Spehro Pefhany wrote:
I just don't use LDO where at all avoidable. And they usually are
avoidable, meaning no tunnel of death :-)


LDOs have other pathologies which often aren't mentioned in the
datasheet or possibly the chip guys didn't even know about them. My
worst one was a LM29-something. Client insisted on keeping it against my
advice. Phssst ... *BANG*. Hmm, we had babied the ESR so what the hell
....? Call into the mfg. The engineer there grew concerned and suggested
a phone meet with the chip guys present. One could here some shuffling
there in their room. More and more people came in, papers were perused,
lots of mumbling in the background. Suddenly one guy let of an "Oh
drat!" Turns out this LDO didn't "like it" if the source impedance got
too high and we were feeding it from a week isolated path.


No imagine scenario #2: Joe Digital sees there's a nice 3.3V rail and
happily sprinkles the usual 0.1uF caps here and there plus a big fat
10uF ceramic just to make sure. This brings the combined ESR down way
below the minimum ... phssst ... *BANG*.

--
Regards, Joerg

http://www.analogconsultants.com/

"gmail" domain blocked because of excessive spam.
Use another domain or send PM.

On Jun 15, 12:00*am, Joerg <notthisjoerg...@removethispacbell.net>
wrote:
As a rule with the 7085 we would always just use a normal 100uF NON-
ESR electrolytic near the regulator. We found that when driving long
lines the 78xx series could oscillate wildly if one used the normal
datasheet 100nF. Then is just became a habit to use the 100uF. Never
had trouble doing that.

Rocky

Joerg wrote:
Thank you both for the explanations and references. I understand about
half. I will have to take another stab at it later.

Alan Nishioka

In message <h-Odnf55oJeji8nVnZ2dnUVZ_tLinZ2d@giganews.com>, kittykgl
<kittykgl@gmail.com> writes
present.

OK, a story...

Many years ago (20 or more) I got the job of repairing and testing
customer returned linear voltage droppers (24v to 12V DC for use in
trucks) which used a 7812 driving the base of one or more 2N3055 like
transistors (various types, BD, 2SC, 2N, MJ etc...) depending on the
'rated' output current.

A good proportion appeared to work perfectly well measuring the output
with an old AVO meter and a fairly heavy dummy load, the ones that
didn't were shorted pass transistors or other silly faults.

One unit had a label attached saying FM radio goes off when connected.
I thought the silly trucker had shorted his accessory feed or
disconnected it fitting the dropper but no. I tested it with the meter
and my resistor bank load and it appeared to perform fine but I turned
on my workshop radio, sure enough I couldn't hear *anything* apart from
static when this unit was turned on.

None of these devices had the recommended capacitors or any kind of
noise filtering on the input/output of the 7812 and they were
oscillating wildly with lots of RF being generated (and possibly
amplified by the pass transistors) as my 'scope proved. Fitting the
datasheet recommended caps on the input and output killed the
oscillation stone dead.

Measuring with and old AVO indicated no problem, measuring with a Fluke
meter (8022B IIRC) gave me a different voltage depending on where about
on the output wire I probed, I can only assume I was finding sub
wavelength points.

I retrofitted caps to all the fifty or so in the batch and then to the
several hundred the importer had.
--
Clint Sharp

On Sun, 15 Jun 2008 11:07:33 +0100, the renowned Clint Sharp
<clint@clintsmc.demon.co.uk> wrote:


It may not have even been the regulators. The emitter follower
configuration is also prone to oscillation if the load has substantial
capacitance (Colpitts), so you have several chances for trouble there.
I don't think you could make that old slug 2N3055 oscillate at
FM-radio frequencies with a sledgehammer (it could barely do HF
audio), but some of the old 2SC power parts have pretty high ft.




Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
speff@interlog.com Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com

On Jun 15, 1:55*pm, Spehro Pefhany <speffS...@interlogDOTyou.knowwhat>
wrote:
problems. They had a much better HF response and would readily
oscillate at RF frequencies. This was an issue in some electronic
iginition systems we built.
Rocky

On Sun, 15 Jun 2008 07:55:07 -0400, Spehro Pefhany
<speffSNIP@interlogDOTyou.knowwhat> wrote:

The 2N3055 specifications are not very demanding, so there have been
rumors, that many manufacturers relabeled chips as 2N3055 if it was
out of specs for the intended high quality/high price part.

I have also heard some decades ago the story that some out of specs
VHF power transistor was relabeled as 2N3055, but this caused a lot of
stability problems in a circuit layout not suitable for VHF power
transistors :-).

Of course, this might be an urban legend.

Paul

On Sun, 15 Jun 2008 05:25:14 -0700 (PDT), the renowned Rocky
<RobertGush@gmail.com> wrote:

The Motorola ones (MC7805, not LM7805) have weasel words in the
datasheet that should alert the careful (ie. paranoid) designer.



Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
speff@interlog.com Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com

On Jun 15, 7:13*pm, Spehro Pefhany <speffS...@interlogDOTyou.knowwhat>
wrote:
Rocky

In message <ql0a54tl4b8albblih1cjimkk2uuk9kecb@4ax.com>, Spehro Pefhany
<speffSNIP@interlogDOTyou.knowwhat> writes
mounted in IBM XT PSU cases, switchable for different loads and a DC fan
running on the higher current settings. I suspect rather low
capacitance.
50-55MHz so I suspect that was the source, whether or not the
transistors were amplifying it I have no idea and I can't remember what
transistors were fitted to that particular dropper as the manufacturer
covered them to prevent the collector shorting out.

I didn't remove the covers on many unless I actually needed to replace
transistors (I was testing and repairing 30+ a day..) I did see a fairly
mixed bag of transistors, I guess the factory was fitting whatever came
cheapest so it might have been something with fairly high ft.

fitted... I used to make good money repairing those too.

--
Clint Sharp

On Fri, 13 Jun 2008 21:42:49 +0200, Anton Erasmus
<nobody@spam.prevent.net> wrote:

Well, that clears it up. While 2.2mF is literally the right
capacitance, in the USA I rarely see the prefix milli for capacitance
- such a value is usually written as 2,200uF, as is shown on the
schematic.

What made me think it was 2.2uF is that: it wouldn't be unusual to
see a capacitor of perhaps 2.2uF right at the input of a regulator,
especially if the regulator had some distance of wiring from that
2,200uF power supply capacitor. This makes a lower impedance on the
regulator's input, which also keeps it from oscillating.

Joerg wote of just this in another post in this thread:

LDO's certainly appear to be more sensitive to source and load
impedance than regular "High DropOut" linear regulators, but all
linear regulators are high-gain devices, and are likely to oscillate
if connected to the "wrong" impedances.