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LED anchor light
The use of LEDs for marine navigation light applications
Although it might seem that the marriage of LEDs to marine navigation lights is a perfect match of technology to a particular need, it has been only relatively recently (early to mid 1990's) that light-emitting diodes (LEDs)which were suitable in intensity, beam pattern, and color have become available for this application.
Deep Creek Design, Inc. was the first to market a LED anchor light, (now made by FirstStar LED) in 1998. Others soon followed and now there are many companies offering such a product.
What makes a LED anchor light different from a 'normal' anchor light, that is one that uses an incandescent light bulb, is the use of light-emitting diodes. This makes the light very efficient and reliable with an improved functionality in terms of operation at normal levels of intensity with lower or higher levels of input voltages. Other features such as day light sensing and automatic shut off may be built into the LED anchor light as well.
LED light technology
As the name light-emitting diode indicates, LEDs are a type of semi-conductor electrical component known as a diode, and like a normal diode they function sort of as a "one way check valve", that is they conduct power and will light up only when the power is of the correct voltage polarity, that is to say only when the electricity is going the proper 'direction' through the LED. Applying the wrong polarity may damage the LED so it is important to protect it from voltage of the wrong polarity.
LEDs produce light from electricity moving from one type of semi-conductor crystal to another type of crystal within the LED. These tiny crystals of semi-conductor material are at the heart of an LED and they are surrounded by a miniature reflector and this in conjunction with the LEDs lens assembly forms the emitted light into the desired beam pattern. Almost all LEDs make light of only a very narrow color range, for instance pure red or green. The color emitted depends on the electrical properties of the crystals used. So called "white light LEDs" usually make their white light by actually emitting blue light that is then converted into white light by phosphor chemicals in the epoxy lens covering the LED in much the same way that a fluorescent light converts UV light into white light by the phosphor powder coating the inside of the tube. You can get LEDs that produce any almost any color of light you might want, and by combining colors from several LEDs you can get any color the eye can see.
Like other semi-conductors such as transistors, LEDs are very sensitive to changes in applied voltage. With a normal incandescent light bulb if you change the voltage you apply to it then the current through it will change more or less proportionally, in accordance with Ohm's Law (where electric current is equal to the voltage measured across the 'external electric load' that is using electrical power divided by the electrical resistance of that 'load' that the current is flowing through, in this case the 'load' being the light bulb), but unlike a 'load' that obeys Ohm's Law, if the voltage applied to a LED changes by only a small amount the current through the LED will change by a lot more than you would expect if you were using Ohm's Law. For example with a particular LED, when it is run at the voltage it is rated for plus a voltage increase of only ten percent the current will change by 100 percent not just ten percent! Because of this special property of LEDs, a very slight change in input voltage to a LED based light could mean the current through the light's LED array would increase from a very safe level to an excessive amount that will burn the LEDs out, so the electrical power supplied to the LED array in the light must be regulated.
This job of supplying regulated power to the LEDs is the function of a specialized power supply that both regulates the power to the LED array and also may perform other control functions such as dimming, day sensing, and LED color selection. The special type of power supply used for the purpose of supplying power to and controlling LEDs is generally known as a LED 'Driver'. Not all 'drivers' are created equally, one very important difference is the 'drivers' electrical efficiency or that is how much of the power that is supplied to the light is wasted by the 'driver' and never makes it to the LED array. The cheaper the light, the less efficient its 'driver' will be, and also the less protection that 'driver' will offer the LED array from voltage fluctuations, such as power surges and transients (see Voltage spike). By using a more efficient 'driver' more of the LEDs potential can be utilized making more light for less energy from a smaller area LED array and this can actually reduce the overall cost for the amount of light you get.
Most LED navigation lights will have the 'driver' built into the light, but some LED navigation lights on the market use an external 'driver' to provide the power to one or more LED arrays. This can be a cost effective solution but it is generally not as good as each light having its own 'driver', one major disadvantage being that if the external 'driver' fails all the lights it supplies will go dark.
LED lights that have a wide input voltage range and that are rated as "constant power" (electrical power, that is watts, is volts multiplied by amps)indicate that a far more efficient 'driver' design is being used. For example if the specifications say something like "this light will use three watts with an input voltage of 6-34 volts DC" or "this light has a special circuit to optimize power usage and prolong battery life" the light has a better type of 'driver'. This type of 'driver' design is known by its full name as a switch mode DC-DC converter, (see also switched mode power supply).
