lights - Wisely on Diigo

The white light produced by a white LED is fluorescent light, at
least in part.
Typically, bright yellow fluorescent phosphors are activated by a
high-efficiency blue LED, and the result includes both yellow and
blue colors.
Even though the result is white-ish, it does not cover the full
incandescent spectrum, so filtering may not be particularly useful

              
            

Normally, however, when manufacturers talk about LED "failure"
over time, they are not talking about LEDs actually going dark, as
many of the Philips LEDs do.
Instead, LEDs normally do not die, but simply get dimmer to a point
where the result is no longer acceptable.
In these Philips strings, one might hope that would mean 25,000 hrs
for each LED, or 25,000 hrs/168 hours-per-week =
 149
weeks or about 3 years of continuous full-power operation.
Unfortunately, the high rate of LED total failure in these Philips
strings has obscured any LED fading issues.

              
            

The Philips 60-LED multi-color strings are arranged in two
circuits of 30 LEDs each with a total forward voltage of about
75 volts.
The LEDs are off until at least 75 volts is applied.
Abstractly, one could have any number of circuits, so we could
see 30, 60 and 90 bulb strings which are virtually the same
except for bulb count and length.
Other manufacturers apparently use circuits with 35 LEDs, thus
having a forward voltage of about 87.5 volts, which should be a
little more efficient.
Since red LEDs have less forward voltage, they can occur in larger
numbers, such as a single circuit of 50 with a total forward
voltage of about 100 volts.

              
            

Just like the incandescent strings, the Philips strings can
be cut in the middle (with the power off), where there are only
2 wires.
The first half can be used after insulating the cut wires with
shrink tubing or electrical tape.
The second half can be used by finding another fused plug, perhaps
from an old incandescent string, and splicing that into the cut
wires.
Or maybe the second half could be used for light replacements.

              
            

When the LEDs conduct, we have about 30 LEDs eating up about 2.5V
each, on average, so little current flows until the AC gets above
about 75V.

When the AC wave goes negative, presumably the sum of all the LED
reverse voltages exceed 168V so that significant reverse current
does not occur.

Not every circuit which uses LEDs is necessarily efficient.
The LED light strings described here are efficient largely because
they use many LEDs in a series circuit.
That reduces the line voltage by about 75 volts before it gets to
the current-limiting resistor.
That means only a modest fraction of the total line voltage is
wasted heating the resistor.
With this efficient topology, we can use the line voltage directly,
without voltage-changing transformers or power supplies.
And we can use just a single resistor for the whole circuit, instead
of needing a separate resistor for each bulb.

              
            

Alternately, a power supply with current control could produce
high-voltage DC, which would eliminate light pulsing.
That would come at substantial cost, and introduce potential
reliability issues when compared to the resistor alternative

Yet another possibility might be to use a transformer power
supply and run the string on low-voltage DC, probably with LEDs in
parallel.
That would produce continuous non-blinky light.
But a transformer for each string would be expensive, and a larger
transformer to handle multiple strings would be even more expensive.
Since a lower voltage design implies higher current, the light
string wires would handle more current, although the present wires
are probably thick enough anyway.
Such a design probably also implies an extra resistor for each lamp
and much lower efficiency.

              
            

Dimming

In general, all lights last longer if they are dimmed, and
there are several possibilities:

The simple incandescent trick of using a power diode to restrict
lighting to one side of the AC wave does not work with LED strings
that already use only one side of the AC wave.
One could add a power resistor in series with the string, but
the resistor will get hot, and the desired resistor value will
change as we daisy-chain more strings.
Although it is possible to connect two LED circuits in series,
that seems unlikely to work very well since it would take 150V just
to start conduction, producing brief pulses of light.
LED strings can be dimmed using conventional triac lamp dimmers,
which is probably the overall best solution.

