LED COB lights

In this chapter we build an LED COB grow light from 8 LEDs and an adjustable driver aimed at high efficiency.

The goal is to make a light with the least number of components mounted on a frame made from L profile aluminum. The module has a single power input compatible with a standard 3 pin power cable. The LEDs are connected to a constant current driver, cooled by pin fin heat sinks that are rated up to 50W passive heat dissipation allowing usage of up to 2100mA LED drivers (Even more with some added airflow).

Update

Looking for a new light source? Check out the LED Strips light that outshines this build in every way: It's more efficient, lighter, and cheaper.

Since I wrote this chapter the Pin fin heatsinks have become way too expensive for this light design. If you still want LED COB based lights, check out this all-in-one lightbulb version of the same Citizen CLU048 cobs from this guide: Amazon.com They have an integrated LED driver and fit in a standard E27 Base.

All LEDs are wired in series by connecting the driver’s positive side to the positive side of the first LED. The same LED`s negative side connects to the next LED`s positive side, continue this until you reach the last LED, connect the remaining negative LED terminal to the negative side of the driver. In the below wiring diagram the potentiometer and resistor are only needed if an externally adjustable driver is used. Get the externally adjustable driver if you are planning to install the Arduino-based monitoring and controlling system too.

LED COB

8 pcs, 3500K, Citizen CLU048-1212C4 Version 6

Pin fin heat sink

8 pcs, 133mm diameter, 70mm height Up to 50 W heat

LED holder

8pcs, Ideal holder 50-2204CT

Thermal paste

Amazon.com, Amazon.de

LEDs

The LED COBs we are focusing on are in the size range of 25x25mm to 35x35mm with a typical forward voltage requirement of 36V per chip. The exact voltage requirement depends on the current the driver provides and the LED model. Always check the manufacturer’s specification for the calculations. Here are the specs for Citizen CLU048 LEDs as an example.

In this build I used Citizen CLU048 LED COBs with 28x28mm size and 2760 mA maximum supported current. The below graphs show how the required voltage and relative luminous flux change based on the provided current. Notice that as the current gets higher the required voltage increases and the efficiency decreases. The luminous flux does not increase at the same rate as the current: for example doubling the current from 1050mA to 2100mA only increases the light output by 75%, the rest of the energy is transferred to heat.

Driver

Selecting the right driver depends on the current you want to run the LEDs and the forward voltage required by the chip at the selected current. We are going to be using constant current drivers that can adjust their output voltage to always push a predefined current through the circuit. To calculate the total required voltage multiply the number of LEDs with the forward voltage at the selected current. Below are some sample drivers with typical current ratings and the number of CLU048 LED COBs they can run. Look up the forward voltage for your selection of LED COB and driver in the manufacturer's specification and adjust the calculations accordingly.

It's easy to see that using a single driver 1050mA is the maximum current that can be used with 8 LEDs, above this current either the number of LEDs needs to be decreased or a second driver has to be added. At the end of this guide, there is a chapter on how to wire multiple drivers. In this build, I’ll aim for high efficiency with a single 185W - 700mA internally adjustable constant current driver. The forward voltage of one LED at this current is 34.3V making the total required voltage 274.4V, this voltage is within the 143 - 286 V supported range of the driver.

The takeaway from these samples would be that if the sum of the forward voltage exceeds the maximum supported voltage you need to find a higher wattage or lower current driver or decrease the number of LEDs. Also, make sure to always connect at least as many COBs in series that their total required forward voltage reaches the driver’s minimum supported voltage. If you connect less than this the driver won't be able to scale back the voltage enough and fry the LEDs.

Adjustability

Not adjustable: While every HLG series is adjustable in some way, not all drivers have this option. These drivers always operate at their rated current without an option to dim them.

Internally adjustable (A series): The top driver on the photo. Under the rubber cap in the top right corner, it has a built-in potentiometer for adjusting the light intensity with a screwdriver. This is done by changing the current the driver provides between 30% and 100%. The water-resistance rating is lower due to the extra opening (IP65 ).

Externally adjustable (B series): Notice the extra cable coming out from the driver’s bottom right side. That is connected to a 100K ohm linear potentiometer used for dimming the lights. It has a higher waterproof rating: IP67 and a wider range of dimming: 15% to 100%, but requires soldering.

While the better water-resistance of the externally adjustable driver sounds great, you will have a hard time finding a waterproof potentiometer. And why would you spend extra money and effort to make a dimming circuit when you can easily hack an external knob on an internally adjustable driver?

