In this chapter you will find an overview of the basic LED properties, how to select between the different LED versions and how to find a matching LED driver. If you are looking for the DIY build steps check out the LED Strips or LED COB pages.

LED properties

Lights have a significant part in a plant’s life, when designing a grow light the goal is to provide a uniform light coverage with high PPFD values. But wait, what the heck is PPFD? It measures the amount of photons passing through a specific PAR region every second...simple? Not really. Explaining all the parameters of horticulture lighting metrics is way beyond the scope of this guide. Read the below two articles and you will be up to speed in no time:

I highly recommend check out Dr. Bruce Bugbee's in-depth videos on cannabis grow lights:

So how do we compare lights and what are the typical parameters to look for?

Color temperature: Different temperature LEDs provide different spectrum of light, their Kelvin temperature (K) helps us to categorize this. A lower K rating means the light is shifted towards the red (620 nm) color spectrum. As the K rating gets higher, the blue (450 nm) spectrum is starting to get stronger. Have a look at the spectrum output of the different Kelvin temperature rated Bridgelux EB Gen3 LED strips. Notice the peaks at 450 nm and 620 nm:

The above graph shows how PAR (Photosynthetically Active Radiation) changes depending on the light wavelength. It is easy to see the peaks at Violet/Blue and Red, these are the spectral ranges where photosynthesis is most efficient. The different colors of light can have a powerful effect on plant photosynthesis and morphology, such as shape, height, etc. Check out Dr. Bruce Bugbee's video on how this effects cannabis growth. In summary:

Blue (380-430nm): Inhibits cell expansion and prevents stem elongation, results in shorter, bushier plants. This is especially important indoors where the available height is limited. In cannabis the blue photon fraction does not affect yield or cannabinoid (THC, CBD) concentration significantly. Absorbed by the top layer of the leafs.
Green (480–560nm): Facilitate human vision, helps to diagnose plant problems. Green photons are also used in photosynthesis, penetrates deep into the canopy and leaf material, reaching leaves at lower branches.
Red (620-750nm): Drives photosynthesis, very efficient (+15% better than blue photons). Absorbed by the top layer of the leafs.
Far-red (700-750nm): Enhances cell expansion, makes cannabis grow taller and increases flower weight. Not directly photosynthetic, has synergistic effect when combined with shorter wavelengths (More on this: Emerson effect, Dr. Bruce Bugbee - Far-red: The Forgotten Photons). Penetrates deep into the leafs.

As cannabis growing is more focused on the flowering phase and flower weight, its best to select full spectrum white LEDs in the temperature range of 3000-3500K that has a color shift to red but still provide enough blue light range (6-10% of total output) to limit height. Stay away from only blue and red LED lights, as the marginal efficiency benefit does not worth loosing the ability to visually diagnose nutrient deficiencies.

Bonus eye candy: Check out the different absorption profiles of light on a leaf irradiated with direct and diffuse light:

Light absorption profiles

650nm red, 532nm green, 488nm blue
Image from: Functional Plant Biology - 2010, Craig R. Brodersen and Thomas C. Vogelmann

Forward voltage: LEDs require a certain amount of voltage to turn on. If you connect multiple LEDs in series the required forward voltage is multiplied by the number of LEDs connected in series. Most LEDs used for growing plants have a forward voltage within 20-72 Volt range depending on the internal wiring of the LED chip, quantum board or LED strip. Sometimes manufacturers make different voltage versions of the same chip, for example 72V and 36V versions of a chip. They will have the same light output only the current flowing through the chips varies. In general, it's better to go with the lower voltage versions of the same type LED as you will be less limited in the LED driver selection.

Efficiency: To increase the output of an LED we need to increase the current flowing through the chip. In an ideal scenario doubling the current would give us twice as much luminous flux, but in the real world we get diminishing returns. The harder you drive an LED the less efficient it will be, this lost energy will be transformed into heat that requires heat sinks to dissipate. LED strips are most efficient an their nominal current, but some manufacturers specify an overdrive current where LEDs still operate reliably and provide more light with a slightly lower efficiency.

Physical dimensions: LED strips come in different length PCB boards, typical options are ~250 / ~500 / ~1000 mm long and 20-30mm wide. Based on the layout of the LEDs they can be single or double row. All strips have a positive and negative connector for DC power input. Driven at low current (700mA) they do not require additional cooling, however they still require a frame for physical support as the PCB board the LEDs are soldered on is flexible. Later on we will build an aluminium frame that doubles as a heat sink and can support strips up to 1400mA.

Matching LEDs and drivers

LEDs require regulated current, so do not start hooking them up to a power supply you have lying around even if the output voltage matches. The LEDs would take all the current the power supply can provide, overheating and burning out in the process. This is why we need LED drivers, providing a predefined, regulated current and voltage to the chips. Depending on which parameter is regulated on a driver, they can be either constant current or constant voltage drivers.

