On Mon, Mar 11, 2013 at 7:52 PM, Mike Ryan <mryan001@tampabay.rr.com> wrote:
>
> The landscaping lights turn out to be 12 volt AC powered and not 12 volt DC
> powered.
I have a home security system that is 12 volt AC powered; there is a
wall-wart transformer that steps it down from line voltage. In the
"can", the current is rectified and used to charge a sealed lead-acid
battery.
Obviously, if the lights themselves are LEDs, then something,
somewhere is rectifying the current (possibly the LEDs themselves,
although usually they won't withstand a large reverse bias).
The forward voltage on an LED is usually on the order of a couple of
volts, and there must be a current limit to prevent instant burn out.
Of course, if you wanted a completely linear solution, you could start
with a transformer with a 10:1 turns ratio to go from line voltage to
12 volts, then use a diode bridge rectifier and a resistor upstream of
the LEDs. That would be very quiet, and about 17% efficient (with
most of the remaining 83% of the power consumed to warm up the current
limiting resistor). Therefore, I would say that even if the current
is distributed to the lights as 12 volt AC, it is very likely there is
a switch-mode device in the circuit upstream of the LED, and that that
device is the source of your RFI. It very well might be integrated
into the LED bulbs themselves (like the electronic ballasts in compact
fluorescent bulbs), in which case you are going to have to snuff it at
every bulb.
--
Charles M. Coldwell, W1CMC
Belmont, Massachusetts, New England
"Turn on, log in, tune out"
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