Internally adjustable driver - Cap hack

Converting an internally adjustable driver into an externally adjustable is by far the easiest solution for dimming. You only need a nail, a tire air valve cap, and some grease. You will also need to use glue, a metal saw, and a rasp.

First, take the rubber cap from the internally adjustable driver and carefully poke a 2mm thick, minimum 4cm long nail through its center. The adjuster knob will be made from a car tire valve cap. Fill the cap with glue then use a twisting motion to insert the nail’s head into it. Once the glue dried cut off the excess overflow glue and fit the assembly into the driver. At this point, the length of the nail will probably be too long.

Measure how much to cut off for the cap to be level with the driver, add 2 mm extra as the end will need to be filed off to fit into the internal potentiometer. In my case, the nail had to be cut down to 4 cm from which 3cm was sticking out from the cap. Once the nail is cut to size using a rasp shape the end like a flathead screwdriver. Before pushing the assembly back into the driver apply some grease on the nail to help it turn freely inside the rubber cap.

Externally adjustable driver

With externally adjustable drivers dimming can be achieved the easiest by connecting a 100KΩ (Kilo Ohm) linear potentiometer between the dimmer cables. Setting the resistance to 100KΩ or above will output maximum power, dialing back on the resistance dims the lights. Do not use the dimmer to turn off the light completely by setting the potentiometer to 0Ω as the dimming circuit is not designed for that. To protect from accidental over-dimming it is best to have a 10KΩ resistor in series with the potentiometer, this limits the dimming to 10%. If you would like to explore the other dimming options or would like to dim your light completely, please refer to the HLG manual here. Go with the externally adjustable version if you are planning to upgrade to an Arduino-based controlling and monitoring system.

Metal film resistor

10K Ohm resistor (10 kΩ)

Amazon.com, Amazon.de

Potentiometer

100K Ohm (100 kΩ), Linear, Mono

Amazon.com, Amazon.de

Solid core wire

20 cm, AWG 18 / 0.82mm2 thickRemaining from light assembly

Amazon.com, Amazon.de

Wiring

The potentiometer and a resistor need to be connected in series between the two dimmer cables. As the dimmer cables are interchangeable the only thing we need to get right is the potentiometer wiring. Connect the center leg to one of the dimmer cables, and one of the side legs to the resistor. Depending on which side leg you choose, the dimming will happen clockwise or counterclockwise. To avoid dimming under 10% that could damage the driver a 10KΩ resistor is connected in series with the potentiometer.

Euro style wire connectors are added to the cables to ease attaching the module to the LED driver’s dimmer cables. The potentiometer was mounted on the frame using a 2mm wide zip tie.

Heat sinks

I’m going to be using passive pre-drilled pin-fin heat sinks with LED holders. This way I just need to pop the LEDs into the holder, apply thermal paste, fasten it on the heatsink and push the solid core wires into the holder’s connectors. The thermal paste improves cooling surface contact and heat transfer. Put a thin layer of paste on the chip and on the heat sink too, then use a slight twisting motion before fixing the LED in place to spread the paste evenly and to ensure there are no air pockets left.

The positive side of LED’s is marked on the chip as +, make sure to align it with the holder’s + mark. For now, just install the LED COBs on the heat sinks, we will deal with the wiring at a later chapter.

Manufacturers specify the maximum case temperature of their LEDs around 85°C / 185 F, keeping the temperatures down increases the lifetime of an LED. Unfortunately, large coolers are expensive so choosing reasonably sized coolers can cut total cost significantly. As an example, the 50W rated, 13cm diameter, and 7cm tall heat sinks are way oversized for 700mA. A 15W passive cooler could easily handle the heat one LED generates at this current. Moving to 2100mA would validate the large heat sinks when 80W power is consumed per LED.

Manufacturers suggest calculating with a 70% heat factor, meaning 70% of the energy used by the LED will get transformed to heat. At 700mA (0.7A) the forward voltage of one LED will be 34.3V making the power consumption 34.3 x 0.7 = 24W. 70% of these 24 watts will be converted to heat, meaning we need to channel 16W of heat. Increasing the current to 2100mA raises the forward voltage to 38.2V and the wattage to 38.2 x 2.1 = 80.22W, meaning 56W of heat needs to be channeled away from the LED. While the 70% heat factor is a worst-case rating it's still better to oversize the coolers a bit and spend a little more money on them than burning out the LEDs or severely decreasing their lifetime by running them too hot.

