COB LED Basics: A Beginner’s Guide

After sifting through my posts, I realized that I’ve neglected to write something that’s a little more suitable for somebody who’s brand spankin’ new to the world of LED COBs. This guide will serve as a short introduction to the basic elements of COB LED lighting systems, and is a good place to start if you’re looking to make the switch from other types of lighting, or if you’re new to indoor gardening altogether.

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 LEDs themselves
  • Heat sinks that the COBs are mounted to
  • LED drivers that power the COBs
  • The wires that interconnect the COBs and drivers.

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

1. COB LEDs

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. LEDs 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’s characteristics that can be found in the data sheet include:

  • Color temperature (how “warm” or “cool” is the color 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?)
Front side of a CREE CXB3590 COB. The yellowish pads you see in the corners are the positive and negative connection points.

Front side of a CREE CXB3590 COB. The yellowish pads you see in the corners are the positive and negative connection points.

Back of the COB. This is a ceramic substrate.

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 COBs 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 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!

COBs, holders, and an LED driver.

Reflectors

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

2. Heat Sinks

Despite being very energy-efficient, high-powered COB LEDs 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 COBs and keep them running as cool as possible. Heat sinks are most often made of aluminum, 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.

A Vero 18 COB mounted to an actively cooled heat sink.

The fan blows air over the fins, removing hot air.

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.

A passive classic fin heat sink.

A CXB3590 COB mounted to the same passive heat sink.

Passive pin-fin heat sinks, pre-drilled for COBs.

Thermal Interface

If you were to try and 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.

Thermal paste applied to the back of the COB – you don’t need much!

Smoothed out.

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 COBs can use. The electricity coming from your outlet is alternating current (AC), and this does not work for COB LEDs – 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 COBs. 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 COBs.

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

A 200 watt constant current LED driver.

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 COBs to one another and then connect the COBs back to the driver for power. See the post on wiring LED COBs in series and parallel for more information on how to hook up the COBs, 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.

An LED driver with a dimming potentiometer, AC power plug, and 18 gauge output wire extensions connected.

 

Well, that’s it for the basics of a COB LED system! As always, you should definitely 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.

10 Comments

  1. Hi, great info. So how can i pick drivers and cobs what do I look for and have the best light possible? thanks

  2. Is it ok to have a driver thats over rated? For example would I be able to use the same driver for a 3 cob system for a single cob system?

    • LEDGardener

      June 8, 2017 at 9:24 pm

      Hi Payl, that depends mainly on whether the driver is a constant voltage driver or a constant current driver.

      If it’s constant voltage, as long as it has enough current capacity, you could use it for one COB or multiple COBs wired in parallel (e.g. – you have a 36 volt driver that can produce 6 amps, and you’re using COBs that draw 2 amps each at 36V. You could hook up 1 COB, which would draw only draw 2 amps from the driver, or you could hook up 3 COBs which would draw 2 amps each for a total of the full 6 amps from the driver. This is one nice thing about CV drivers, but they tend to be trickier to use and there’s a better chance of cooking your COBs if you’re new to electronics.

      If it’s a constant current driver, it will have a specific range of voltage that it will work properly in, called the constant current range. For example, if you have a constant current driver that’s rated to put out a steady 2,100 milliamps of current between a range of 36 volts and 72 volts (like the Mean Well LPC-150), then you could use 1 COB on it, or 2 COBs, on it wired in series (when wired in series, their voltage adds up, so you’d have 36+36 = 72V with 2 COBs in series), but not 3 COBs on it, as that would be 36V+36+36 = 108V, which is outside of the constant current range of that driver.

      This can work the other way too, where your driver might have a range that 3 COBs fit in (say, 72V-144V), but it won’t work for a single COB since the single COB voltage of 36V is too low. I’d recommend going with a constant current driver if you’re just getting started. You can get one for like $20 for a single COB, then upgrade to a bigger one if you ever buy more COBs.

      This is sort of a simplified answer as there are other variables and different ways to get around these problems like getting creative with series/parallel wiring, but hopefully it helps!

      • Do not connect COBs to CV drivers. Your example above does not work and would absolutely fry the module. LEDs are current driven devices and will consume as much current as you give them there is nothing in the circuit that tells it to stop consuming at 2 amps.

        • LEDGardener

          August 10, 2017 at 2:44 pm

          If you put in a little more research, you’ll find this to be completely untrue. LEDs can absolutely run on CV without issue, so long as you aren’t overdriving them and pushing them to the point of thermal runaway. Here are some examples of my own work:

          http://ledgardener.com/cob-leds-work-constant-voltage-cccv-drivers/

          http://ledgardener.com/constant-current-vs-constant-voltage-drivers/

          Just the other week I hooked up a single Quantum Board to a 600W constant voltage driver. I ran it for hours, at nearly max current, and it was completely stable. That driver was capable of producing 11 amps, and the max of that board is only 3 amps. I’ve also been running a couple Vero 18s on a constant voltage driver for months – that driver is powerful enough to destroy them both, but again – no issue.

  3. Hello bud, I am about to wire up my cob system, I have 20 anodised heat sinks joined up to an bare aluminium frame, I also have 4 meanwell hlg drivers on the aluminium frame.
    I am going to connect each drivers AC sides to C14 sockets one each and run a 1 to 4 way c13 plug to drivers.

    My question is about grounding the aluminium frame for protection from a short circuit, I have took precautions such as rubber sleeved grommets in the aluminium and heatshrink on wires.

    Should I just run a wire from the aaluminium to the ground wire of one the c13 splits and have that split always connected? Wouldn’t this cause a loop with the grounds of all the drivers?

    Would appreciate your help

    • LEDGardener

      August 5, 2017 at 8:25 am

      Hey there, good question. I think I’ll ask one of my electrician friends to be sure. I wonder if the frame would be somewhat grounded by the metal case of each of the drivers, if you’ve got drivers mounted to it. The resistance between the frame and the case may be higher than ideal though – I’m not certain.

  4. Good info and thanks. My question is where to buy and what type of thermal pad to use. Im using vero 29 7’s. I don’t want to use paste so i can upgrade down the line. I’m using screws.

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