Hi Everyone, There seem to be a lot of questions and misinformation floating around about LEDs, so we thought we’d share some answers to problems we encounter most frequently. The answers are based on our day-to-day experience working with LEDs and are by no means scientific. Here goes: Q: What ratio of royal blue:cool white LEDs should I use? A: Everybody has different preferences on how blue they like their tanks to look. Generally speaking, a 1:1 ratio using CREE XR-E LEDs will give you a 14k look, while a 2:1 ratio will put you closer to 20k. If you are using CREE XP-G’s then you would need more blue to achieve the same look as the white XP-G LEDs are roughly 30% brighter at the same power. Q: How far above the water should the LEDs be placed? A: It depends on what your set-up allows for, but we recommend at least 6-8” above the water if you are going to use lenses to avoid “spotlighting” at the top of the tank. Q: What angle lens should I use? What do the angles mean? A: The larger the angle, the wide the spread of light there will be. Thus, a lens that is 80 degrees will spread out much further than a lens that is only 30 degrees. However, the 30 degree lens will allow the light to penetrate deeper into a tank. Most CREE LEDs without any lenses will be about 110-120 degrees. Which angle lens to use depends on the depth of the tank. As a rough guide: if your tank is less than 20” tall, use 80 degree optics (or none at all), for a tank 20-30” tall, 60 degree optics would probably be best, and for a tank with a height in the high 20’s or 30”+ go with 40 degree optics. There are more angles available than just the 3 we listed so again just use this as a rough guide. Q: How many LEDs do I need to replace my MH fixture? A: 24 CREE XR-E LEDs will give higher PAR values than a 250W MH fixture. There isn’t an exact formula for this, but you can take that as a rough measurement and extrapolate to fit your desired lighting needs. Q: What are the minimum and maximum number of LEDs my driver can power? A: To calculate this, you need two pieces of information. First, you need to know the minimum and maximum voltage of your driver (ex. 9-48V). Second, you need to know the voltage drop across each LED (ex. 3.3V). Note this is only applicable to constant current drivers, not constant voltage drivers. Example: Driver: Mean Well LPC-35-700, 9-48V LEDs: CREE XR-E, ~3.3V each 9V driver min voltage/3.3V per LED = 2.72 LEDs minimum. Rounded up = 3 LEDs minimum (we can’t use less than 9V, so we round up to 3 LEDs) 48V driver max voltage/3.3V per LED = 14.54 LEDs maximum. Rounded down = 14 LEDs maximum (we can’t use more than 48V, so we round down to 14 LEDs). Q: My LEDs flashed briefly and went out. What does that mean? A: 99% of the time it means your LEDs are fried, usually due to too much voltage and/or current going through them. Check to see if you have at least the minimum number of LEDs in your string. Also, make sure your driver has been attached to the LED string, i.e., ENSURE your wiring is COMPLETE BEFORE you plug the driver into the wall. Q: What’s the difference between the MeanWell ELN-60-48D and ELN-60-48P? What else do I need to dim them? A: Both drivers will require an external 10V power source for the dimming circuit. A lot of people like to use a 9V wall wart as it’s easier to come by. In addition, the dimming on the D is controlled with a 0-10V regulator, while the P requires a PWM controller. For those that want a knob they can turn to control dimming, you’ll want to go with the D driver. The 0-10V regulator is normally controlled through a potentiometer that can be purchased at Radio Shack for $3. In addition, these drivers are compatible with the various reef controllers on the market (ReefKeeper, Reef Angel, etc). Just look at the spec sheet to see if they are 0-10V or PWM compatible. Q: Do I need a fan? A: An extremely important but often overlooked part about building a quality LED fixture is cooling. In order to achieve the stated 50,000 hour lifespan of the LEDs, it is very important to make sure the LEDs are sufficiently cooled. While most heat sinks on the market work well, the addition of a single fan will drastically lower the temperature of the LEDs. Keeping the LEDs cool should extend their useful life because heat is the enemy of all semiconductors. A modest fan on top of one of our heat sinks will keep the temperature of the fixture down to just slightly above room temperature (room temp = 72°F) – not even warm to the touch. Q: How big of a heat sink do I need? A: This depends on several factors such as how high from the water you’ll be hanging your lights, what degree optics (if any) you’ll be using, the dimensions of your tank, but generally speaking you do not need the heat sink to be the same footprint as your tank. For example, if your tank is 48” you do not need a 48” heat sink. Keep in mind that light will be coming down at an angle towards the water, so there will be a good amount of spread to the light (obviously the larger the angle the better the spread). As an example, we used a 4.25” x 23” heat sink on a 30” x 18” tank at MACNA using 60 degree optics at about 8” above the waterline and had no problem whatsoever with coverage. Q: How do you wire LEDs together? How are they connected to the driver? A: Please see the attached picture for an easy visual aid on how to wire LEDs in series. Generally speaking, you’ll attach the LEDs to each other + to -, and from the driver to the LEDs + to + and – to -. LED wiring diagram - 3reef Photos If there are any other questions out there feel free to ask them and we can answer them as a group. Also, if you’ve experienced anything different than what we’ve stated feel free to share that experience. A lot of this is more art than science so the more experiences we have to draw upon the more beneficial it will be for everybody. Updated FAQs for dimmable drivers (specifically Mean Well ELN series): Q: I am trying to adjust the V (voltage) and I (current) of my driver but I am getting inaccurate results when I turn SVR2. A: The driver must have a load. Without a load you cannot measure the current or voltage output of the driver accurately. Connect the minimum # of LEDs to the driver before attempting any measurements. Q: What is the minimum # of LEDs for my driver? A: ELN-40-48-D: 8 LEDs ELN-60-48-P: 8 LEDs LPC-35-700: 3 LEDs (Assuming you're using 3W CREE LEDs. Otherwise please see our original post for how to calculate minimum loads) Q: What do SVR1 and SVR2 do on my D or P model driver? A: SVR1 adjusts maximum voltage. SVR2 adjusts maximum current. Q: Can I run my D or P model driver without a dimming control unit (ie. potentiometer or PWM controller)? A: Yes, attach a constant voltage power source of 10V (a wall wart will work) to the driver and set SVR2 to your desired output current. It's kind of the poor man's way to dim but it will work.