Frame

With this many LEDs, individually mounting each heat sink is not practical. Cable management and height adjustment would be bothersome without a frame. Above 4-5 LEDs, it's worth considering a framed design. L-profile aluminum can provide a cost-effective frame for the lights, that is relatively easy to work with. I chose 2mm thick, 15 x 15mm aluminum L profiles for the build and got 7 meters in total length.

L profile aluminum

15 x 15 mm, 2 mm thick, 6 x 80 cm + 2 x 50cm

Amazon.com, Amazon.de

Fine thread eyelet hook+nut

4 pcs, 45 mm length, 4 mm diameter (M4)

Amazon.com, Amazon.de

Fine thread screw+nut

8 pcs, 10 mm length, 4 mm diameter (M4)

Amazon.com, Amazon.de

Zip tie

10 pcs, 30 cm long, 5 mm wide

Amazon.com, Amazon.de

My goal was to have two support bars that are held up by carabiner ratchets and place cross bars between them for holding the coolers. For my 52 x 86cm box area I cut the support bars to 50cm and the heat sink holder bars to 80cm.

Prepare the frame for drilling by placing some old wood boards under it. After measuring, and making sure the middle row is centered use a 4mm drill bit to make the holes for the screws that will keep the frame together. Use fine threaded M4 size screws and hooks along with M4 hex nuts. To avoid scratching the sides of the box, place self-adhesive rubber pads around the edges of the light assembly.

To ensure the heatsinks do not slide around once they are in position put zip ties between the guide rails and pull them tight. This will press the rails against the heatsinks keeping them in place even upside down. To increase the clamping force once the zip ties are fastened push them close against the heatsinks where the bars cannot flex as much. The driver is also fastened to the center of the frame using zip ties.

Wiring

AC side

The power connector on the AC side is made from a PC power cable and a waterproof screw locking connector. Choose an AC cable long enough to reach the power outlet even at the lowest light height. The waterproof connectors require no soldering and it is very easy to (dis)assemble them using a single screwdriver. Simply chop off the C13 connector from the PC power cable, strip back the insulation and match up the color codes of the cable with the LED driver AC input. Inside the connector, there are marks where to connect the different wires: L - Live Brown wire, N - Neutral Blue wire, G - Ground Yellow/Green wire. Only work on electric devices after disconnecting them from the power source first!

LED Driver

Meanwell HLG-185H-C700A, 185W700mA Constant CurrentInternally adjustable

LED Driver

Meanwell HLG-185H-C700B, 185W700mA Constant CurrentExternally adjustable

3 pin connector

IP68 waterproof ratingScrew connectors, 3 pins

Amazon.com, Amazon.de

AC power cable

2 meter / 6 ft, Wall plug, C13 connector

Amazon.com, Amazon.de

3 pin connector

IP68 waterproof ratingScrew connectors, 3 pins

Amazon.com, Amazon.de

Grounding

The housing of the LED driver is connected to ground, to ensure the frame of the LEDs is also grounded we need to add a short wire connecting the LED housing to the aluminum frame. Grounding the frame is an important safety measure, do not skip it.

DC side

On the DC side, the LEDs are interconnected by AWG 18 / 0.82mm2 solid core wires and the driver is connected using fast release Wago connectors. The LED holder support wires between AWG 18 (0.82mm2) and AWG 24 (0.2mm2), try to get the thicker 0.82mm2 / AWG 18 gauge wires as they are easier to push into the connector. Be careful not to damage the insulation while routing the wires between the LEDs.

Splicing connector

2 pcs, 2 pins, Wago 221-412

Amazon.com, Amazon.de

Solid core wire

2 meters / 6 feet, AWG 18 / 0.82mm2 thick

Amazon.com, Amazon.de

When using a constant current driver the LEDs are connected in series. The best is to start with the (+) DC cable of your driver and wire it to the (+) pole of the first LED COB. Then link the (-) side of the same LED to the (+) side of the next LED, daisy chaining all your LEDs using this process. When you get to the last LED in the line, simply connect the (-) LED side to the (-) DC side of the driver.

Mounting

For hanging the assembly I used carabiner ratchets with a 180cm long rope. One end of the rope is tied to a carabiner and the other end is routed through the ratchet. To adjust the height the rope is pulled further through the ratchet, shortening the distance between the two carabiners and lifting the lights.

Screw eye hook

2 pcs, 19 mm head diameter

Amazon.com, Amazon.de

Carabiner ratchets

2 pcs, 180 cm long

Amazon.com, Amazon.de

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.

