Back to Basics: LEDs: Part 5: Dimming

LED Dimming

LED dimming is a fundamental aspect of modern lighting systems. Rarely do you see LED systems just doing ON and OFF these days. Mood lighting, custom fades and ambiance is all a rage now. It’s all about achieving the desired brightness for a particular application. The key reason why dimming is essential is that it increases energy efficiency and also keeps extending its lifespan by not driving it at its full brightness.

There are different kinds of dimming drives possible for LEDs. Before we get into that, it’s important to understand what dimming ratio is. This is a ratio number you see in most LED driver datasheets. The dimming ratio in LEDs refers to the ratio between the maximum brightness (luminance) of the LED at full power and its minimum brightness when dimmed. It essentially measures how effectively an LED can be dimmed. A higher dimming ratio indicates a better performance in terms of achieving lower brightness levels when dimmed. For instance, a dimming ratio of 1000:1 means the LED can dim down to 1/1000th of its full brightness.

It also corresponds to the dimming resolution of an LED driver which refers to the number of distinguishable steps or levels at which an LED light can be dimmed. It represents the granularity of control over the brightness levels. Meaning, that even if you have a high dimming ratio and low dimming resolution, you may be able to get to the lowest dimming ratio but the gradation from low to high may not be smooth and your eye might be able to pick it up when you go from high to low. The higher dimming resolution allows for finer adjustments to the light output. Some LED types of drives can achieve only some range of dimming ratios. We will look into those in detail next time.

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Back to Basics: LEDs: Part 4: Matrix Topology Drives

LED Matrix Drives
LED Matrix Drives

In our exploration of LED drives, we’ve looked into series and parallel setups. Now, let’s check the fusion of both: the Matrix Drive. This advanced configuration intertwines the best series and parallel designs, offering unique advantages for various applications. The image shows how to wire it up. In this setup, each row is shorted, creating individual paths for the current flow.

The advantage here is that, if an LED fails as an open circuit, the rest of the LEDs in the column are not affected as the current has a different path to flow. Current variation in arms is kept to a minimum. Now when an LED fails as a short circuit, the entire row turns OFF, but still rest of the LEDs in the columns are ON and with no increase in current per branch. So it does give you some sort of protection to avoid a chain reaction of all LEDs failing.

The major drawback of this method is that since there could be variations in Vf of the LEDs and each LED is in parallel to one another, you will see that the brightness intensity of LEDs won’t be uniform and there may be hotspots & dim spots in the lighting. So you have to choose LEDs with minimal Vf variation from the manufacturer.

Overall if you have no constraints over cost, series LED drives might be the best bet. If there are quite a lot of those LEDs, keep adding more series drivers. If cost is a concern(buck drives are cheaper) and you don’t have much concern with the lifetime of LEDs consider parallel drives. If you choose this, better to go with more LEDs in columns to reduce the current increase if one of them fails.

So now you know all about the nuances of series, parallel, and matrix LED drives and each has its pros & cons. Choose one based on your application and requirements. Happy designing!

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