# LED University

### How do I know how many LEDs I'll need?

There are various ways to calculate the number of LEDs you’ll need, and we’re always happy to recommend the correct kit for you. As a very general rule of thumb, for reef tanks without SPS you can use the following formula when using our LEDs:

(Length of tank in inches x Width of tank in inches)/22

For example, if you have a 40g breeder and your footprint is 36” x 18”, you would need:

(36” x 18”)/22 = roughly 28-30 LEDs

For tanks with SPS you can use the following formula:

(Length of tank in inches x Width of tank in inches)/18

For example, if you have a 75g tank and your footprint is 48” x 18”, you would need:

(48” x 18”)/18- = roughly 48 LEDs

For freshwater or planted tanks you can use the same formula, only divide by 35-40 instead of 18-22.

For example, if you have a 75g tank and your footprint is 48” x 18”, you would need:

(48” x 18”)/35 = roughly 25 LEDs

### What Current Should I Run My LED’s At?

LED TypeRecommended CurrentMaximum Current
Cree XP-E (Blue, Royal Blue, Green) 800mA-900mA 1000mA (1A)
Cree XP-G/XP-G2 (Neutral and Cool White) 1200-1300mA 1500mA (1.5A)
Cree XM-L/XM-L2 (Neutral and Cool White) 1600-2800mA 3000mA (3A)
Cree XT-E (Royal Blue) 700-1300mA 1500mA (1.5A)
Philips Cyan 800-900mA 1000mA (1A)
Philips Rebel Deep Red 660nm/UV 600-700mA 700mA (0.7A)
Moonlight 350mA 350mA (0.35A)

### How do I know which drivers to use?

The first decision to make is whether you want a dimmable or a non-dimmable driver. Dimmable drivers have a few advantages:

• Tweak color temperatures to your liking (if you have multiple drivers)
• Acclimate tanks more easily
• Compatible with many reef controllers (ie. Apex, Reef Angel, etc) for sunrise/sunset simulations

Once you know which type of driver you need, you’ll need to look at the current (mA) output of the driver. For non-dimmable drivers they will only output one current level. For example, the LPC-35-700 driver will output 700mA at all times, regardless of how many LEDs are attached to the driver (assuming you’re running one string in series off the driver, as opposed to say multiple parallel strings).

Dimmable drivers on the other hand have variable current levels. Please check the product description page and/or technical data sheets for the current range. For example, the ELN-60-48D driver outputs a current of 0-1.3A. Since the driver can output a max of 1300mA, this driver may be too powerful for some LEDs (ie. Red and UV LEDs can only handle 700mA) unless you dial down the maximum output current via the driver’s SVR2 knob (Mean Well drivers only).

To figure out how many LEDs a driver can run, you’ll need two pieces of information: the driver’s voltage range and the voltage required by the LED at the given current level that you would like to run the LEDs at. As an example, the LPC-35-700 driver has a voltage range of 9-48V. The XP-G Cool White LED requires 3.05V at 700mA (remember this is the current output of the LPC-35-700 driver). Thus, the minimum number of LEDs required is 9V divided by 3.05V. 9/3.05 = 2.95, thus we round up and the minimum number of LEDs is 3. For maximum LEDs, we divide 48V by 3.05V and get 15.74. We round down and get 15 LEDs. Typically we play it safe and thus on our product page we list the range on the LPC-35-700 driver as ~3-14 LEDs. Keep in mind the voltage required by each LED can be quite different so you may need to do these calculations for each of your strings.

### What angle lenses should I use?

What angle lens you should use depends on a couple of factors, namely the height of the lights above the water, the height of the tank, and the spacing of the LEDs. As a general rule of thumb please use this table as a guide:

Tank HeightLens AngleRecommended Height Above Water
Up to 22” 80 degree 7-12”
22-30” 60 degree 10-15”
30”+ 40 degree 15”+

### How do I glue the lenses on?

First, ensure that there are no solder balls next to the LED chip. This will prevent the lens from sitting flat. The LED chips are square and the lens cut-outs are also square. If the lens isn't sitting properly the first time, try rotating it 90 degrees until the lens sits flush with the board. Once this happens, use thermal adhesive (Arctic Alumina) or plastic epoxy to glue around the edges of the lens. This will prevent adhesive from touching the LED directly.

We also suggest NOT putting adhesive on the bottom of the lens and pressing the lens down. While this is easier/faster, it does often lead to adhesive sticking to the LED chip. This may cause damage to the LED, and when removing a lens almost always ends with the LED chip coming off the board and being ruined.

We typically do not recommend using super glue or adhesives with fumes as they may cloud or damage the LED (we've done it in the past ourselves in a pinch, but it's generally not recommended).