Dual drivers

Sometimes a single driver cannot provide enough voltage to run a large number of LEDs. As samples, I have selected high current drivers as they have lower maximum voltage support and it is more common to run into the issue of not having enough voltage to drive all LEDs with them. Please note that all of these configs are way too powerful for a grow box, they are more suited for a grow room where there is a much larger area to cover. In this sample build, I`m going to be using two 700mA drivers, simply because that is what I had at hand and I can still demonstrate every part of a multi-driver light with them.

A good example of a dual-driver light would be an 8 LED COB light, driven by two 320W - 2100mA drivers. In this case, the drivers are fully utilized to their maximum capacity where they have the highest efficiency, and the passive 130mm wide, 7mm tall coolers can still deal with the heat without active fans.

Above you can see how the completed driver and light assembly looks like. The driver assembly has one AC cable coming in, and two DC cables leaving the junction box. Within the junction box, we have the wire splicing connectors, power buttons for the drivers, and the potentiometer for adjusting the externally dimmable driver. The light assembly has 10 LEDs wired in two circuits of 5-5 LEDs.

Assembly

The junction box and drivers were mounted on a hardwood sheet to simplify assembly and installation to the grow box. When fastening the drivers make sure they have 5-10 mm free space under them. The gap will provide some additional airflow to keep the drivers cool, I have used 4mm thick self-adhesive rubber feet. The junction box between the drivers is to hide away the wiring for the drivers, power buttons, and dimming.

Junction box

16 x 10 x 9 cm

Amazon.com, Amazon.de

Wood screw

12 pcs, 1 cm long, 4mm diameter (M4)

Amazon.com, Amazon.de

Euro style wire connector

6 pairs

Amazon.com, Amazon.de

AC power extender

2 pcs, C14 and C13 connectors

Amazon.com, Amazon.de

AC comes in, DC goes out

Let's have a peek inside the junction box. Left is the AC power side where one power cable comes in and splits into two to power the drivers. On the right side are the two DC power cables leaving the junction box going leading to the light assembly. The assembly was made modular using PC power extension cords. The input side has a C14 power plug and the two outputs are C13 plugs. This way it's easy to disconnect the lights from the drivers.

Closer look on the AC side: The incoming AC power cable contains 3 wires: Live (ACL, blue), Neutral (ACN, brown), and Ground (yellow/green). These are bundled together with the matching Driver A and B power inputs using 3 splicing connectors. Note: At a later step the Ground wire will be further split to provide grounding for the aluminum frame.

The DC side: The DC cable has two wires: one Red(+) and one Black(-) wire. They are connecting straight to the power cables leading to the light assembly using Euro-Style Wire Connectors. The ground wires are not used on the DC side cable. Dimmer wires are not yet connected to a potentiometer. Notice the two unconnected green ground cables heading towards the LED lights, these will be connected to the incoming AC ground.

It's important to wire the (+) and (-) cables correctly to the LEDs as the polarity is not interchangeable. The LED will not turn on if wired incorrectly.

Wiring lights with 2 or more drivers:

Wiring multiple circuits are no different from the single driver setup, the only difference is that I need to make more loops. In this sample build, one circuit is cabled with brown wires and connected to driver A and the second circuit uses blue wires and connects to driver B. The LED COBs are interconnected by AWG 18 / 0.82mm2 solid core wires and the connector is made from the C14 end of a PC power extender.

Below you can see the combination of the 2 circuits at work:

The potentiometer can be mounted on the side of the junction box. Minimum setting resistance: 10.2KΩ, Maximum setting resistance: 108.9 KΩ

The potentiometer is soldered to increase resistance clockwise, this increases the light intensity.

Power switches [optional]

This section is only needed if you would like to have an option to individually control the circuits, normally both circuits are controlled by a timer that powers the junction box. To achieve independent control install power rockers on the side of the junction box, get round switches as they are easy to fit with a drill.

AC power rocker

2 pcs, round

Amazon.com, Amazon.de

Euro style wire connector

2 pcs

Amazon.com, Amazon.de

Heat shrink tube

Amazon.com, Amazon.de, AliExpress

Soldering the power rockers, and insulating with shrink tubes.

Splitting the incoming AC cable: We have 3 incoming lines: Live (ACL, blue), Neutral (ACN, brown), and Ground (yellow/green). With the tho power switches in place, the live (brown) wires are no longer directly connected to the driver's brown AC inputs, rather through the power switches.

Before AC power switch After AC switch installed

Not a big change, now the ACL (brown) cables are going to the drivers through the power rockers.

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