Constant current: These drivers push a fixed current through the LEDs connected in series to them by varying the output voltage in a predefined range. To determine the maximum number of LEDs a driver can power you need to divide the driver’s maximum output voltage with the forward voltage of one LED. You can use less LEDs as long as the total forward voltage required is more then the driver’s minimum voltage output. The driver will adjust the output voltage to make sure it keeps pushing the pre-defined current through the circuit.

Constant voltage: These drivers keep a fixed voltage between their positive and negative terminals and the current the driver can provide is split between the LEDs connected in parallel. This sounds good, but there are possible pitfalls. If one LED fails the rest will have to take up more current. If you are driving the LEDs around the maximum supported current this extra current could cause the remaining LEDs to overheat and fail. The second possible problem is called thermal runaway. As an LED chip heats up its resistance will decrease allowing more current to flow through heating the chip up even more and dropping the resistance further down. This means the worst cooled LED will get the most power and in case of an insufficient thermal contact or failed active cooler this can lead to uneven light output or even a burnt out LED.

To sum it up: There are builds with constant voltage drivers, but using them requires protection circuits and higher technical knowledge. I would not recommend them to beginner DIYers. Selecting the right driver depends on the current you want to run through the LED strips and the total forward voltage required by the strips at the selected current.

Selecting an LED strip

When selecting what strips to get the Samsung H / L / Q and Bridgelux EB series were among my options, the Bridgelux strips were chosen due to their price. Costing half as much as the Samsung strips while having a marginally lower efficiency was a trade-off I could easily live with.

When selecting the color temperature the usual options are 3000K (warm / fruiting) / 3500K / 4000K (cool white) / 5000K / 5700K (daylight / vegetative). You can mix any color temperature strips in a light, all other parameters of the strips are the same. I have selected a color temperature of 3500K and couple 5000K strips with a ratio of 4:1 in favor of the more fruiting focused 3500K lights, while still keeping some blue spectrum for the vegetative growth.

Bridgelux EB Gen 2 LED strips

Different Kelvin temperature strips

The below graphs show how the required forward voltage and light output changes in contrast to the current. The 100% light output is measured at 700mA nominal current, notice that at the 1400mA overdrive current the flux only changed to ~190%. This is due to decreased efficiency, the price we need to pay for overdriving the LEDs.

Depending on the LED strip length the supported current and forward voltage can change. Before proceeding to select a LED driver you need to decide the strip length and current you would like to use. With a low number of strips its best to use the overdrive current to get the maximum out of a single strip. If efficiency is your goal you need more strips and drive them at the nominal current.

Forward voltage depends on the provided current, increasing the current also increases the forward voltage required to turn on the chip. For this build I have selected the 560mm EB series strips, and will be building two lights from them: One efficient using 14 strips running on nominal current and one 8 strip version running on overdrive.

Selecting a driver

As one of my goals was high efficiency I already decided to skip the cheap Chinese LED drivers and go with the more expensive but better quality branded drivers. I knew I wanted a constant current version of the Meanwell HLG series due to their high 92-95% efficiency, good reviews and wide range of supported input voltages (US and EU). I just had to find the right one. In an effort to navigate through the different versions, I armed myself with the model encoding information of the HLG series and started digging here.

Below are some sample drivers with typical current ratings and the number of 560mm Bridgelug EB Gen2 strips they can run. Look up the same parameters for your selection of LED strip and LED driver in the manufacturer's specification and adjust the calculations accordingly.

Required Voltage = [Number of Strips] x [Forward Voltage at selected current]
Driver minimum supported voltage < Required Voltage < Driver maximum supported voltage

If the sum of the forward voltage exceeds the maximum supported voltage you need to find a higher wattage driver or one with a lower current. Optionally you can split the wiring into multiple circuits with individual drivers. Always connect at least as may strips in series that their total required forward voltage reaches the driver’s minimum supported voltage. If you connect less strips the over-voltage will destroy the strips (successfully tested that..).

Always check the manufacturer’s specifications to get voltage, temperature and overdrive current parameters. When overdriving make sure the strip case temperature never exceeds the specifications (In this case 85°C / 185 °F).

Timer

Lights need a timer where you can set when they should turn on and off. Both analog and digital timer versions are equally good solutions, but in my opinion, digital timers are better as they are more precise and tend to last longer. Digital timers can be adjusted down to the minute, while analog ones are usually adjustable by 30 minute increments. As long as they can provide a daily schedule adjustable by hours they are fine. Look for a timer that has a built in on-off switch, that way you don’t need to bother wiring a power switch to the light assembly.

Google Sites