Technical Information

Most LED's have their characteristics specified at a current of 20 mA. If you want really good reliability and you are not certain you don't have worse-than-average heat conductivity in your mounting, heat buildup in wherever you mount them, voltage/current variations, etc. then design for 15 milliamps.

Now for how to make 15 milliamps flow through the LED:

First you need to know the LED voltage drop. It is safe enough to assume 1.7 volts for non-high-brightness red, 1.9 volts for high-brightness, high-efficiency and low-current red, and 2 volts for orange and yellow, and 2.1 volts for green. Assume 3.4 volts for bright white, bright non-yellowish green, and most blue types. Assume 4.6 volts for 430 nm bright blue types such as Everbright and Radio Shack. Design for 12 milliamps for the 3.4 volt types and 10 milliamps for the 430 nm blue.

You can design for higher current if you are adventurous or you know you will have a good lack of heat buildup. In such a case, design for 25 ma for the types with voltage near 2 volts, 18 ma for the 3.4 volt types, and 15 ma for the 430 nm blue.

Meet or exceed the maximum rated current of the LED only under favorable conditions of lack of heat buildup. Some LED current ratings assume some really favorable test conditions - such as being surrounded by air no warmer than 25 degrees Celsius and some decent thermal conduction from where the leads are mounted. Running the LED at specified laboratory conditions used for maximum current rating will make it lose half its light output after rated life expectancy (20,000 to 100,000 hours) - optimistically! You can use somewhat higher currents if you heat-sink the leads and/or can tolerate much shorter life expectancy.

Next, know your supply voltage. It should be well above the LED voltage for reliable, stable LED operation. Use at least 3 volts for the lower voltage types, 4.5 volts for the 3.4 volt types, and 6 volts for the 430 NM blue.

The voltage in most cars is 14 volts while the alternator is successfully charging the battery. A well-charged 12 volt lead-acid battery is 12.6 volts with a light load discharging it. Many "wall wart" DC power supplies provide much higher voltage than specified if the load is light, so you need to measure them under a light load that draws maybe 10-20 milliamps.

Next step is to subtract the LED voltage from the supply voltage. This gives you the voltage that must be dropped by the dropping resistor. Example: 3.4 volt LED with a 6 volt supply voltage. Subtracting these gives 2.6 volts to be dropped by the dropping resistor.

The next step is to divide the dropped voltage by the LED current to get the value of the dropping resistor. If you divide volts by amps, you get the resistor value in ohms. If you divide volts by milliamps, you get the resistor value in kilo-ohms or k.

Example: 6 volt supply, 3.4 volt LED, 12 milliamps. Divide 2.6 by .012. This gives 217 ohms. The nearest standard resistor value is 220 ohms.

If you want to operate the 3.4 volt LED from a 6 volt power supply at the LED's "typical" current of 20 ma, then 2.6 divided by .02 yields a resistor value of 130 ohms. The next higher popular standard value is 150 ohms.

If you want to run a typical 3.4 volt LED from a 6 volt supply at its maximum rated current of 30 ma, then divide 2.6 by .03. This indicates 87 ohms. The next higher popular standard resistor value is 100 ohms. Please beware that I consider the 30 ma rating for 3.4-3.5 volt LED's to be optimistic.

One more thing to do is to check the resistor wattage. Multiply the dropped voltage by the LED current to get the wattage being dissipated in the resistor. Example: 2.6 volts times .03 amp (30 milliamps) is .078 watt. For good reliability, I recommend not exceeding 60 percent of the wattage rating of the resistor. A 1/4 watt resistor can easily handle .078 watt. In case you need a more powerful resistor, there are 1/2 watt resistors widely available in the popular values.

You can put LED's in series with only one resistor for the whole series string. Add up the voltages of all the LED's in the series string. This should not exceed 80 percent of the supply voltage if you want good stability and predictable current consumption. The dropped voltage will then be the supply voltage minus the total voltage of the LED's in the series string.

Do not put LED's in parallel with each other. Although this usually works, it is not reliable. LED's become more conductive as they warm up, which may lead to unstable current distribution through paralleled LED's. LED's in parallel need their own individual dropping resistors. Series strings can be paralleled if each string has its own dropping resistor.

Copyright: Don Klipstein, Jr. 01/01/00

Diagram of a white LED

How does a LED work? This is a very simple explanation of the construction and function of LED's. White LED's need 3.6VDC and use approximately 30 milliamps of current, a power dissipation of 100 milliWatts. The positive power is applied to one side of the LED semiconductor through a lead (1 anode) and a whisker (4). The other side of the semiconductor is attached to the top of the anvil (7) that is the negative power lead (2 cathode). It is the chemical makeup of the LED semiconductor (6) that determines the color of the light the LED produces. The epoxy resin enclosure (3 and 5) has three functions. It is designed to allow the most light to escape from the semiconductor, it focuses the light (view angle), and it protects the LED semiconductor from the elements.

