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Please note this article discusses the application of only one of the two types of LED drivers, which are “switch mode” and “linear”.  Switch mode drivers are considerably more energy efficient than linear drivers.  Since many people choose LEDs for their high energy efficiency, I do not use linear drivers in any of my products as that would defeat the purpose of using LEDs in the first place.  So the context of this article is in that of the switch mode drivers.

Voltage (V for Volts) is pressure or strength.  Current (A for Amps and mA for milliamps; 1000mA = 1A) is flow or work being done as a result of the voltage.  Voltage is impressed upon the circuit and the result is current flow.

Now given that, LED drivers are strange beasts.  They operate in an unconventional manner in that they regulate the current, not the voltage.  They are constant-current drivers.  They change the voltage to whatever is necessary to maintain a specific current level.  A 1000mA driver will always drive the LEDs at 1000mA total, regardless of the voltage required to do so.

On the LED side of the driver…

Connect LEDs in series to add their voltages.  The current remains the same.

Connect LEDs in parallel to divide the available current between them.  The voltage remains the same.

Many applications, including the example below, use both series and parallel to obtain optimum use of the LED drivers and power supply.

Consider the following example:

• 18 LEDs rated at 3.2V and 350mA each
• A 1000mA “buck” LED driver (a buck driver steps the voltage down)
• A 24V/1.5A power supply

One LED requires 3.2V at 350mA to produce its rated output.  Therefore, two LEDs in series require 6.4V at 350mA.  Two in parallel require 3.2V at 700mA.  And three in parallel require 3.2V at 1050mA to achieve their rated output.  Go back and re-read those sentences a few times to understand the relationships.

Since the LED driver delivers a constant 1000mA, we must use LEDs in parallel to divide the available current and prevent burning out the LEDs.  Three LEDs in parallel will divide 1000mA into about 333mA per LED.  So far we’re only driving 3 LEDs and we need to drive 18.

Since our power supply pushes 24V, we can use LEDs in series to add their voltages to take advantage of the available voltage from the power supply.  When connecting LEDs in series, the string of LEDs must add up to a few volts less than the power supply for the drivers to work correctly.  We can drive up to 6 LEDs in a series string for a total of 19.2V, which is perfect for a 24V supply.

Again, since the LED driver delivers a constant 1000mA, we must put strings of 6 LEDs in parallel to divide the available current and prevent burning out the LEDs.  Three strings of 6 LEDs in parallel will divide 1000mA into about 333mA per string.  Now we have 3 strings of 6 LEDs for a total of 18 LEDs. (CAUTION: strings in parallel must be identical.  Never run a string of 5 LEDs and a string of 6 LEDs in parallel. Also never run strings of different colors in parallel because different color LEDs have different electrical properties.)

So three LEDs in parallel use 333mA each and three strings of 6 LEDs in parallel use 333mA each.  But the strings of 6 will be considerably brighter, even though both cases are using 333mA.  Why?

Power (W for Watts) is how much work is being done, based on the product of the voltage and current.  For LEDs, work is light output and heat.

Watts = Volts x Amps

Three parallel LEDs at 3.2V and 1000mA use 3.2W.  But three parallel strings of 6 LEDs at 19.2V and 1000mA use 19.2W.  Remember above I stated the drivers will adjust the voltage to maintain a specific current level?

On the power supply side of the driver…

We have 19.2V of LEDs and 1000mA of current running through them.

Our 18 LEDs in 3 strings of 6 are using 19.2W of power.  We multiplied the voltage of the strings by the total current flowing through them to determine this number.  Therefore, we need 19.2W of power coming into the driver from the power supply.  So you rearrange the formula above like this:

Amps = Watts / Volts

Our power supply is 24V.  We picked that voltage because it’s very common and easy to come by.  Doing the math, 19.2W at 24V gives us 800mA.  Think about that for a moment.  We are pushing 1000mA through the LEDs, but only drawing 800mA out of the power supply.  This is because power is the product of voltage and current.  Since our power supply pushes 24V and we only need 19.2V to light the LEDs, we gain some current to make the power in and power out roughly equal.

1000mA / 3 strings = ~333mA per string

3.2V x 6 LEDs = 19.2V per string

19.2V x 1000mA = 19.2W total LED power

19.2W / 24V = .8A (800mA) from the power supply

IMPORTANT: The drivers aren’t 100% efficient (they’re about 80-90%), and you never want to run a power supply near 100% capacity.  So you have to add 25%-50% to the current rating (Amps) for losses and overhead when calculating for what size power supply to use.  That’s why we’re using a 24V/1.5A supply when we only need 24V/0.8A.

For a Power LED Shield that is fully loaded with LEDs, I recommend a 24V/6.5A supply even though the shield will only draw 4-4.5A from the supply.  This allows the power supply to run cooler and gives you some room to add fans or power the Arduino board as well.

In general, a LED aquarium light consists of a power supply, LED drivers, LEDs, heat-sinks and fans.  Also common is a micro-controller, such as the Arduino to automate schedules and control dimming.  Many go as far as having an LCD display with menu and input for changing and setting the micro-controller program.  For those wanting a very nice, fully automated system, the Krusduino project fulfills that.  Here I will discuss things from the perspective of an Arduino running a program such as Krusduino or a self-rolled program.

