wrote in message
...
On Mon, 31 Mar 2008 12:19:50 -0700 (PDT), "
wrote:
On Mar 31, 12:34 am, wrote:
Try hehttp://ogmtechnical.blogspot.com/
Thanks for that link. Speculating between the lines and being a tad
skeptical of company self-reporting it sounds like they may have unit
to unit differences in RFI and hence a manufacturing/QA problem.
Sounds more like there are variables in how they are installed.
Bingo! We have a winner!
PWM
regulators are made in the gazillions for the automotive market. As I
recall the gent in the PS letter was quoted as saying (more or less)
"gee we've installed countless LEDs in cars and we never have had a
problem with RFI there, it must be a user thing." But, boats aren't
cars. Rather like gasoline engines that need to be modified for
marine use with things like spark arrestors and bilge blowers because
of potential problems in boats, LEDs for marine use should take
particular care with RFI because of potential problems. PWM
regulators are great but because of their timing circuitry are
potential RFI emitters.
The current used to power an LED mastlight is TINY. Even if the regulator
WERE a
bit noisy, it would be so weak that it likely wouldn't have any effect at
all
unless maybe the regulator was mounted directly on the antenna whip. The
regulation in my anchor light is completely surrounded by the grounded
metal
bayonet base.
If the switching signal (PWM) is run into an inductive load in a poorly
designed circuit you could have tens or even hundreds of volts as a
transient signal. It's not the current but dI/dt that makes the voltage. One
does not need an antenna to radiate. A plain old wire can work just fine.
Since this is all so variable, it points right at the installation factors.
By the way, radio amateur QRP distances show over 1,000 miles range with a
single NAND gate as the output driver. Figure PWM waveform is 5V @ 5mA =
25mW-- 14 dBm into 50 ohm. VHF radio sensitivity typically 0.25 uV into 50
ohm ---1.25e-15 Watt or -120 dBm. Say the PWM operates at 100 kHz so the
1500th harmonic has to be down -134 dB to be undetectable on the VHF. The
Fourier expansion of a square wave PWM shows the harmonic coefficients
decreasing as 1/n, n being the harmonic number so the VHF in-band harmonic
is 1/1500 of the original signal or about 33 dB down. That leaves about 100
dB of required attenuation (if all the DC input power were radiated) not to
interfere with the VHF. FCC requires the harmonics to be much lower, which
they are in order to comply. Automotive testing is rigorous and thorough and
it takes a real screw up to get the thing to radiate and cause interference
in the boat. The lesson here is if you don't understand electrical
installations, hire an experienced (even licensed) technician to do the work
It may save your life.
By the way, some remarks that Roger made have reminded me that
changing the bulb in your nav lights from the manufacturer's tested
type to anything else will take the lamp out of compliance. I'm not
lawyerly enough to know what having non-USCG approved lighting does
to a skipper's liability. It is likely that many LED bulbs will not
comply with Annex I section 8 despite being visible at the ranges
required by rule 22... It's also likely that USCG stamped LED lamps
don't meet annex I.8...
The USCG doesn't test devices and approve them. They publish standards,
and a
laboratory tests your product and charges you a fee to attest that it
meets the
standards. If you have an unapproved device and it becomes an issue in
court,
you can still retroactively have the device examined and declared
compliant.
Good info on the retroactive part. You can't do that for FCC.
A kerosene lamp is perfectly valid as an anchor light as long as it meets
the
standards. No actual "pre-approval" is needed to use it. You just have to
have
confidence that it can meet the criteria. My LED anchor light is USCG
approved
because I say it is. No one has asked me to prove it, but I'm sure I could
hire
someone to validate my claim if it was ever needed.
Meanwhile, just hope that when you have to defend your unit with a stamp
of
approval on it from some lab, the opposition doesn't take the wind out of
your
sails by claiming it was partially obscured by a blob of bird **** on one
side
and therefore didn't meet the standards at the time of the incident.
Let's put windshield wipers on all navigation lights!