As you can see, the entire unit is totally embedded in epoxy. This is what make LED's virtually indestructible. There are no loose or moving parts within the solid epoxy enclosure.

Therefore, a light-emitting diode (LED) is essentially a PN junction semiconductor diode that emits light when current is applied. By definition, it is a solid-state device that controls current without heated filaments and is therefore very reliable. LED performance is based on a few primary characteristics:

Color

LED's are highly monochromatic, emitting a pure color in a narrow frequency range. The color emitted from an LED is identified by peak wavelength (lpk) and measured in nanometers (nm ).

Wavelength in nm

Peak wavelength is a function of the LED chip material. Although process variations are ±10 NM, the 565 to 600 NM wavelength spectral region is where the sensitivity level of the human eye is highest. Therefore, it is easier to perceive color variations in yellow and amber LED's than other colors.

LED's are made from gallium-based crystals that contain one or more additional materials such as phosphorous to produce a distinct color. Different LED chip technologies emit light in specific regions of the visible light spectrum and produce different intensity levels.

                                                                White Light

When light from all parts of the visible spectrum overlap one another, the additive mixture of colors appears white. However, the eye does not require a mixture of all the colors of the spectrum to perceive white light. Primary colors from the upper, middle, and lower parts of the spectrum (red, green, and blue), when combined, appear white. To achieve this combination with LED's requires a sophisticated electro-optical design to control the blend and diffusion of colors. Variations in LED color and intensity further complicate this process.

Presently it is possible to produce white light with a single LED using a phosphor layer (Yttrium Aluminum Garnet) on the surface of a blue (Gallium Nitride) chip. Although this technology produces various hues, white LED's may be appropriate to illuminate opaque lenses or backlight legends. However, using colored LED's to illuminate similarly colored lenses produces better visibility and overall appearance.
Intensity

LED light output varies with the type of chip, encapsulation, efficiency of individual wafer lots and other variables. Several LED manufacturers use terms such as "super-bright," and "ultra-bright" to describe LED intensity. Such terminology is entirely subjective, as there is no industry standard for LED brightness.



The amount of light emitted from an LED is quantified by a single point, on-axis luminous intensity value (Iv). LED intensity is specified in terms of millicandela (mcd). This on-axis measurement is not comparable to mean spherical candlepower (MSCP) values used to quantify the light produced by incandescent lamps.

Luminous intensity is roughly proportional to the amount of current (If) supplied to the LED. The greater the current, the higher the intensity. Of course, there are design limits. Generally, LED's are designed to operate at 20 milliamps (mA). However, operating current must be reduced relative to the amount of heat in the application. For example, 6-chip LED's produce more heat than single-chip LED's. 6-chip LED's incorporate multiple wire bonds and junction points that are affected more by thermal stress than single-chip LED's. Similarly, LED's designed to operate at higher design voltages are subject to greater heat. LED's are designed to provide long-life operation because of optimal design currents considering heat dissipation and other degradation factors.
                                                                   Eye Safety Information

The need to place eye safety labeling on LED products is dependent upon the product design and the application. Only a few LED's produce sufficient intensity to require eye safety labeling. However, for eye safety, do not stare into the light beam of any LED at close range.

                                                                             Visibility

Luminous intensity (Iv) does not represent the total light output from an LED. Both the luminous intensity and the spatial radiation pattern (viewing angle) must be taken into account. If two LEDs have the same luminous intensity value, the lamp with the larger viewing angle will have the higher total light output.



Theta one-half (q½) is the off-axis angle where the LED's luminous intensity is half the intensity at direct on-axis view. Two times q½ is the LED's full viewing angle; however, light emission is visible beyond the q½ point. Viewing angles listed in this catalog are identified by their full viewing angle (2q½ °).

LED viewing angle is a function of the LED chip type and the epoxy lens that distributes the light. The highest luminous intensity (mcd rating) does not equate to the highest visibility. The light output from an LED chip is very directional. A higher light output is achieved by concentrating the light in a tight beam. Generally, the higher the mcd rating, the narrower the viewing angle.

The shape of the encapsulation acts as a lens magnifying the light from the LED chip. Additionally, the tint of the encapsulation affects the LED's visibility. If the encapsulation is diffused, the light emitted by the chip is more dispersed throughout the encapsulation. If the encapsulation is non-diffused or water clear, the light is more intense, but has a narrower viewing angle. Non-diffused and water clear LEDs have identical viewing angles; the only difference is, water clear encapsulations do not have a tint to indicate color when the LED is not illuminated.

