Can’t believe it has been almost 5 months since I posted. I got samples from several flex strip manufacturers, data on hundreds more. Here’s the scoop on flex LED strips:

Quality varies considerably not all strips are the same, not even close. Price is no indicator of quality unfortunately. Some were OK, many poor quality (“waterproofing” epoxy not fully cured). Decided I need to design my own. Here’s what you need to consider when designing an LED lighting system.

1. Which LEDs are you going to use, don’t just say Cree or SSC or any “brand” there are hundreds of models of LED. Look at thermal performance as well as optical. If you don’t want to mess with lensing or much more optics than simple diffusers, choose carefully. Luminit has a really cool diffuser, holographic based design, nano-technology well applied.  Check it out if you can afford the good stuff.

2. HB LEDs (High brightness, also HP or High Power) are not usually suitable for flex boards. Too much heat to remove. Common are 0.2W SMD5050 and less than 0.1W SMD 3528. Newer SMD3535 that run 0.5 watt may be possible but you will need a good thermal analysis. Anyone out there have access to a thermal simulator? I can’t afford ComSol but it would do it. Meanwhile stick with 1/4 watt or less on flex.

3. Flex board layout for good thermal design is not difficult, but it looks nothing like the strips you get today from China. If you plan on sticking the flex strip to some metal surface, be sure it is clean… really clean, and large enough to dissipate the heat at a low temperature. Even the 3M glue will let go if it gets too warm for too long. Consider alternative backup adhesive.

4. Consider how you are going to drive the LEDs: Constant current is the best choice for a series string, but if you get too long the flex board can’t withstand the voltage across it. Beyond 7 LEDs in series (~24V) you probably need to think about rigid boards. Once you realize you need parallel circuits you have to balance current between the parallel sets. Voltage variation between LEDs and the response to temperature will lead to “current hogging” by one leg and thermal runaway unless you design in some appropriate feedback.

5. Once you’ve picked a series/parallel design and the number of LEDs to get the output you need (use a few more and don’t drive them so hard, you will get more light per watt at lower drive, look at the data sheets, that lumens per watt curve is a curve not a line. It’s called droop, but that’s too much detail for here.

6. OK so now you need a power supply capable of xx watts, see if you can find one that has appropriate approvals, ratings and protection for less than $0.50/watt. Maybe need to do your own supply too? Well that’s another expedition for another day. Stay tuned to learn about CC CV PFC and more lighting power acronyms.

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