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    Have a Lighting Question?  Ask the Expert!


      This thread has been closed to new questions.

      However, we welcome you to Post Your Question about Lighting in the element14 Community Lighting group. You'll find many fellow members and experts who have just the answer you're looking to find! 


      Thank You, Your Friends at element14 Community

      David Cox

      David Cox

      David is a Channel Technical Manager with Cree, Inc.
      His core expertise includes Solid State Lighing, Power, System Design, Embedded Design, System solutions.

        • 1. Re: Have a Lighting Question?  Ask the Expert!

          I have a question for David.  I'm am trying to develop a driver circuit for driving two Cree MC-E's at 350 mA.  My power source is eight nimh AA batteries in series (8 - 11 volts operating range).  There are several different ways to arrange the total of 8 leds and am wondering if any particular arrangement can be expected to give the best performance.  Right now I am considering three different driver arrangements:

          (a) one buck driver powering 4 parallel strings of two leds (driver current = 1400 mA)

          (b) two buck drivers with each driver powering 2 parallel strings of two leds (driver current = 700 mA)

          (c) one boost driver powering one string of 8 leds (driver current =350 mA)


          Any suggestions on what configuration is best suited for this application?



          • 2. Re: Have a Lighting Question?  Ask the Expert!


            Thank you for using CREE!

            To keep your design simple and maximize Battery run time I would choose option C.

            To reduce your system losses - do not use the Spring type battery holders - they have very high resistance.. use either blade holer or flying leads..


            Good Luck and let me know how your design turns out

            • 3. Re: Have a Lighting Question?  Ask the Expert!

              Hi Mr. David,

                              I am from INDIA, you know the country which recently won the Cricket World Cup. HUH finally! it gives a great amount of satisfaction. Ok so actually i am an Electronics Engineer & I am palnning to set up a LED light manufacturing unit. I am motivated to do so because LED's are the BUZZ word today & secondly what better way to help my society, as millions of indians still living in the dark ages.

                             I am not much worried about the electronic driver design as i have some years of design experience also on the LED front i am getting great help from CREE india, i will be recieving sample LED's from CREE in a few days to speed up my design.

                             Now the thing worrying me the most is the manufacturing set up and how to bring it all together as i don't have prior experience of manufacturing, like the machinery required , then should there be a Surface Mount Tech Machine, How to design and manufacture Dichroic reflectors, than what about the front end software tools required for designing , simulation , thermal analysis etc

                             In short the only thing i have right now is the design of drivers and the LED's & the thing i need is a complete end product. I will be grateful if you can provide some inputs.


              Thanking You


              Muhammad Asif.

              • 4. Re: Have a Lighting Question?  Ask the Expert!


                Congratulations on INDIA winning the Cricket World Cup!


                Cree would love to help you succeed in your efforts to develop Solid State Lighting in INDIA.


                We have posted a lot of our knowledge base on Cree's landing page on Element14 (see link below)




                Check out the Application notes, Reference designs and Tools Section


                From a system design stand point, check out Cree's online System design tool: Product Characterization Tool (PCT)



                This tool allows you to enter you System Design requirements (System Lumens, Optical Efficiency, Electrical Efficiency)

                then helps you evaluate the right LED drive current to optimize your design.

                For all valid LED drive currents, the PCT quickly and easily does the following:

                • Shows all critical LED design parameters, such as luminous flux and efficacy, for selected LED models
                • Performs de-rating calculations based on junction temperature (Tj)
                • Calculates LED valuations, such as lumens per dollar (lm/$)
                • Estimates simple LED design parameters, such as the number of LEDs required and total power consumption

                The PCT uses specifications and characteristics from LED data sheets to accurately compare LED parameters with changes in flux, price and junction temperature. The PCT can compare up to three LED models side-by-side to aid in making critical design choices.


                To get to market fast you may want to look at Cree's more integrated Lighting modules and LED Array products like the LMR4, LMR2 (coming soon), MPL, CXA20


                Cree also works with complimentary suppliers and system design experts to help you get to market faster..


                Understanding what problem you are trying to solve is one of the most important starting places..  How much light is need in your application


                Cree published a very good application note which documents the design process the system..

                here is the link..



                Hopefully this is a good starting point on your journey to changing the lighting world in INDIA!



                David Cox

                • 5. Re: Have a Lighting Question?  Ask the Expert!

                  Would you clarify deltaCCT?

                  • 6. Re: Have a Lighting Question?  Ask the Expert!

                    Thank you for the great question!


                    Delta CCT = the Change in CCT over angle – (CCT = correlated color temperature)



                    when we talk about delta CCT we are usually talking about the how much the color of the white light changes over the viewing angle of the LED (smaller is better ). This is critical for some applications like linear lighting and a don’t care for outdoor lighting


                    • 7. Re: Have a Lighting Question?  Ask the Expert!

                      Dear David,

                      what size LED die are usually used for low-, medium-, and high-power LEDs?

                      Thank you.