Overall visibility can be enhanced by increasing the number of LED chips in the encapsulation, increasing the number of individual LED's, and utilizing secondary optics to distribute light. To illustrate, consider similar red GaAlAs LED chip technology in four different configurations:



In each case, the amount of visible light depends on how the LED is being viewed. The single chip may be appropriate for direct viewing in competition with high ambient light. The 6-chip may be better suited to backlight a switch or small legend, while the cluster or lensed LED may be best to illuminate a pilot light or larger lens.
Operating Life

Because LED's are solid-state devices they are not subject to catastrophic failure when operated within design parameters. DDP® LED's are designed to operate upwards of 100,000 hours at 25°C ambient temperature. Operating life is characterized by the degradation of LED intensity over time. When the LED degrades to half of its original intensity after 100,000 hours it is at the end of its useful life although the LED will continue to operate as output diminishes. Unlike standard incandescent bulbs, DDP® LED's resist shock and vibration and can be cycled on and off without excessive degradation.
Voltage/Design Current

LED's are current-driven devices, not voltage driven. Although drive current and light output are directly related, exceeding the maximum current rating will produce excessive heat within the LED chip due to excessive power dissipation. The result will be reduced light output and reduced operating life.

LED's that are designed to operate at a specific voltage contain a built-in current-limiting resistor. Additional circuitry may include a protection diode for AC operation or full-bridge rectifier for bipolar operation. The operating current for a particular voltage is designed to maintain LED reliability over its operating life.

                                          Precautions While Working With LED's

We cannot assume any responsibility for any accident or damage caused when the products are used beyond the maximum ratings specified herein.

The user of these products must confirm the performance of the LEDs after they are actually assembled into the user's products/systems. It is strongly advised that he user design fail-safe products/systems. We will not be responsible for legal matters which are caused by the malfunction of these products/systems.
LED Lamps

                                                        Static Electricity and Surge

Static electricity and surge damage LEDs. It is recommended to use a wrist band or anti-electrostatic glove when handling the LEDs. All devices, equipment and machinery must be electrically grounded.

                                         Lead Forming

The leads should be bent at a point at least 3mm from the epoxy resin of the LEDs.

Bending should be performed with the base firmly fixed by means of a jig or radio pliers.

                                    Mounting Method

The leads should be formed so they are aligned exactly with the holes on the PC board. This will eliminate any stress on the LED's.

Use LED's with stoppers or resin spacer to accurately position the LED's. The epoxy resin base should not be touching the PC board when mounting the LED's. Mechanical stress to the resin may be caused by the warping of the PC board when soldering.

                                       Do not bend

The LED's must not be designed into a product or system where the epoxy lens is pressed into a plastic or metal board. The lens part of the LED must not be glued onto plastic or metal. The mechanical stress to the leadframe must be minimized.

                                                                     Soldering

Solder the LED's no closer than 3mm from the base of the epoxy resin.

For solder dipping, it may be necessary to fix the LED's for correct positioning. When doing this, any mechanical stress to the LED's must be avoided.

When soldering, do not apply any mechanical force to the lead frame while heating.

Repositioning after soldering must be avoided.

	Soldering Iron	Dip Soldering	Reflow Soldering
Lamp LED	300° C (max) 3 sec (max)	260° C (max) 5 sec (max)	Not Allowed
Chip LED	300° C (max) 3 sec (max) w/ Twin Head iron	Not Allowed

Cleaning

Avoid exposure to chemicals as they may attack the LED surface and cause discoloration. When washing is required, "isopropyl alcohol" is to be used.

The influence of ultrasonic cleaning on the LEDs differs depending on factors such as oscillator output and the way in which the LEDs are mounted. Therefore, ultrasonic cleaning should only be performed after making certain that it will not cause any damage.

Emission color

LED emission wavelengths vary. LEDs are classified by emission color into different ranks. When a large volume of LEDs are purchased, LEDs with different color ranks will be delivered.

Packaging

The lead frames of the LED's are coated with silver. Care must be taken to maintain a clean storage atmosphere. If the LED's are exposed to gases such as hydrogen sulfide, it may cause discoloration of the lead frames. Moistureproof packing is used to keep moisture away from the chip type LED's. When storing chip type LED's please use a sealable package with a moisture absorbent material inside.

LED Home Lighting has provided this copyrighted information to be used for basic knowledge of LED's. We cannot confirm all information as being 100% correct so please use said information accordingly. We cannot be held liable for any mistakes. Thank you.