In this design, as the supplied input voltage goes up, the 'driver' will convert the supplied input power (voltage X current) into different voltage and current quantities on the 'drivers' output, providing a steady amount of output power that is just what the LEDs need, regardless of what the input voltage does (within limits). The power supplied to the LEDs will remain constant (at a given LED temperature, that is) so the input current the light draws goes down as the input voltage supplied to the light goes up. If the light is rated at three watts for example if the input voltage is 5 volts the light will draw .600 amps, but when the input voltage is 15 volts the light will now draw only .200 amps of current, so the light is using the same power no matter what the input voltage is. The outstanding advantage of this design is its extreme efficiency. If properly designed it can also raise the overall reliability of the light as the LEDs are very well protected since they see no change in either voltage or current applied to them from the driver. Some of the disadvantages are a possibility of radio frequency interference (RFI) or Electromagnetic Interference (EMI) in other electronic devices if the light is not properly designed, also stability and reliability under all circumstances can be more costly to achieve, and with the extra components need to make this type of 'driver' it will not be the most compact design all of which means this type costs more money. It is also possible to make similar "switch mode" designs that supply a constant voltage to the LEDs instead of a constant power level(or more accurately, a constant current level through the LEDs). There are some good reasons for such a design, as with the constant voltage design, if several LEDs or LED groups are driven in a parallel circuit and one or more LED burns out or "opens" the rest will not then receive too much current and also burn out, as they likely would if the array was powered by a driver that supplied a constant current to the LED array.
Lights that draw the same amount of amps on their input regardless of how much input voltage is supplied (within limits) use what are known as "constant current" drivers. This type of 'driver' design is also known as a 'linear regulator' or 'resistive type' regulator. This is because although it uses a semi-conductor (not a resistor) to regulate the power to the LEDs, it is very similar to an automatically variable resistor or rheostat in function. This type of driver is not nearly as efficient as the "switch mode" design. An example of this design may have specifications that say something like "this light uses .600 amps at 5-40 volts DC". In this second example, the light may supply a steady unchanging amount of current to the LEDs just as was true in the first example, but instead of converting the input power supplied to the 'driver' to just what the LEDs need, the 'driver' just turns any power in excess of what the LEDs need into heat, so the light will use more and more power as the input voltage supplied to it rises above what is needed, even though the LEDs will not be using more power or getting brighter. If the input voltage is 5 volts the power the light uses might be .6 amps X 5 volts, and if the input voltage is 15 volts the power the light would then use the same .6 amps but now it would be multiplied by the input voltage of 15 volts, resulting in a huge power increase that is all wasted, turned into heat by the 'driver'. The advantages of this design is that it is very simple, cheap, and rugged. However it is also very wasteful and will create a lot more heat than a DC-DC converter, and may need a heat sink potentially making it less compact. Also in general it does not do as good a job of controlling the power to the LEDs as the DC-DC converter does.
The very cheapest models will use a simple 'current limiting' resistor in a series circuit with the LEDs to supply power to them. This is a design that does a very poor job of protecting the LEDs from high normal voltages and keeping them bright at low normal voltages and is very wasteful of electrical power because as the input voltage goes up the voltage supplied to the LEDs also goes up resulting in a much higher current flow through the LEDs and much higher power usage by the LEDs, unlike one of the lights in our previous examples that used proper 'drivers' that kept power flow through the LEDs constant. This design is much less efficient, and in general makes a lot less light and may fail prematurely. Although manufactures are using better 'driver' designs more often than ever now, a lot of 'bottom line' models or very low power lights still use current limiting resistors to supply power to the LEDs.
Other driver types
There are a few other types of 'drivers' one notable example being another very efficient design that does not convert input power but instead supplies power to the LEDs as pulses of varying width by using a form of Pulse-width modulation (PWM). It may supply nearly constant average current to the LEDs, with many of the advantages of the switch mode type such as high efficiency and very little heat made, and it has some other advantages the other types do not have as well, such as being more compact for its power output and able to make the LEDs seem brighter to the eye than they really are by pulsing them, a brightness enhancement phenomenon due to the human visual perception Broca-Sulzer effect 2 and related to Bloch's law. There are some disadvantages as well. A PWM does not have as low an input voltage range as a DC-DC converter type might. The LEDs must also be matched more closely to the load with this type of driver, which is to say that the voltage rating of the LEDs must be closer to that of the supplied input power than would be the case with the DC-DC converter 'driver' for example. There are a few other types of drivers as well but most lights on the market today are either a DC-DC switch mode, linear/resistive, or simple current-limiting resistor type. Again of these three the DC-DC switch mode converter is by far the best choice in terms of protecting the LEDs and the light's overall electrical efficiency. The DC-DC converter type will often be more bulky and when you can see it, it is comprised of many individual parts, coils, transistors or IC's, capacitors, etc. A lamp or luminaire that uses this type will often cost more per light although it may be cheaper in the long run due to its more efficient use of energy and extended lamp life.
