Jim WA9YSD writes...
> I had them out another time to help with white noise. I found out
> that those new stop light replacements are LEDs. They generate lots
> of noise. Nothing they can do about it. You Think?
Here's a provision of the New York State DOT specification for LED
traffic signal indications:
5.4 Electronic Noise. Units supplied to this specification shall meet the
requirements of the
Federal Communication Commission Title 47, Subpart B, Section 15.109(b)
regulations
concerning the emission of electronic noise by Class A digital devices.
This is a typical entry in LED signal head specifications, and all
products on the market at least claim to meet this requirement, for
whatever that's worth.
The few schematics I've reviewed (the actual schematics tend to be
trade secrets and the guts of these things are usually potted to make
quick reverse engineering a bit challenging) show that they run on AC,
using a transformer or running a string of diodes in series. They
usually run diode pairs or pairs of strings of diodes in parallel and
oppositely polarized to illuminate on both sides of the AC wave form.
They also often have filtration to prevent parasitics and to improve
the efficiency. None of them run on digital controls, such as with
square-wave inputs. They are all fed by 120VAC from load switches that
are usually something like a bit switching transistor. The load
switches receive a 24VDC discrete signal from the controller (or
controller interface).
It doesn't seem to me that these would really generate a lot of RFI
unless something was wrong, but I have not studied that aspect of them
(nor am I qualified to go very far down that road).
One thing is for sure: All signal heads use or will soon use LED
displays. They typically consume 7-25 watts instead of the usual
67-watt bulb (for mostly obsolete 8-inch heads) or 133-watt bulb for
typical 12-inch heads. Also, they have a useful life approaching
100,000 hours, versus 8000-10,000 hours typical of the high-end
traffic signal bulbs. The only two problems with them are 1. drawing
enough current to keep conflict monitors happy, and 2. not getting
warm enough to prevent blowing snow from building up.
Rick, KR9D
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