Introduction
In starting sales of a new PWM LED Controller (The Storm and Storm X), we discovered the prevalence of consumer confusion about the various LED dimming schemes that are in use today. This article aims to provide a succinct and informative introduction to the topic of LED dimming as applied to aquarium lighting.

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Skyler’s PWM LED Reef Tank

Background
LEDs are different than most electronic gadgets in that they are constant current devices. This usually means that the current must be regulated in some way. For simple low-power LEDs, a series resistor can be used to limit the current to the manufacturer spec. For high power LEDs, the current is regulated by an LED driver. You can buy an LED driver that is rated to drive the current that you want for your LEDs. Once you hook up the power, viola, there’s light! But as you’ll know once you turn it on, today’s high power LEDs are blindingly bright. Left unchecked, high power LEDs at full brightness can disorient and stun your fauna, bleach your corals, cause excessive heat, and reduce the life expectancy of the LEDs themselves.

Voltage & Power Supply Selection
In order to build out a string of LEDs for your reef tank, one of the most important things you need to know is your LED string forward voltage, Vf. Each string should be wired in series (positive of one LED to negative of the next LED, and so on) and driven by a single driver. Then the total forward voltage is the forward voltage of each LED multiplied by the number of LEDs.

Suppose for example that you are running a string of 10 Cree XT-Es. You want to look at the manufacturer’s datasheet and find a chart that plots Forward Voltage vs. Forward Current, which is on page 7 of the XT-E datasheet. Let’s say our constant current driver is rated at 1000 mA, then from the chart we know that the forward voltage is about 3.18V, multiplied by 10 is 31.8 Volts. Most LED drivers are fairly efficient, so it is usually safe to assume an efficiency of 90% when converting the input voltage to the output voltage used by the LEDs. Using this assumption we divide 31.8 Volts by 90%, arriving at a value of 35.3 Volts. Rounding up to a standard power supply size, we select a 36V power supply.

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Cree XT-E Datasheet – Current vs. Forward Voltage

We also need to know the wattage of the power supply, which is an extremely simple formula. Power (watts) = Current (amps) multiplied by Voltage (volts). The current is 1A, and the Voltage is 36V, so the total power dissipated by the LEDs full-on is 36 Watts. Suppose you’re running two of these strings of LEDs, then you simply add the power dissipations getting 72 Watts. You’ll want to select a power supply with 30% or more overhead, so you’re looking at ~50W for one string or ~100W for two strings.

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Multiple Series-LED Strings

Dimming
To gain the full benefit of LEDs they need to be dimmable somehow. There are two methods used to dim LEDs – (1) reducing the current through the LEDs and (2) a method called PWM. The former is almost never used and has the disadvantage of causing a color shift in the LED’s output. The latter, PWM dimming, is the de-facto method of choice.

What is PWM? PWM stands for pulse width modulation. It’s a fancy way of saying that the LED is turned on for some time, and off for some time, all really fast – typically from 490 to 1000 times per second. Because the LEDs are switching on/off so quickly, we simply perceive the proportion of on time as the brightness. The proportion of on time is called the duty cycle, ranging from 0% (off) to 100% (fully on). The advantage of PWM is that it lets us control the LEDs in the native language of computation, ones and zeroes. And because each time the LED sees “one” it is fully on at the rated current, there is no spectral shift even at very low brightness levels.

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Basic PWM (Source)

Control
We now turn our attention to the precision control of LEDs through the use of a microcontroller. A microcontroller can be used to tell the LED driver when to turn on, and when to turn off. It can therefore control the duty cycle assumed by the LED driver and by extension the brightness of the LEDs. With a little programming, the controller can control on and off times, brightness levels, and produce effects such as clouds and lightning.

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Storm Controller Sunrise/Sunset PWM Control Scheme

A good reef aquarium lighting controller will give you control over sunrise and sunset times, ramping profiles of each LED string, and of course control over the brightness levels. The CORALUX Storm and Storm X controllers all these functions, plus moon phase simulation to give your aquarium the best possible approximation of what nature intended.

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LED being controlled via PWM

Standards
For various reasons, several standards have emerged for how the controller communicates with the driver. There’s 10V Analog, used by devices such as the Neptune Apex Controller and accepted by the Mean Well ELN-60-48d driver. There’s 10V PWM, which is used by some controllers and accepted by the Mean Well ELN-60-48p driver. And there’s 5V PWM which is what the Storm and Storm X LED Controllers output. The 5V standard is accepted by the Mean Well LDD-H LED Drivers as well as most PWM buck drivers out there. It is imperative that both your controller and your driver speak the same language. There are converters that can translate between the standards.

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CORALUX LDD-4 PWM Driver PCB Board

Conclusion
The topic of PWM LED dimming as it applies to reef aquarium lighting can seem confusing at first, but a little time spent learning the basics can go a long way towards ensuring a successful LED project build. If you’ve read and understand this article, you have a good foundation on the principles of LED dimming. It is a vast topic with many approaches, but with a little research you can prepare yourself to build your own reef aquarium LED system. Fiat lux.


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