What I don’t talk about here is how many of what color or brand or type of LED; which optics to use or how to achieve the right aesthetic.  That is because all that is COMPLETELY SUBJECTIVE and opinions will vary widely.  This is just a guide on the electronics, cooling and sizing.

UPDATE: The Power LED Shield V2 uses up to 1200 mA RDC-24 LED drivers from RECOM. I’ve updated the content of this article to align with the capacity of the Power LED Shield V2.

1. How many LEDs? Drivers?

Each RCD-24 driver module can drive up to 9 LEDs.  You can put multiple identical strings in parallel for a total of 18, 27 or 36 LEDs on a single channel.  But the output of the driver will be divided evenly into each string.

For example, on a 1200mA driver…

• 9 LEDs in a string:  each LED will get 1200 mA
• 2 parallel strings of 9 LEDs:  each LED will get 600 mA
• 3 parallel strings of 9 LEDs:  each LED will get 400 mA
• … and so on.

The brightness of each LED will be reduced accordingly with the drop in drive current.  Note, however, that LEDs are more efficient at lower drive currents, so the brightness will not drop as fast as the current.

Sticking with 9 LEDs per driver, the Power LED Shield has four RCD-24 drivers on it, for a total of 36 LEDs per Power LED Shield.  Not all drivers have to be used.  In fact the product page allows you to order your shield with any number of driver modules up to four, and in any combination of capacities.

2. How many LEDs on a heat-sink? How big of a heat-sink?

I recommend no less than four square inches of heat-sink per LED.  A common heat-sink size is 12″ x 8″, which will hold 24 LEDs.  If you’re running a nano or pico tank, scale down from there to maybe 6″ x 8″ (12 LEDs), 6″ x 4″ (6 LEDs) or 12″ x 2″ (6 LEDs).  If you can’t find a heat-sink of those dimensions, just go for something that works for you and make sure you do not go any more dense than four square inches of heat-sink per LED.

For maximum utilization of the capacity available in the Power LED Shield, run 36 LEDs on a 12″ x 12″ heat sink.

3. Fans? How many? Temperature controlled?

Here I diverge a little from popular belief (or maybe just popular desire).  As much as I like temperature controlled fans, I do not like them on LEDs.  The overall lifespan of the LED is directly related to its operational temperature.  So I feel the fans should always be running when the LEDs are powered on.  Maybe a good compromise is to vary the speed of the fans with the brightness or temperature of the LEDs, but only turn the fans off when the LEDs are off.

If you are sticking with the 12″ x 8″ heat-sinks, then one reasonably powerful fan per heat-sink will do you well.  Find a fan whose diameter is at least half the width of the heat-sink; preferably more.  Try to balance air flow and noise.  Smaller heat sinks can use smaller fans.  ALWAYS USE A FAN or you may kill your LEDs prematurely.

Mount the fan so it is blowing down onto the heat-sink from about 1/4-1/2 inch above the heat-sink fins. You need that gap for efficient air flow.

IMPORTANT: Most fans run on 12V power.  But the Power LED Shield handles up to 36V.  The most common power supplies I’ve seen in use for Power LED Shield projects are 12V, 24V and 36V.  You must use pairs of identical fans wired in series for a 24V supply, or triples of identical fans for a 36V supply.

4. Pico? Nano? Or a BIG tank?  How many heat-sinks and Power LED Shields?

Nanos and picos are usually good with one 12″ x 8″, 12″ x 4″ or smaller heat-sink.  Sometimes a few small heat-sinks are necessary for good coverage.  In any case, you can usually get away with one Power LED Shield.

For large tanks, a 12″ x 8″ heat-sink will light 18″ x 18″ to 24″ x 24″ of water surface and a 12″ x 12″ heat sink will cover a bit more.  Orient the heat-sink to your preference, for best appearance.

• 48″ L x 15″ W: 2 Power LED Shields driving 48 LEDs on two 12″ x 8″ heat sinks
• 72″ L x 18″ W: 3 Power LED Shields driving 108 LEDs on three 12″ x 12″ heat sinks
• 96″ L x 36″ W: 6 Power LED Shields driving 216 LEDs on six 18″ x 8″ heat sinks

For deeper water, use optics that focus the LED light into a narrower beam.  That’s all I am going to say about it because there is a ton of personal preference and variation due to the kind of organisms in your tank.

5. How to power all this? Amps, Volts and wire gauge (oh my!)

Each Power LED Shield will draw about 5A when fully loaded with LEDs.  You can run a power supply for each shield, or you can run one big power supply for everything.  It’s up to you.  For individual supplies, I recommend a 24V or 36V, 6.5A switching power supply.  If you are going to use one big supply, account for each Power LED Shield, plus some room for fans, the Arduino and about 15%-25% more for overhead.

The LEDs can be connected with 22 Ga wire.  If the wires are longer than about 2ft total, step up to 20 Ga wire.

For individual power supplies to each Power LED Shield, 22-18 Ga wire is good.  For wires over about 2ft, go with 18 Ga.

For one big power supply, you have to break out the wiring to each shield because you can’t fit that heavy of a wire into the screw terminals on the Power LED Shield.  Run heavy feed wires from the power supply to a distribution block, then run 22-18 Ga wires from the distribution block to each Power LED Shield.

The fans can be connected with 24-22 Ga wires.  Sensors should all use 28-24 Ga.

Look here for the current carrying capacity of wire gauges:  Powerstream Wire Size

There you have it.

If I missed anything, please report it to me at “mark at chester family dot org”.