nothing whatsoever wrong with it. I didn't admit to the
possibility of partial damage which could manifest under other load
conditions and that made me completely blind to fixing the problem.
Lesson learned - I hope!

 

[Cursitor Doom]

On Sun, 17 Jul 2016 08:34:01 -0700, Jeff Liebermann wrote:

Assumption is the mother of all screwups. If it helps, I think everyone
has the same problems with bad assumptions. It's our way of simplifying
our lives. Our brains are pre-programmed to look for patterns and
repetitions, so we generalize based on the available but usually
insufficient evidence. Don't fight it. Assumption and generalization
work well if properly used.

I wouldn't have minded so much, but a while ago we had a thread on the
golden rules of troubleshooting and my contribution was, of all things,
"assume nothing" (!)

I could go on forever on how I do troubleshooting. However much of it
is more intuitive than logical. That's guaranteed to produce more
confusion than enlightenment, so go with your first guess. It's usually
right.

So it's more of an art than a science? I'm not a professional technician
so I don't do these sort of things on a daily basis. Consequently my
reasoning skills are nowhere near as strong and well-developed as someone
who's fully immersed in the business.

 

[Jeff Liebermann]

On Sun, 17 Jul 2016 11:27:18 -0000 (UTC), Cursitor Doom
<curd@notformail.com> wrote:

False assumptions have always plagued me. In this instance I falsely
assumpted that because the reg was functioning fine under a heavy passive
load, there was nothing whatsoever wrong with it. I didn't admit to the
possibility of partial damage which could manifest under other load
conditions and that made me completely blind to fixing the problem.
Lesson learned - I hope!

Assumption is the mother of all screwups. If it helps, I think
everyone has the same problems with bad assumptions. It's our way of
simplifying our lives. Our brains are pre-programmed to look for
patterns and repetitions, so we generalize based on the available but
usually insufficient evidence. Don't fight it. Assumption and
generalization work well if properly used.

In my life, I've noticed that first impressions are usually correct.
That applies mostly to people, but also to things. I just look at the
circuit, the components, the PCB, or the schematic, and ask myself
"what's the most likely culprit". Never mind logic, deduction,
measurement, and flow charts. Just intuition based on experience. For
example, I always look for bulging capacitors, previous repairs,
crappy soldering, corrosion, bad wire connections, loose parts, signs
of overheating, loose screws, and an assortment of common problems
that can be found by inspection. Do it often enough, and it becomes
automatic[1].

We have one big advantage with repair. We can assume that the circuit
was designed correctly and that it did work at one time. All we need
to do is find the one part that died. However, this was not really a
repair task, but rather a construction or engineering task, which
cannot make the assumption that the circuit was previously working.
So, I ask myself, what is the most likely screwup someone could do and
what were the likely causes of the circuit behavior? The behavior
made no sense, so went through my history of all the things I've
destroyed on the bench. I started with improper construction and
ended with the effects of overvoltage and overload while looked for a
pattern. Overvoltage and temporary wiring errors are my big problems.
Often, the combination blows things up.

If intuition doesn't pay off, I make measurements. I suggested that
you use a scope to look for oscillation and current spikes. It would
have helped identify the exact symptoms, but would not have pointed
directly to the LM317T.

I could go on forever on how I do troubleshooting. However much of it
is more intuitive than logical. That's guaranteed to produce more
confusion than enlightenment, so go with your first guess. It's
usually right.

Good luck.


[1] I've been looking at resistor color codes for much of my life. I
don't look at the colors individually and decode them into numbers and
multipliers. I look at the entire resistor, automatically recognize
the pattern, and (for example) 10K pops into my head. The same can be
done with defective PCB's and products. Something that looks "odd" is
often the culprit.

--
Jeff Liebermann jeffl@cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

 

[Cursitor Doom]

On Sun, 17 Jul 2016 13:10:53 -0700, Jeff Liebermann wrote:

10K hrs is about 5 years of total immersion. I've found this to be true
for most everything I've tried, except my crappy piano playing. Well
over 10K hrs of practice does not compensate for a fundamental lack of
talent.

Tell me about it! Having a natural gift for such activities makes a
massive difference.
BTW and FWIW, there's a very interesting discourse on the correct
approach to troubleshooting in Robert Persig's Zen and the Art of
Motorcycle Maintenance. And it's not restricted merely to motorcycles,
either:

https://www.amazon.com/Zen-Art-Motorcycle-Maintenance-Inquiry/
dp/0060589469/ref=sr_1_1?ie=UTF8&qid=1468793289&sr=8-1&keywords=zen+and
+the+art+of