Contents
Introduction
An LED (Light Emitting Diode) COB (Chip On Board) light is a type of LED that is made from an array of smaller LEDs combined into on package. They are useful because they can be bigger, brighter and simpler than single LEDs.
The Problem LED COB Lights Solve
Imagine you want to create a 20W LED light. You could purchase a pack of 20 1W LEDs, then figure how to mount them all, how to wire them all together, and depending on how you’ve connected them (all in series? all in parallel? a mixture of series and parallel?), you’d need to pick the correct spec driver to power them. You also need to consider how well balanced all the individual diodes are, and how them impacts your choice of series and parallel.
You could make your life easier by instead purchasing seven 3W LEDs, you now have fewer chips to mount, fewer connection and fewer options, but it’s still a complicated build.
Wouldn’t life be simpler if you could just use a single 20W LED? Finding such a single LED would be difficult, if not impossible (single LEDs tend to max out at around10W). If you did find one, it would require a high current, power supply of over 5A (assuming a forward voltage around 3.7V).
COBs are an array of small LEDs, packaged together in a way that lets us treat them like one big LED. When we build a light using a COB LED we don’t have to worry about the tiny LEDS they’re constructed from, we just read the specs (e.g. 36V, 800ma) and hook the cob up to a suitable LED driver. One chip to mount, two connections to the driver (positive and negative), life is simple again.
Cooling an LED COB Light
In our previous example we picked a hypothetical 20W LED. By using a cob we were able to use a single LED while still reaching our 20W target. But putting that much power into a small package brings a problem: cooling.
LEDS are way more efficient than incandescent lights, but they still dump more than half of the energy (Watts) they consume as heat. My rule of thumb is easy, just assume the total power going in (say 20W) needs to be cooled. Cooling is a black art anyway, unless you’re using an off-the shelf cooler, you’ll need to use some trial and error to get something that works well.
Why is cooling LEDs important?
Cool LEDS is important for efficiency and to ensure your light keeps working. In an extreme example, running an LED at high power (> 0.1W) without a heatsink will likely cause the LED to overheat and burn out. The light will break and can’t be fixed.
Using too little cooling is nearly as bad – it might take longer, perhaps several hours of continuous use, but the LED can still be damaged by overheating. The datasheet for your COB LED should tell you what temperatures it can cope with. The Cree CXB1507, for example, can only be used at full power up to 85C.
If you’re lucky, the datasheet will also tell you how efficient the COB LED is at different temperatures. Using the Cree CXB1507 as an example again, it’s around 10% more efficient (depending on the exact model) at 25C than at 85C.
Options for Cooling LED COB Lights
To cool a COB LED you will almost certainly need to attach it (with thermal compound) to something thermally conductive, like metal. An aluminium (copper is even better) heatsink is great, but I’ve also had good luck with a sheet of copper. Lower power lights (say 10W) can make do with a modest passive (no fan) heatsink, while higher power lights (closer to 100W) need very large heatsinks and/or active cooling with a fan.
Whatever cooling option you choose needs to be open to the environment, i.e. a heatsink is no use if it’s sealed inside a box. When designing a high power light, the cooling should be one of the first considerations.
I’m in the process of designing and building a small desktop LED plant light using a single 18V CXB1507 COB LED. It’s capable of being driven at 13.5W, but I’m choosing to run it at a modest 6W so I can cool it passively. Even so, I’ve built the light out of copper so I can use the light’s frame as a heatsink to keep the chip cool.
Choosing a COB LED (power)
There are three basic power specifications you need to know when choosing an LED driver for a COB LED:
- the COBs forward voltage
- it’s maximum current
- it’s maximum power
These will usually be easily visible on any advert, and sites like Digikey or Mouser will let you search using these parameters.
The LED’s power (in Watts) is simply the forward voltage (in Volts) multiplied by the current (in Amps), you may have seen this written as: P = IV and it’s the basic formula for electrical power. This is very useful if you want to drive your COB at a lower power (e.g. the power of an 18V LED run at 350ma is 18 * 0.35 = 6W). In practice the voltage will vary a little as the current is changed, but you can assume it’s fixed. If you know the current you can work out the power and vice versa.
Choosing what Current to Drive a COB LED
You can think of the max current (and power) specs on a COB LED like the maximum speed of a car. Yes it is possible drive a car at 150Mph (okay, maybe not my beat up mini, but a theoretical car!) but that doesn’t mean you should aim for that speed. Similarly my CXB1507 has a max power of 13.5W, but I don’t have to drive it that hard.
Thankfully, by carefully picking your LED Driver (power supply) you can choose to drive your LED at a lower current, and hence a lower power. As mentioned above, there’s a linear relationship between current and power, so halving the current halves the power etc. The current (and hence the power) is determined by your choice of LED driver.
While staying within the limits the max current and power limits of your chosen LED, you should pick a drive current that’s commonly available, and doesn’t exceed the limits of your chosen cooling solution.
Constant Current vs Constant Voltage LED Drivers
When chosing a driver for your COB LED, you’ll want to pick one that is Constant Current. This means that it will always drive the load (your LED) at a fixed current and will vary the voltage it outputs (within a max range) to achieve the desired current.
