# LEDs used on Limelight

What are the leds used on limelight? They are very bright and powerfull. I couldn’t find any information about them? How many lumens are they and what is their product number?

you should be able to find generic 1W led chips all over the place - here’s 10 pieces for \$5

What circuit should I use to drive them?

you should be able to just apply power and then let them work, you might want resistors to change the brightness

you should be able to just apply power and then let them work

Don’t do this - you will burn your LEDs out!

The IV curve (drawn current versus applied voltage) of LEDs is super nonlinear. Around the operating point/rated current draw of the LED, the slope on the curve is usually extremely high. That means that even a slight variation in the voltage of your power source can drastically increase the current drawn by the LED. For instance, in the image below, if the rated forward voltage and current of the LED is 50 mA at 2.1V, a perfect power supply would supply 2.1V exactly and yes, you could drive the LED directly off of the supply (assuming perfect manufacturing tolerances). But, if your power supply varies even in the slightest, let’s say just to 2.15v (a 5% increase, well within tolerance for many supplies), for the LED represented by the below image, you will more than double the current through the LED. That’s going to near instantly burn our your LED, which is not a fun time.

So what do you do? You add eithera resistor in series with the LED to limit current (the more current the LED tries to pull the more the voltage drop across the resistor is), or, for higher power LEDs (when you don’t want to deal with the power loss/heat dissipation on the resistor), you use a constant current power supply, which precisely modulates the input voltage so that the LEDs are drawing some reference current.

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Yes, constant current drive is required.

Also, for 1 watt or higher LEDs, proper electricomechanical design is key to dissipate the heat, otherwise they last about 10 seconds at full power. Read the data sheet and application notes.

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The data provided for the part that @solomondg linked is incomplete. It does not give the thermal impedance to the heat sink. It does have a line " Working temperature: less than 60 degress (the led need to use with heatsink)". Scrolling down, there is a fairly long section showing the recommended heatsink configuration (with a fan).

How bright do you (@ExplosivesTR) need the LED to be? You can do a parametric search on the DigiKey website. Click on “More Filters” then sort their offerings by the Millicandela Rating, colour, and viewing angle.

This is what we used in 2017.
You can probably find comparable items from other sellers on other sites.
Generic 3W green LEDs: https://smile.amazon.com/gp/aw/d/B01DBZIIPY
Heatsinks: https://smile.amazon.com/gp/aw/d/B06XKTRSP7

The LEDs were wired to a constant current / constant voltage supply similar to this: https://smile.amazon.com/dp/B07PBDXKD6/

The current limit was set by using a multimeter in series with the LEDs, set slightly lower than rated to what was bright enough for our use on field but not too obnoxious (was still uncomfortable to look at for any duration of time - we made a cover for the pits)

Three LEDs were positioned around our camera. They were left on for the duration of the match without any issues throughout the season w/ heat.

If you’re designing for on-robot use, and build a very bright array like the Limelight, you’ll want the ability to easily change the brightness of your array.

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