Back to Basics: LEDs: Part 11: Light Pipes

One of the harder problems I face when I design products with LEDs is how to get this light to the user on the product surface, esp. the indicator LEDs. How do you make them look good in a product? Enter Light Pipes. These are fundamentally mechanical parts made usually of transparent acrylic or polycarbonate to guide light from your LED source to the product surface. They operate based on Total Internal Reflection. Light enters at an angle greater than the critical angle, undergoes multiple internal reflections, and exits on the other side.

LightPipes come in two types, Rigid & flexible. Rigid ones are common in consumer products where the LED’s position is fixed, and the path to the surface is relatively straight. It can be mounted directly on a PCB in a straight line, or at a 90-degree angle in cases where a straight path isn’t feasible (Check images). For 90-degree LightPipes, a slight turn radius is advisable to minimize light losses. On the other hand, Flexible light pipes, typically crafted from optical fibers, can be bent into various shapes to achieve the desired light output.

The important consideration point is the light entering point between the LED and the pipe. Your goal is to ensure maximum light enters the pipe and minimize leakages. You can do that by having an opaque coupler or by reducing the distance between the LED and pipe to an absolute minimum(but not touching). Usually width of the input of the light pipe should be wider than the beam angle of the LEDs, so for these applications always prefer using LEDs with a lower beam angle for larger light coupling.

And there you have it! Wrapping up this LED series. It’s been fun. I’ve tried to cover all things LED-related, making it as enjoyable and informative for you as it was for me putting it together. Hoping you loved it. If I missed anything, give me a shout. See you next week with something else.

If you liked the post, Share it with your friends!

Back to Basics: LEDs: Part 10: LED Thermals

When it comes to LEDs, it’s not just about the light they emit; it’s about how they handle the heat! LEDs are sensitive beings; they don’t bode well under excessive heat. Heat adversely affects their efficiency, light output, and overall longevity. If you’ve ever seen a decline in LED brightness over time, heat is the problem. Excessive temperatures can lead to a phenomenon known as “thermal runaway,” where the LED’s performance spirals out of control, shortening its lifespan and compromising its luminosity.

LEDs generate heat primarily through the forward voltage across the semiconductor junction. Higher the current, more the heat produced. Heat output can be assumed to be around 80% of the input power given to the LEDs(rough estimate). Do note that forward voltage decreases as the junction temperature of an LED increases. Temperature increase can cause colour shifts in emitted light too. There will be a graph in the LED datasheet called the temperature derating graph, which plots LED current vs LED temperature. It gives a safe operating region for an LED for a given temperature. Always keep within its limits.

So how do you keep your high-power LEDs cool? Heatsinks are the most common solution. They come in all shapes and sizes. Thermal heatsink design is simple but too long to explain here. Dave from EEVblog has a few videos on how you can do that, with thermal resistances from the heatsink’s datasheet, to keep your LED junction temperature in check. A commonly overlooked aspect by newbies is LED thermal vias and connecting them to the large ground planes. Surprisingly, you can utilize PCBs, both Aluminium and FR4, as effective heatsinks, provided they are sufficiently large. For critical applications, consider integrating a PTC thermistor. Typically positioned near the LED at a test point, it actively measures real-time heat, enabling the regulation of driver current via feedback. This, in turn, acts as a safeguard against thermal runaway, ensuring the stability and long life of your LED system.

If you liked the post, Share it with your friends!
1 2 3 53