Monthly Archives: May 2014

How to Construct a LED Luminaire  

There are many aspects that have to be taken into account when constructing a LED luminaire. In this post I will give you some tips how to build a LED luminaire and which things have to be considered.

There are five steps for designing LED luminaires:

  1. The definition of lighting requirements; how much you need light in certain environment/space.
  2. Defining some design constraints and special requirements, for example related to optics or withstanding high temperatures.
  3. Optical, thermal or electrical design.
  4. The number of LED components is then calculated based on the lumen quantity defined in the specification.
  5. Finally, all components to construct the final luminaire, or prototype in the first stage, have to be defined and specified.

In this post, we concentrate on this last step, how to construct LED luminaires from lighting components rather than the step-by-step luminaire design process itself.

When you start the construction of a luminaire from lighting components, the first thing that you should decide is which kind of LED you use in your luminaire. Basically there are three different options:

  • High-power COB LEDs
  • Mid-power LEDs
  • Low-power SMD LED diodes.

In this post, I’ve divided the options to two different groups: high-power COB LEDs and mid- and low-power LEDs, which usually are SMD LED components.

High-power LEDs

COB Luminaire

Luminaire using Citizen COB LED

High-power LED option comes usually into question when there is a need for large luminous output. Citizen Electronics’ COBs are powered from 3W COB to 80W COB with typical power consumption values. The luminous flux ranges from about 100 lumens to 13 000 lumens with typical current and even to nearly 20 000 lumens with maximum operating conditions. When constructing a luminaire using COB LEDs there is a need for some kind of heat management because of large power density on small area that has to be conducted away from COB LED. The power density can be even 22 W/cm2 on light emitting area of about 8.5 cm2. Many times price target sets the limit for cooling, and active cooling methods can’t be used. This requires the use of passive heat management. In another words, you need some kind of heat sink. Typical material is some aluminum alloy with a fin-shaped structure in order to increase the overall surface area of the heat sink element. Additionally, some kind of thermal paste or other thermal interface material is inserted between the LED and heat sink in order to increase heat conduction from the LED to the heat sink and ultimately to the luminaire body.

Of course you need also LED driver to feed electric current to the COB. When talked about COB LEDs, many times you can drive only one or two LEDs with the same driver. Esko Väisänen explained the selection process of LED driver in his post: How to choose the right LED driver.

In addition to the COB LED and the LED driver, you may also need some optics, such as lens or reflector to increase light quality coming out of your luminaire. Current feeding to your COB LED can be realized either by soldering wires to the COB or using special sockets or connectors to supply power to the LED.

Mid-power and low-power LED

LED Module, which can be used in luminaire needing good light distribution.

LED Module, which can be used in luminaire needing good light distribution.

When constructing a luminaire using mid-power or small-power LEDs, heat management is not such a big problem as in the case of high-power LEDs. Normally the key target for luminaires using mid- or small-power LEDs is the light distribution or color consistency. Applications range from store lighting through emergency lights to decorative lighting, even to LED bulbs. Diffuser plates, for example opal diffuser or prismatic diffuser, can be used in these kinds of luminaires. The luminaire in which small- or mid-power LEDs are used include usually PCBs. The alternatives are usually FR-4 or aluminum metal-core PCB (MCPCB) on which SMD LED diodes are assembled. Usually, if the total power consumption of the LED module exceeds 10W, the aluminum MCPCB is used in order to better conduct heat away from the PCB. Depending on the case, some kind of extra aluminum profile can be used to conduct heat away. If the luminaire has a closed structure, you should also take care of the heat inside the closed luminaire. Heat should be distributed and conducted away from the LED so that the LED does not get too hot. FR-4 can be used for lower power modules by using enough copper pour to spread heat uniformly. FR-4 boards are also cheaper than aluminum PCBs and are used if price becomes the key issue and power consumption is not the problem. In addition to the PCB and the LED diodes some kind of connectors are necessary to connect the LEDs into the outer world electrically.

Also these luminaires need a LED driver to give current to LED components. In addition to heat formed inside the LED components themselves, heat forms also in the LED driver operation. This causes problems if the luminaire structure is closed as discussed earlier. If you use many modules within one luminaire, you will need some kind of cabling between the modules. When designing for example a linear luminaire, there may be a need for some cabling throughput. This will usually mean that cabling has to go on the other side through some plastic or metallic profile. Some additional screw holes may be possibly needed for attaching the metallic profile into the luminaire body.

If you have any questions or comments, please feel free to leave them on the comments field.