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Q: Should a diffused or non-diffused LED be used when backlighting?
A: One common mistake designers make is using a diffused LED for backlighting. A diffused LED distributes the light over a wider area, diminishing much of the LED's output. The goal is to have as much of the emitted light as possible directly reach the front panel. Therefore, non-diffused LEDs are more effective because their light is focused in a narrow beam. Waterclear or untinted styles are best to use because they do not have a diffusant that reduces light output.
Q: Can LEDs be driven directly from a voltage source?
A: Direct operation from a voltage source is not recommended even though the LEDs appear to be functioning as required. The Forward Current (If) Vs. Forward Voltage (Vf) graph shows that a slight change in Forward Voltage will cause a large change in Forward Current leading to a possible electrical over-drive condition. This over-drive condition will reduce LED reliability. Even if over-drive conditions are not reached, swings in If will result in significant luminous intensity variations. The preferred method of operation is the use of a constant current LED driver.
Q: Can the LED light output be increased by increasing the forward current?
A: Increasing the forward current through an LED generally will increase the amount of light emitted. However, the increased power dissipation in the LED causes increased die temperatures, which have an adverse effect on device performance and longevity.
Q: What happens when one resistor is used to limit the current through several LEDs connected in parallel?
A: The forward voltage of an LED varies between devices with temperature and with forward current. When connected in parallel, the voltage across each device is identical which causes uneven current sharing between the LEDs resulting from temperature and device variations. This results in brightness variations between the parallel-connected LEDs at best and, in extreme cases, can lead to premature device failure. Therefore, each LED connected in parallel should have it own current limiting resistor.
Q: How do LEDs respond to different temperatures?
A: In general, lower die temperatures result in increased luminous efficiency, higher forward voltage for a given forward current, increased device lifetime and a shift in the output spectrum toward shorter wavelengths. Increased die temperatures have the opposite effect. The temperature coefficients of forward voltage and wavelength are typically on the order of 2mV/°C and 0.1nm/°C respectively.
Q: How is Luminous Efficacy used?
A: Incandescent light bulb manufacturers use luminous efficacy as an alternative to efficiency. Efficiency is a unit-less ratio of input to output. Thus, efficiency for an incandescent light bulb can be calculated by dividing the output radiated radiometric energy in watts, by the electrical input energy. However, this does not give a good indication of how effective the incandescent bulb is at converting energy into visible light. Therefore, "luminous efficacy" is used to refer to the ratio of visible light emitted by a light source to the amount of electric power consumed by the source, in which case a more precise term would be "luminous efficacy of a source." However, "luminous efficacy" may also refer to the ratio of radiated energy in the visible spectrum to total radiated energy, and in that case the more precise term would be "luminous efficacy of radiation". LED manufacturers use luminous efficacy to convert photometric lumens to radiometric watts. Which usage is intended isn't always clear, and furthermore the terms "efficacy" and "efficiency" are often confused by virtue of phoenetic similarity and inconsistent usage in published sources. In general, one needs to take great care to determine what is being referred to when either term is used.
Q: Are there any simple methods to drive LEDs?
A: A limiting resistor in series with the LED can be used. The resistor prevents the LED from being overdriven and allows the designer better control over the current flowing through the LED. The design steps are as follows: From the Relative Light Intensity (Iv) Vs. Forward Current (If) graph, determine the If at the Iv desired. From the Forward Voltage (Vf) Vs. Forward Current (If) graph, determine the Vf at the If found above. The value of the series limiting resistor is (Vs-Vf)/If where Vs is the supply voltage across both the LED and the series resistor. If there are more than 1 LED connected in series, then the resistor value is [Vs-(Vf1-Vf2-....-Vfn)]/If. Parallel configuration of LEDs are not encouraged because of "current-hogging."
Q: Does a 100 mcd LED always emit more light than a 20 mcd LED?
A: There is no definitive answer. A millicandela rating is determined by an on-axis measurement of peak intensity at a specific current, not by measuring total light output. Since the light emitted by a diffused LED is spread over a wide viewing angle, the on-axis value may actually be less than a non-diffused LED from which all light output is concentrated in a narrow beam. Thus the total light output of a 20 mcd LED may be greater than that of a 100 mcd LED though the on-axis reading is less.
Q: Does epoxy color determine emitted light color?
A: No, but it will appear somewhat brighter. The eye can detect an increase or decrease in light only when its intensity is doubled. In applications that use many LEDs in close proximity, the intensity of the devices should be closely matched to provide visual uniformity. A good rule to follow is: the dimmest device should be no more than half as bright as the brightest LED. Greater disparity is detectable as bright and dim spots.
Q: What is heat sink grease?
A: Heat sink grease is a thermally conductive material used to improve heat transfer from objects (such as electronic components) that generate heat to another object (called a heat sink) that acts to dissipate that heat into the surrounding atmosphere. Heat sink grease works by filling small gaps between mating surfaces that would otherwise impede the transfer of thermal energy between the two objects.
Q: How do I use heat sink grease?
A: Make sure the area for grease application is clean and dry. Apply a small amount of the grease to the bottom of the heat sink and spread evenly, and attach the heat sink. Remember to use as little as possible, as using too much can decrease the efficiency of the heat sink.