In multi-color strings, the blue, white and red may be most apparent,
because they typically have the highest efficiency and thus produce
the most light from a tiny trickle of current.
The dimming is far beyond what is needed to extend lamp life, but
having lights be partially lit can be an issue in other ways.
And the same issue occurs with most devices using triac power
control, such as:

Lamp dimmers,
X-10 remote control modules,
Photoelectric outdoor light timers, and
Anything other than mechanical contacts.

Using less power than a 4W bathroom nightlight, a 3.9W

white rope light around a mirror provides enough light to actually
read pill bottle labels at night.
Two 4.8W
 strings of white LED lights on a colored wall
behind a bed add to small and distant incandescent light to provide
enough light for reading.
A single white string hung in 4 lengths in a white wall corner
converts a wasted corner into an effective reading nook.
For computers using a small Uninterruptable Power Supply (UPS),
a single 4.8W
 white string should provide more than enough
keyboard light to shut the computer down during nighttime power failure.

              
            

An unusual design advantage of these particular strings is that
they use only one side of the AC power cycle.
When we connect several strings in a daisy-chain connection, we
can rotate the plugs and cause any string to use either half of
the cycle.
There is no apparent visual difference, until we start fooling
with the power.

With appropriate power diodes, it is easy to build a controller
to switch each side of the AC cycle on and off, and so
independently control two different groups of LED strings
from the plug end alone without separate wiring.

              
            

Faceted Mini White
051 04 1783

Another string of 60 lights, in two series circuits, rated at
4.8 watts.

One of these strings, in 4 passes at the corner of a
light-colored wall, is enough to provide reading light at night.
Two of these are almost enough for reading light against a darker
wall behind a bed.
Four of these strings light a patio.

We got one box that had no fewer than 4 bad white lamps.
This was probably due to someone trying to repair a problem in
the string, which was a socket that was put in backwards in
manufacturing.
Any light inserted in that socket would not only not light, but
would also be ruined, and I saw it happen.
White Rope
051 04 1787

9ft of vinyl pipe with white LEDs inside about every 2 inches, for
a total 54 LEDs, in what looks like 2 circuits, rated at 3.9 watts.

We put 4 of these on the roof of our entryway, replacing a
string and a half or 150 incandescent mini lights.
The LED ropes at 15.6W are substantially brighter than the
incandescent strings at 60W.
We also hope they are easier to clean and that the bulbs will last
longer.
We put one of these around a bathroom mirror, thus achieving useful
light at about the same power as a 4W nightlight.

We got one box that had a length out, so we took the box back
and exchanged it for another.
Fortunately, that one did work as expected.

As I looked at the dark bulb, the rest of that circuit suddenly
brightened, and it was clear the dark LED had just died.
Trying the bulb in a known working socket showed the formerly-working
bulb was now bad.
So I had actually watched an LED fail due to reverse current flow.
The failure took 5 or 10 seconds, and for that time the rest of
the circuit was somewhat dimmer than it should have been.
That should be a tip-off for the future.

Inspection of the lamp wire showed that the polarity or direction
of the dark socket was the opposite or reverse of other sockets.
One possible fix might have been to simply reverse the one LED in that
socket.
However, since that would surely set up a sequence of similar failures
in the future, it is fortunate that is not really possible.
The socket connections are at different heights and the LED leads have
been cut to different lengths.
The correct solution is to reverse the socket.

LED forward voltages exhibit substantial variation, even for
the exact same color in the exact same string.
Here are few actual values for 3 bulbs of each color from these
strings, measured at 20.1mA DC:

RED: 1.95V, 1.96V, 2.10V,an average of 2.00V
ORANGE: 1.95V, 2.13V, 1.96V, an average of 2.01V
YELLOW: 2.06V, 2.18V, 2.09V, an average of 2.11V
GREEN: 3.11V, 3.14V, 3.15V, an average of 3.13V
BLUE: 3.28V, 3.16V, 3.24V, an average of 3.23V
WHITE: 3.26V, 3.32V, 3.40V, an average of 3.33V

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