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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
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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.
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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.
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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.
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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.
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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.
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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
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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.
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- 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.
Dimming
In general, all lights last longer if they are dimmed, and there are several possibilities:
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- Lamp dimmers,
- X-10 remote control modules,
- Photoelectric outdoor light timers, and
- Anything other than mechanical contacts.
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:
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- 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.
- Using less power than a 4W bathroom nightlight, a
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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.
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- Faceted Mini White
051 04 1783
Another string of 60 lights, in two series circuits, rated at 4.8 watts.
![[Patio Light]](http://www.ciphersbyritter.com/radelect/lites/PATIO.JPG)
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.
- Faceted Mini White
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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.
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- 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
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:
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