                      • 8. Re: Have a Lighting Question?  Ask the Expert!


                        thank you for the question:

                        From the element14 webinar I gave on March 29th 2011 slide 18 titled LED Chips: Size Doesn't Matter

                        I show power die from Cree's ETC and EZ chip families that range from 0.35x0.47mm to 1.4x1.4MM in size

                        The link to the on-demand presentation is here: https://newark.webex.com/newark/lsr.php?AT=pb&SP=EC&rID=49322112&rKey=23487de9a1e644c9


                        from Cree's small chip family the chip size is 200x200μm


                        hope this answers your question

                        • 9. Re: Have a Lighting Question?  Ask the Expert!

                          This portion of the presentation conveys the message to focus on the application and use the right LED @ the drive current needed to solve your lighting problem at the right price.


                          so if you are designing a distributed lighting appliction use products from Cree's distributed lighting portfolio MX , and ML-E ML-B families of products


                          if you are doing MR16 Design Cree created a product that solves this really hard problem MT-G for the 35W and 50W LED replacement and XLamp® XM-L™ EasyWhite LED for 20W LED replacement


                          Cree invented the XM-L for the next generation of designs needing high lumen packages like high bay, street lights, low bay  to reduce the inital Cost and Total Cost of ownership


                          we are focused on creating the right parts for each lighting application to enable the market to adopt Solid State lighting NOW.

                          • 10. Re: Have a Lighting Question?  Ask the Expert!

                            What are the chances that CREE will start putting more information in their datasheets, or at least offer some guidance on what the datasheets don't say?


                            Take the X-Lamp XR-E for example. Any absolute maximum ratings? no just Characteristics, any recommended operating conditions? no just characteristics.


                            Characteristic maximum current 1A (> 5000K white), 0.7A (others). DC pulse characteristic 1.8A, 1kHz, 10% (all). What about other frequencies and duty cycles? What about pulse ratings for photo-flash type applications? What about current ripple ratings at 10s or 100s of kHz as typically produced by switchmode drivers? What about guidance on the consequences of over/under driving? You don't need 50,000 hours life from a photo-flash for example.


                            You make the things, you must know a lot about them but seem to leave your customers guessing.

                            • 11. Re: Have a Lighting Question?  Ask the Expert!

                              Inverter vs Driver?


                              My electrical engineering senior design project is going to be a power generating bicycle with a number of different lighting systems along with some other features. I'm thinking about using electroluminescent wire for lighting the bike frame, so I know I'll need an inverter.


                              My question is, what is the difference between an inverter and a driver? When I look at different websites that sell EL wire, they all seem to use the terms inverter and driver interchangeably. Is a driver just an inverter with a built in power supply?


                              The generator on the bike will provide enough power to energize all the electronics, so I don't want something that is battery operated, but most of the products I've seen are. Will I just have to rig the inverter/driver to be able to incorporate it into our power system?


                              I apologize if I posted this in the wrong location. Let me know if I should have posted this elsewhere.

                              • 12. Re: Have a Lighting Question?  Ask the Expert!

                                you are luck!

                                I happen to have lots of experience with power -


                                Inverter  takes DC and makes AC - so for CFL and EL products you need to have AC signal - many people call the products that convert the power from DC to AC drivers or inverters ..

                                if you want to use the most efficient light source you will need to use LED technology - which will require a different kind of driver - dc/dc constant current driver-

                                Element 14 has many suppliers that you can help wih your power requirements

                                Good luck on your project

                                • 13. Re: Have a Lighting Question?  Ask the Expert!


                                  Thanks for the advice earlier on LED configuration and battery holders.  I made some measurements on my battery holder and you are correct that there is a lot of resistance in the springs.  Right now I don't know of any suitable off the shelf battery holders that don't use springs so at least for now I think I have to live with it. 


                                  After looking at LED configuration some more, I'm in the process of building a board with two MCEs configured as four parallel strings of two LEDs. This configuration has the advantage of also being compatible with two XM-L LEDs so I'll be testing it both ways.  Thanks again for the help and I'll let you know how it goes.



                                  • 14. Re: Have a Lighting Question?  Ask the Expert!

                                    You are in Luck - Cree Agrees with you - We are trying to make it easier to adopt Solid State Lighting..


                                    if you look at the latest Cree Data sheet for the


                                    Cree® XLamp® XM-L EasyWhite™ LEDs



                                    Cree choose to bin this part close to the target application operating conditions (MR16 20W LED replacement)

                                    •Binned at 85°C with single EasyWhite bin per CCT

                                    customers no longer have to guess or do math to figure out what LED CCT and Flux at 85C Tj

                                    we even added a chart labeled "RELATIVE CHROMATICITY VERSUS CURRENT AND TEMPERATURE (CRI 80)"


                                    to help customers understrand how their design will function in Pulsed Over-Current above the max rating:

                                    Pulsed Over-Current Driving of XLamp LEDs: Information and Cautions




                                    Please give us feedback on what you need to help you solve your lighting problems

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