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LED University

How Do I Calculate How Many Watts An LED Is?

The formula to calculate the max wattage on an LED is simply the maximum current (measured in Amps), multiplied by the forward voltage of the LED (at max amperage). This wattage however is the theoretical maximum and in most cases is NOT what the LED is actually drawing from the wall. Make sure to multiply the drive current (ie. 1.4A) by the forward voltage (ie. 36V on most cobs) to see what the actual theoretical draw is (1.4A x 36V = 50.4 Watts). As always these are theoretical calculations and a kill-a-watt meter should be used to accurately measure wattage.

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:

  • Soft ramping up/down of light to avoid shocking plants
  • Can be programmed to turn on at only specific times of the day using a controller, without the use of a wall timer
  • Ability to dim down light when plants are young or don't need full power

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. Typically speaking we look at the maximum drive current and try to ensure that all the LEDs on your string are capable of handling this amount of current, even if you plan on dimming. This is a fail safe in case your dimmer fails, or is accidentally turned all the way up.

To figure out how many LEDs a driver can run you’ll need two pieces of information: the driver’s output voltage range and the forward voltage required by the LED at the given current level that you would like to run the LEDs at. As an example, the HLG-320H-C1400B driver has a voltage range of 114-229. The CREE CXB3590 COB LED requires ~35V at 1400mA. Thus, the minimum number of these LEDs required is 114V divided by 35V. 114/35 = 3.26, so we round up and the minimum number of LEDs is 4. For maximum LEDs, we divide 229V by 35V and get 6.54. We round down and get 6 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 MA Driver Should I Use For My COBs (Assuming All 36V COBs)?

1050mA- Use this current if reducing heat and electricity consumption is your first priority. COBs will be cooler and more efficient but you’ll need more of them so it will cost a little more. This is usually not recommended if you have a thick canopy to penetrate unless you're also using a secondary reflector/lens.

1400mA- This is becoming the industry standard as it's a great balance of initial cost, heat, efficiency, and canopy penetration. 

1750mA- For a slight reduction in initial cost, and a slight increase in canopy penetration. Will be slightly warmer and less efficient than 1400mA.

2100mA- This is the best option if you’re on a budget. It reduces the number of COBs you need because you're driving the LEDs harder. It won't be quite as cool or efficient as 1400mA but is still a huge reduction in heat and electricity consumption over HPS. Make sure you have some active cooling if running your COBs at ~75W+.

How Much Wattage/How Many COBs (36V CXB3590) Do I Need For My Space?

2' x 2': 150W required for full coverage. For example 4 COBs @1050mA, 3 COBs @1400mA, or 2 COBs @2100mA.

2' x 4': 300W required for full coverage. For example 8 COBs @1050mA, 6 COBs @1400mA, or 4 COBs @2100mA.

3' x 3': ~340W required for full coverage. For example 9 COBs @1050mA or 7 COBs @1400mA.

4' x 4': 600W required for full coverage. For example 16 COBs @1050mA, 12 COBs @1400mA, or 8 COBs @2100mA.

Which Driver Do I Need To Run "X" Number Of COBs (36V Version)?

HLG-185H-C1050B: 3-5 COBs
HLG-185H-C1400B: 2-4 COBs
HLG-240H-C1050B: 4-6 COBs
HLG-240H-C1400B: 3-5 COBs
HLG-240H-C1750B: 2-4 COBs
HLG-240H-C2100B: 2-3 COBs
HLG-320H-C1050B: 5-8 COBs
HLG-320H-C1400B: 4-6 COBs
HLG-320H-C1750B: 3-5 COBs
HLG-320H-C2100B: 3-4 COBs
HLG-480H-C1400B: 5-9 COBs
HLG-480H-C1750B: 4-7 COBs
HLG-480H-C2100B: 4-6 COBs

How Far Above The Canopy Should I Hang My Light?

These recommendations are meant to be a starting point, and the optimum distance will depend on several factors such as strain, canopy density, ambient temperature, etc.

COBs @38W (1050mA) without reflectors: 8-10"
COBs @38W (1050mA) with reflectors: 10-18"

COBs @50W (1400mA) without reflectors: 10-15"
COBs @50W (1400mA) with reflectors: 14-24"

COBs @65W (1750mA) without reflectors: 12-18"
COBs @65W (1750mA) with reflectors: 16-28"

COBs @75W (2100mA) without reflectors: 15-20"
COBs @75W (2100mA) with reflectors: 22-32"

How Do The LDD Drivers Work? How Are They Different Than The HLG Drivers?

LDD drivers are different than HLG 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. A typical mono LED draws about ~3V, so we can run 45/3V = ~15 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. The Storm, Storm X, Bluefish Mini and Bluefish controllers are all compatible with LDD drivers. Only the Storm w/ 10V converter and Bluefish are compatible with HLG drivers.

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 transferring properly to the heatsink.

How Hot Should My Heatsink Be?

We recommend keeping your heatsinks under ~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).