COB LED Basics

COB LED systems are actually quite simple – there are only a handful of different parts, and they all go together pretty easily. The main components of a COB LED system are:

· The COB LED themselves

· Heat sinks that the COB are mounted to

· LED drivers that power the COB

· The wires that interconnect the COB and drivers.

Not so bad, right? Let’s delve a little further in.

1. COB LED

The main component of a COB LED lighting system would obviously be the lights themselves. The term COB stands for “Chip On Board”, and refers to the fact that each unit is actually several LED (Light Emitting Diode) chips mounted together on the same substrate, which is often ceramic or metal. LED are semiconductors that produce photons of light when electrons flow across a junction and fill “electron holes” on the other side.

COB manufacturers release data sheets for their products that dictate each unit’s specifications, as well as minimum, typical, and maximum values for a number of different variables. A sample of Cree’s data sheet for the CXB3590 can be found here. A few examples of a COB characteristics that can be found in the data sheet include:

· Colour temperature (how “warm” or “cool” is the colour of the light?)

· Typical forward voltage (what will the voltage be across the COB at a certain level of current?)

· Maximum current (how many amps of current can you drive the LED with before it dies?)

· Temperature (how does the maximum current rating change, based on the temperature of the COB case?)

· Luminous Flux  (how bright is the light when measured at a given current and temperature?)

· Relative Spectral Distribution (what wavelengths of light are most concentrated in the light produced by the COB? Does it peak in the blue range or red range?)

So, how do these little chips get mounted? Well, if there are screw holes in the board, you can screw them down, but the better alternative is always to get a holder, if possible.

Cob Holders

If you hate soldering or don’t have an iron, you’re in luck. Many COB have matching holders that can be used to mount the COB to the heat sink and also offer a push-in connection for wiring to the COB terminals. The holders fit over top of the COB, and screw down into the heat sink, sandwiching the COB and forcing it to press tightly against the sink. Once it’s in place, you can simply push the wire into the holder’s terminals (the white plastic pieces you see on the silver holders in the picture below), which press down on the corner contact points of the COB. Nice and simple!

Reflectors

If you want to focus your light more, you can add reflectors that attach to the front of the COB (this often requires a separate adapter, though). You can get reflectors in different angles, colours and materials, depending on what you need to accomplish. You can also attach glass lenses or diffusers to the reflectors to protect the COB from dirt, water, and other garden gunk.

2. Heat Sinks

Despite being very energy-efficient, high-powered COB LED do dissipate a lot of power in the form of heat. If this heat builds, it can alter the COB’s characteristics and subsequently damage the unit, so it must be removed somehow. The sole purpose of heat sinks in a COB light system is to pull this heat away from the COB and keep them running as cool as possible. Heat sinks are most typically made of aluminium, which is a great conductor.

There are a number of different shapes of heat sinks, from the tested and true “fin” type, to the snazzy new “pin-fin” style. The purpose of all the fins, whether they’re rectangular or pins, is to increase the surface area of the heat sink as much as possible. More surface area makes it easier for heat to transfer from the COB, through the fins, and into the air.

Aside from the shape of the fins, there are 2 main types of heat sinks: active and passive.

Active Heat Sinks

Active heat sinks are those that use a fan to blow air over the fins to help remove the heat. If you use an actively cooled heat sink, you can get away with a much smaller unit than you could if you went passive. The risk with active cooling is that if the power supply running the fan were to ever die, the heat sink alone may not be able to cool the COB sufficiently.

Passive Heat Sinks

Passive heat sinks are just chunks of metal and nothing else. As mentioned above, passive heat sinks must be much larger than their active counterparts in order to dissipate the same heat. What’s great about passive sinks is that you don’t have to worry about the fan dying and your COB overheating, as there are no moving parts.

Thermal Interface, If you were to try to mount your COB directly to your heat sink with nothing in between, you wouldn’t get very good contact between the two. When mounting to the heat sink, It’s imperative that 100% of the COB is making solid contact. Every minuscule pore and scratch that isn’t making contact is a spot that heat will build up and cause problems. For this reason, you need to use a thermal interface like a thermal pad, or thermal paste on the back of the COB to ensure the best connection possible.

3. LED Drivers

LED drivers are the handy little boxes that take the power from your electrical outlet and convert it to a form that the COB can use. The electricity coming from your outlet is alternating current (AC), and this does not work for COB LED – they require direct current (DC) instead.

LED drivers often come unterminated, which means you will have to wire an AC power plug to the end of the power cable (out of the box, it’s simply 3 bare wires). You will also need to terminate the positive and negative wires that connect to the COB LED circuit, as well as the 2 wires that control the dimming of the unit (some drivers come with a dimming knob built into the unit, but others simply come with 2 leads that you must attach a potentiometer to).

When purchasing an LED driver, you need to make sure that it is capable of producing enough power (in watts) to run all of your COB. You also need to make sure it can produce this power at a certain current (in amps) and voltage (in volts). LED drivers come in 2 types: constant voltage drivers and constant current drivers. Constant voltage drivers hold their rated voltage steady and vary their current output, whereas constant current drivers hold their rated current steady, and vary their output voltage. C.V. drivers are typically used in parallel wired systems, while C.C. drivers are typically used in series wired systems.

If you’re new to LED, I’d suggest starting with constant current drivers, as they are far easier to work with and are generally better-suited for most LED applications. Constant voltage drivers work wonderfully too, but can be trickier to set up and less forgiving if you mismatch the driver and COB.

I would propose reading these posts on selecting LED COB and matching drivers, wiring COB LED in series and parallel, and constant current vs. constant voltage drivers for a more detailed explanation.

4. Wiring

This is the simplest part of the system – everyone has worked with wire in some capacity or another. You won’t need a lot of wire, but you should definitely try to find the right stuff. You will need to connect all the COB to one another and then connect the COB back to the driver for power. See the post on wiring COB LED in series and parallel for more information on how to hook up the COB, and this post for help choosing wire for your system. In most cases, 18 gauge solid core cable should do the trick, but it’s best to check nonetheless. Well, that’s it for the basics of a COB LED system! As always, you should certainly exercise caution when assembling these systems. It really helps to have some working knowledge of electricity, as there are certainly risks involved. I would suggest enlisting the help of a knowledgeable friend for the electrical stuff if it’s not your forte.

If you’re ready to get into the finer details, check out the DIY Guides section for a number of builds, articles, and calculators to help you build your own LED grow light.

If you’ve got a question, head on over to the forum and start a topic!  Our community is happy to help, no matter what skill level you’re at.