Q: How do I drive a cold cathode fluorescent lamp?
A: Miniature fluorescent lamps operate on AC. They are low pressure discharge lamps which are filled with a noble gas (typically argon) and a small amount of mercury in a phosphor-coated tube. They are also vibration and shock resistant, offer variable levels of brightness and consume very little power.
Q: How much infrared is present in JKL ultraviolet lamps?
A: Less than 5% of peak output is present, and is primarily at lamp ends.
Q: Does the fluorescent lamp and inverter produce EMI or RFI?
A: They combine to produce some RFI emission and it may be necessary to shield the inverter after determining results of system testing. The wipe, the interconnections and the high frequency inverter are all sources of potential EMI.
Q: Do fluorescent lamps get hot?
A: When operated at the specified drive current of 5mArms, the lamp ends near the electrodes will be approximately 40°C above ambient. The glass body will be 10° to 15°C over ambient.
Q: Can I power fluorescent lamps directly from the wall outlet?
A: No! You must use a properly matched inverter. All lamp specifications and operating characteristics are based on inverter power.
Q: How do I determine the Wattage of a lamp?
A: To obtain the wattage of any given lamp, multiply the voltage and amperage (for example: 28 volts x .040 Amps = 1.12 watts)
Q: What is the most common cause of incandescent lamp failure?
A: It most often occurs due to a mechanical break in the filament winding and may be accelerated by the amount of stress to which the lamp is subjected. A lamp operating at an elevated ambient temperature or at increased voltage will fail much faster than one used a rated voltage. Selection of a correct filament increases useful life.
Q: How can I Increase lamp life without increasing cost?
A: The most basic way is to reduce or derate input power. In many applications, there is little if any need for a lamp to operate at its maximum rated voltage. An incandescent lamp operating at 80% of its rated power requirement will still deliver 50% of its brightness, while increasing lamp life by 350%! CAUTION: This does not work with halogen lamps. IF you reduce operating voltage below stated performance ratings, you will reduce the benefit of life, brightness, white light, etc. achieved by the halogen cycle.
Q: What is the typical operating voltage of an LED?
A: The typical voltage for most LED's is from 1.8V to 2.2V. A blue LED has a slightly different range from 3.0V to 3.5V. Various LED are produced with an internal resistor for 5.0V, 12.0V and 24.0V operation.
Q: Is an LED a polarized device, and if so, how can I tell the Anode (positive) connection from the cathode (negative) connection?
A: Yes. An LED needs to be operated in a polarized DC circuit. Typically, the cathode of an LED is indicated by a short lead or flat on the flange.
Q: How long should I expect an LED to last?
A: LED longevity depends on a number of factors, with device type and operating temperature being among the most critical. An average green or amber indicator LED may retain more than half its original brightness even after 20 years of continuous operation, while a high brightness LED without proper thermal management may last less than a minute. In general, LEDs that have a blue spectral output or which use a phosphor coating will have a reduced life expectancy compared to other LEDs. However, these LEDs may still last longer than other lighting technologies. Check the specifications of individual devices for more specific information.
Q: Why is there so much confusion about LED brightness levels?
A: There are no industry standards for brightness. This causes significant problems for users. One company may call an LED with a brightness of only 20mcd at 20mA an "Ultra-Brite" while another might offer the same designation to a device with 200mcd at 20mA. You must evaluate the specs and ignore descriptions like "Ultra, Super or Blinding". White and blue LEDs have a significantly lower life rating – approximately 10 percent that of red output LEDs.
Q: What is the peak wavelength of an LED, and how is it measured?
A: LEDs emit light in a narrow band of wavelengths, perhaps 30 nanometers wide. The single wavelength that is most intense (or brightest) in the center of this narrow band is known as the "peak" wavelength. The band gap or energy level of the semiconductor materials used to produce the LED determines its peak wavelength.
Q: What is the difference between Peak Light Wavelength λP and Dominant Light Wavelength λD?
A: The Peak Light Wavelength is the highest wavelength emitted optically from the LED, while the Dominant Light Wavelength refers to the color shade perceived by the human eye.
Q: What is the difference "Luminance" and "Luminous Intensity" ?
A: Luminous intensity is a measure of the spatial density of light energy as perceived by the human eye, and is commonly measured in candela. Luminance is a measure of luminous intensity in a given direction, is often measured in candela per square meter and is commonly used to describe light emitted or reflected from flat surfaces.
Q: Are there any precautions that should be taken when storing LEDs?
A: The reliability of LEDs can be compromised by moisture absorption, which can lead to failures during the soldering process. For this reason humidity control is highly recommended. Please refer to the respective data sheets for additional information.
Q: Are there any ways to extend the life of LEDs ?
A: One of the major factors that influence LED life is die temperature – the lower the temperature the longer the life. In order to maintain low LED die temperature the following should be heeded: Keep current flow small, Minimize ambient temperature, and Design for sufficient heat dissipation.
Q: Is it ok to use LEDs outdoors?
A: Yes, but adequate moisture & dust ingress protection, thermal management, UV filtering, and corrosion resistance (e.g. salt air) must be considered for outdoor use.