[time-nuts] Totally unrelated, but..
cfharris at erols.com
Wed Dec 7 15:56:55 EST 2016
I don't think so. I first ran into a batch of LM340-5's
that were excellent oscillators back in the 1970's... long
before counterfeiting was even remotely possible.
The symptom is the regulator puts out only 4.5 out of 5V.
LM309's were, however, totally immune.
Usually, I had to be really bad to make it happen, things
like using clip leads between the power supply and load
with the LM340-5 dangling in between.
The answer is as simple as a couple of 0.1uf ceramic caps
soldered right at the input and ground, and the output
and ground pins.
LDO (low dropout) regulators are very susceptible to
oscillation. They need to have a couple of hundred uf
of good quality capacitance right on the input and output
leads. Where people usually get in trouble, is in not
knowing that electrolytic capacitors lose most of their
capacitance as the temperature starts hovering around 0C.
The circuit works great on the bench, but fails when out
hanging on a light pole...
Joe Leikhim wrote:
> Could the low noise parts actually be counterfeit, relabeled as such?
> Is the circuit the regulator feeds sensitive to a narrow band of voltage that the
> "good regulator" is outside of?
> Try replacing the regulator with a battery supply and resistor divider to attain the
> working voltage. Move the voltage around. A good potentiometer and stiff filter
> capacitors are recommended so as not to introduce "pot noise".
> Is something corrupting your test procedure? I had a circuit that misbehaved due to
> floating logic pins reacting to static electricity on the work bench. Another time a
> diode was photosensitive.
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