[PLUG-TALK] Power line noise

Keith Lofstrom keithl at kl-ic.com
Fri Jun 10 13:17:59 PDT 2016

On Wed, Jun 08, 2016 at 07:35:13PM -0700, Denis Heidtmann wrote:
> I wonder what is acceptable for noise on the power line (residential)?  I
> had reason to check, so I used a 0.1 uF cap in series with a 100 ohm
> resistor in series with another 0.1 uF cap connected across the 120V line.
> I check with a 'scope across the 100 ohm.  Last night I saw a 4 V pulse
> every cycle of the 60 hz.  This morning it was not there.  There were other
> pulses present, always synchronous with the 60 hz.  It may be that the
> pulse is due to the LED streetlights, since that is the biggest change
> between last night and this AM that I can think of.

The capacitor into the resistor makes a "differentiator",
so a fast 4 V spike or edge passes through while the
60 Hz signal is attenuated 500x.  Look at the unfiltered
with a 10 MΩ 10x probe, it would tell you what part of
the cycle the pulse/edge is happening in.

LED streetlights are a good guess.  Those probably run
the line voltage through a switching power supply to
generate DC power for the LEDs, and a switcher can draw
excess current and "sag" the line when it turns on,
early in the AC cycle.  However, streetlights are
probably on a different feed, unless there is a PGE
power meter on each one.

Another possibility is the charger for a neighbor's
plug-electric car.  Those chargers draw a hell of a
lot more power than a streetlight, and they would be
on the same residential power feed.

Off on a tangent: I recall your neighborhood streetlights
being white instead of yellow.  If they are the same
"blue plus yellow phosphor" LEDs that Dr. Nakamura
described at his PSU lecture last Wednesday (2016/06/01),
they could be pumping out a lot of melatonin-destroying
blue light at night.  Have you checked the spectrum?

Perhaps with good blackout curtains, you would be
getting a full night's sleep rather than playing with
oscilloscopes.  Doesn't help me, but my judgement and
time management skills are inferior to yours.

BTW, for those of you curious about the spectrum of
a light source, the shiny microgrooved surface of
an ordinary commercial CD can be used as a cheezy
diffraction grating. Tilt the reflection until you see
rainbows.  DVDs work too, but seem to pass less blue.


Keith Lofstrom          keithl at keithl.com

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