This is in contrast to the more common Constant Voltage power supplies such as the USD supply you might use to charge your phone. These give a fixed voltage (say 5V) but the current can vary.
While it is possible to use resistors to drive an LED from a constant voltage supply, I wouldn’t recommend it – just get yourself a constant current supply.
What to look for in a Constant Current Supply
By now you should know what the forward voltage of your LED is, and what current (and power) you want to run your COB LED at. Picking an LED driver is then simple:
Choose a constant current LED driver that runs at your chosen current, and supports your LEDs forward voltage.
Choosing a COB LED Driver: Case Study (Meanwell driver for 18V CXB1507 )
I’ll use our trusty 18V CXB1507 as an example: it has a forward voltage of 18V and a max drive current of 750ma (max power 13.5W). But I’m fairly sure the lamp I’m building would get too hot if I run it that power, I’d like to keep it cool enough to comfortably touch (plus I like to run my leds around half their rated capacity for efficiency and a long life). 50% of 13.5W is 6.75W so I’m looking for something in that ballpark.
My favourite LED drivers are made by a Meanwell, I’m fairly familiar with their line up so I quickly spot the following drivers which may be suitable:
The APC-8-250 would work, 18V is within it’s supported voltage range (16~32V), but the power output would be a fairly low 4.5W (18 * 0.25). If I was making a very small lamp, this would be a good option. At 4.5W I would expect my LED would put out over 473lm.
The APC-8-500 would not work, it’s supported voltage rage (8~16V) does not cover our 18V forward voltage, and the power draw at 500ma of our LED would be 9W (18 * 0.5) which exceeds the 8W maximum of the APC-8-500.
The APC-8-350 is the goldilocks option, it supports an 18V load (11~23V) and will result in a power draw of 6.3W which is very close to my 6.7W target. The APC-8 series are also small and cheap, so great for my application. I’d expect to get over 662lm from the CXB1507 at 350ma (6.7W), which is a lot of light for a small desk plant.
Scotty, We Need More Power!
What if I wanted to drive my LED harder? Sadly Meanwell don’t make an APC-12-500 or an APC-16-500, so my options are:
- APC-25-500: 25W, 500ma, 15~50V
- APC-12-700: 12W, 700ma, 9~36V
Both of these would work, but they’re bigger and more expensive than the APC-8 series, they’d drive the LED at 9W and 12.6W respectively, so I’d need to be very careful to ensure my light doesn’t overheat and consider adding a bigger heatsink.
Another option might be a variable power supply (say the IDLC-25-700) but these are bulkier, even more expensive and have the added complication of needing an external voltage reference to dim them. I personally wouldn’t bother, unless you want to test your cooling capacity at a range of currents, but even then, you’re probably better off with a benchtop power supply.
Choosing a COB LED (quality)
So far the discussion has focussed on the practical aspects of COB LEDs and how to power them, but the quality of the chip and it’s light output is equally as important. Some things to consider are:
Colour
These days I stick to white LEDs, but there’s not reason you couldn’t go for blue, red, purple or even green LEDs. For plants I’ve found that any supposed benefit to blue, red or even UV LEDs has been outweighed by the availability of highly efficient white LEDs. They also make your plants look nicer and are thus nicer to live with.
Colour Temperature (CCT)
You’ve probably heard terms such as warm white, cold white, daylight while or neutral white. The CCT is the specific way to refer to these vague terms. Warm white is around 3000K (or below), neutral white is around 4000K while 5000K and upwards is cool white. Daylight White is very cool, around 7000K or so. I’m currently building desk plant lights, and I enjoy warm white light, so I stick to around 3000K.
The CCT will have a minimal effect on your plants, but a bit impact on your enjoyment of the light, so pick something you like. If you’re not sure, buy a few different CCT version of the same COB LED and try them each out.
Colour Rendering Index (CRI)
This relates to how much colours “pop”, i.e. how vibrant they are under the light. Your plants will not care, and even for people it’s very subjective, but it can make the difference between an unpleasant (cheap) quality of light and something that looks/feels much more high end. I personally look for a CRI of 90 or above, but it’s another one you’ll need to try for yourself to see what you feel is worthwhile.
This will be much more important if the light is for a space in which you live, like a kitchen or living room where people will get the benefit. If the light will live in a closed cupboard, then you might be better off with a cheaper or higher efficiency COB LED, rather than one with a high CRI.
Efficiency
This is measured in Lumens per Watt and higher this number is, the better. Higher efficiency means you can run your lights at lower power, meaning fewer issues with heating and lower electricity bills – win win!
Brand Quality
The specs on the datasheet are great, but if you buy from a cheap/disreputable buyer, then you can’t trust those specifications. Personally I mostly stick to Cree LEDs and Meanwell drivers, and I order them from big name suppliers so I can be sure I’m getting what I expect.
That’s not to say eBay or Aliexpress don’t have some great deals (they do!) you just have to be a lot more careful and do your own testing to ensure you’re getting good quality.