### What's the best color ratio to use?

This is one of our most frequent questions, and one that unfortunately we can't really answer since everyone has different preferences for light. Some people like their tank to look really natural and hate a blue looking light, while others love blue and want the 'windex blue' look. If you look at our Onyx fixture you'll see the colors and quantities that we used which we feel provide a balanced look. On that fixture, out of 28 total LEDs we are running 4 UVs, 1 red, and 1 green. For supplemental colors we feel this is a good ratio, especially on the reds and greens which tend to overpower other colors quite easily. If someone wanted to use 6 or 8 UV's instead that is certainly not incorrect. As for the ratio of blue/white LEDs, that is mostly a personal preference. 95% of people are in between a 1:1 to 2:1 (royal blue:cool white) ratio, which is in between ~13-20k.

### How do I know how much wattage my set-up or LEDs are?

To calculate an LED's wattage you can simply multiply the drive current (for example 1.0A or 1000mA) by the voltage draw of the LED, at that specific current (in our example the voltage draw at 1.0A). Using the XT-E royal blue LED as an example, if we look at the datasheet on page 8 we see a graph depicting the voltage draw at various output currents. At 1.0A the LED will draw roughly 3.2V, thus 1.0A x 3.2V = 3.2 watts. If you had 12 of the royal blues all running at 1.0A your LEDs would be drawing about 32 watts total. You can calculate all the actual wattages on your LEDs and add them together to find the total wattage from your set-up. Please note that just because an LED may be '5 watts' does not mean that it always draws 5 watts. While the XT-E royal blue is a '5W' LED our example has just shown that the actual draw is almost always less (unless you're always running your LEDs at max current).

### What’s the difference between the MeanWell ELN-60-48D and ELN-60-48P? What else do I need to dim them?

The difference between the two models of driver is the type of signal they accept for dimming. One accepts an analog reference voltage, the other a PWM waveform.

The "D" model accepts an analog 0-10V signal. This is simply a voltage applied across the DIM+ and DIM- wires between 0-10V. This can be accomplished with a 10V reference signal from our 10V AC Adapter, which outputs a 10V DC signal the "D" model accepts. If you want to vary the intensity from 10V down to 0V (or 0V to 10V), you can wire the adapter to a potentiometer, which when rotated, will vary the voltage from 0-10V and change the intensity accordingly. For example, 1V = 10% intensity, 5V=50% intensity, 10V=100% intensity. If you have a number of drivers you wish to control, you could try our DIY 2 color dimming kit: http://www.rapidled.com/retrofit-kits/

The "P" model accepts a square wave, or PWM (pulse width modulation), signal. The PWM wave must be 0-10V also, but it is either at 0V or 10V, nothing in between like with the analog signal the "D" model uses. So, the PWM signal runs by a on-off-on-off pattern with on being 10V and off being 0V. Our DDC-02 lighting controller provides three daylight modes with sunrise and sunset and a manual intensity adjustment mode: http://www.rapidled.com/drivers/

### What size heatsink do I need?

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, and the dimensions of your tank. 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 a few years ago using 60 degree optics at about 10-12” above the waterline and had no problem with coverage.

### My string lights up but I have a few LEDs that aren't lighting, does this mean my LEDs are bad? How can I fix this?

Not necessarily! If you have a multimeter, you can perform a continuity test, which will probably tell you why certain LEDs are not lighting up: they are grounded to the heatsink somehow. Continuity refers to two things electrically connected. For example, if two parts are connected by a wire, they are continuous.

To perform this test, set the multimeter to measure continuity (usually the 'diode' setting) and ensure no power is applied to your LED string. Next, touch one probe to the heatsink and the other probe to each solder joint of every LED, going around your string until all joints have been examined. If you hear a beep or get 0.000 reading at a particular joint, there is continuity (conductivity) between that joint and the heatsink (this means there is an electrical path between that solder pad and the heatsink, which we call grounding). This solder joint will have to be re-done since the LED string should be completely isolated from the heatsink electrically for the string to work properly.

### How do the LDD drivers work? How are they different than the ELN drivers?

LDD drivers are different than ELN drivers in that they require DC voltage from a power supply and will pass that voltage through to the LED string, while keeping the output current constant. In the case of LDD drivers, 3V will be lost when the voltage is passed through between the input and the output. Using the LDD-700HW and the SE-350-48 power supply as examples, you will be able to get a constant 700mA to all LEDs in your string (the LDD-1000HW would output 1000mA, etc). The SE-350-48 outputs 48V, and after deducting 3V the output voltage will be 45V. As we mentioned earlier a typical LED draws about ~3-3.3V, so we can run 45/3.3 = ~13 LEDs maximum per string. There is no minimum voltage required so you can run strings of even 1 LED, making this a very versatile driver.