Moving on to the LEDs themselves, even though it is true LEDs are very efficient relative to most other light sources it is nevertheless a fact that most of the energy supplied to them is turned into heat, it is just that for the same energy the LED will end up putting a lot more light where you want it than an old fashioned light bulb would for the same electrical power.
Newer LEDs are much more efficient and brighter than LEDs using older technology. But as engineers design more powerful LEDs, heat dissipation becomes a more critical issue as heat is the enemy of an LED that wants to be long-lived. For the moment "cutting edge technology" LEDs can operate continuously at only about three hundred fifty degrees F (which is less than a third of the operating temperature of a typical incandescent bulb). A properly heat-sinked LED will have a useful life of somewhere between 35,000 and 60,000 hours before a significant dimming occurs. (LEDs seldom fail totally, rather they gradually dim over time). (http://www.lumileds.com/pdfs/RD06.pdf). An improperly heat-sinked LED or an LED that is continuously driven (not pulsed)above its recommended operating current will see its useful life plummet. Purely in terms of the cost benefit of greater reliability, for the average user the higher cost of LED lighting can only make economic sense when its useful life far outlasts the typical lifespans of other kinds of incandescent or fluorescent lighting which range from 1,000 hours to as much as 10,000 hours.
Of course other factors such as a higher emitted light intensity for a given application, faster turn on times, very pure color rendition, higher reliability in and lower Mean Time Between Failures (MTBF) rates especially in adverse environmental conditions such as high humidity combined with greater functionality as in for example a larger range of input operating voltages, and the potential for more control functions such as dimming or emitted light color selection, automated functions such as day-light sensing, and even remote control, will weigh heavily in such cost/benefit analysis when using LEDs in many applications.
But in a niche application such as a boats navigation light (and possibly even interior lights), where off-grid power generation is at a premium and the choice is between higher efficiency lighting or more very expensive and bulky solar panels or other costly alternative power generation sources and/or more batteries to store the electrical power, the higher efficiency of LEDs may more readily provide a clear economic benefit when compared to traditional lighting sources.
With LED marine navigation lights there is a greater need for proper optical considerations than with navigation lights using an incandescent bulb, as an LED array emits many conical "beams" of light that must be more of less evenly spread within the desired area. Also 'white light' LEDs do not emit the broad spectrum of light that a 'normal' light bulb does but rather have a very narrow spectrum made up of relatively few colors. LED white light does not behave like white light from an incandescent or fluorescent bulb when it passes through a color filter and lenses. The colors in the white light LED spectrum will be absorbed at differing rates by colored filters, and most of the light will be absorbed by such filters. The result of using white light LEDs through colored filters therefore is dimming and color shifting. Besides making the light less visible and possibly of a hard to distinguish color to the human eye, another concern here is that automatic light sensing equipment out on the open ocean needs to be able to recognize the emitted light to work properly. The solution to the dimming and color shifting problem is to use LEDs that have the proper emitted frequency of light if they will be used through colored filters.
LED main types
There are two main types of LEDs in use today. The older and still more common type look like a small rounded transparent or translucent 'bulb' with a domed top and have two wire leads sticking out underneath that supply power to the internal parts of the LED. These leads will be soldered through holes in the circuit board. These are "traditional" LEDs. They will be less efficient and may be less bright than the newer type of LEDs that mount directly to the circuit board (or a heat sink) and look like a small transparent plastic lens or dome sitting on top of a flat slightly larger rectangular or rounded part. These types of LEDs do not have leads that solder through the circuit board but instead their leads solder directly to conductors or copper traces on the surface of a circuit board. The LED is generally securely fastened to the board or a heat sink with thermal adhesive, hence the name Surface Mount Device or surface mount technology. In the traditional type the wire leads serve many functions, supporting the LED, carrying electrical power and transferring heat, but in the "surface mount" type these functions are split up, resulting in much higher performance.
It is important that the light be protected from the elements. Look for robust water and spray resistant enclosures (IP 67 or 68) made with tough UV stable plastic and coated or potted electronics. Acrylic often is not as UV stable as other plastics such as Lexan or Buterate (Polyvinyl butyral), so lights housed with it may become much dimmer and have degraded optics after only one or two years in the tropics! A good UV stable plastic should last at least five to ten years without serious degradation. A lamp or luminaire that has its electronics potted (encapsulated) is much better than a those that have inferior protection rely only on a 'waterproof' plastic coating on the electronics to keep them working.
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "LED_anchor_light". A list of authors is available in Wikipedia.|