The LDD drivers require a 5V PWM driver for dimming, so you'll often see Arduino based controllers being used as that's the standard dimming output signal for Arduino. Unfortunately our DDC-02 only outputs a 10V PWM signal so they are not compatible. We are looking to add compatible controllers in the near future to our dimming selection.

### What's the difference between Thermal Adhesive, Thermal Grease, and Thermal Pads?

Thermal adhesive is a two part epoxy that will adhere your LEDs to the heatsink surface. This is meant to be a permanent adhesive and would be used if you do not have a drilled/tapped heatsinks or a heatsink with T-slots for screws. The advantage to using thermal adhesive is you do not require a drilled/tapped heatsink, and it's relative low cost if using a lot of LEDs. The disadvantage would be if you ever needed to remove the LEDs it's more difficult than thermal grease. If you do need to remove LEDs with thermal adhesive get a tool with a flat edge (ie. flat head screwdriver or box cutter) and get the surface underneath the LED board. The adhesive is brittle so it will 'pop' off.

Thermal grease is not an adhesive so it can only be used if you are holding the LED(s) down with screws. If using screws, we suggest using nylon washers (or nylon screws) as screws can sometimes lead to grounding issues if touching a solder pad or solder directly. Thermal grease is great in how easily an LED can be removed if it ever needs to be replaced or upgraded.

Thermal pads are like double sided tape. There is a film on both sides that need to be removed, which exposes a sticky surface to attach the LEDs on one side and the heatsink on the other. If using thermal pads you would typically not require screws. If using a small number of LEDs these are a very fast and cost effective way of sticking your LEDs to the heatsink, and is a lot less messy than thermal adhesive.

These three types of thermal material should not be combined, so it's an either/or choice between them.

If using thermal adhesive or grease remember that more is not necessarily better! A small dab per LED is enough, typically you don't want to see it gushing out of the sides of the LED star when setting. Too much thermal material can inhibit thermal transfer and actually prevent heat from transfering properly to the heatsink.

### What's the difference between the Storm and Storm X? How about the Bluefish and the Bluefish Mini?

The main difference between the Storm and Storm X is the number of channels that you're able to control. The Storm provides 6 channels of control while the Storm X provides you with 16. Also, the Storm has 256 dimming levels compared to 4,096 on the Storm X. Practically this means the dimming on the Storm X is smoother, though for most people dimming smoothness isn't an issue even on the regular Storm.

In addition, if you have an analog (0-10V) or 10V PWM driver you can only use the regular Storm with a 10V converter to keep your current drivers. The 10V converters won't work with the Storm X as currently designed/without further modification.

The Bluefish is compatible with 3 types of dimmable drivers: 5V PWM, 10V PWM, and 0-10V analog. The Bluefish Mini is only compatible with the LDD drivers (technically a 3.3V PWM, though sometimes referred to as a 5V PWM). Also, the Bluefish does come with a case while the Bluefish Mini does not (though we do carry aluminum cases for the Bluefish Mini for an additional fee).

There are of course more differences than what we've listed, but on a very high level we've tried to outline the most important ones. If trying to decide between a Storm or Bluefish Mini just ask yourself how much more you would pay for a wireless set-up. If it's important that you can control the lights from your smartphone/tablet and it's worth ~\$40 to you then go with the Bluefish Mini, if that's not important than stick with the Storm. Our customers have had great success with both controllers and functionally they are quite similar (ie. both will do sunrise/sunset, lightning, geo location, etc).

### How many drivers can I dim with my controller/10V adapter?

This will depend on two factors - the dimming circuit draw and the controller output. For example, the usual draw on an ELN-60-48D or -48P driver is roughly ~3-5mA. However, on LDD drivers the draw is typically measured in microamps (1mA equals 1000 micro amps) meaning the draw is so low that you'll almost never be limited by the number of drivers per channel if using LDD drivers.

The second part of the equation will be the mA output of the dimming signal/voltage. For example, output on each channel of a Neptune Apex controller is roughly 20mA. If you are running ELN-60-48D drivers, you should be able to run 4 drivers without any issues (20mA output / 5mA output = 4 drivers).

### How hot should my heatsink be?

We recommend keeping your heatsinks under ~120-130 degrees F. If your heatsink is hotter than 130 degrees F we recommend adding some active cooling (fans) or turning down the current (mA) on your driver(s).

### I received washers with my order, where do they go?

Since the screws we provide with our heatsinks are zinc and heatsinks are aluminum if you're not careful you can ground out the LED. This typically happens when the screw is touching solder/a solder pad. By placing a nylon washer in between the screw and LED this will help prevent that from happening. The washer must go above the LED, just below the screw head.