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:

• APC-8-250: 8W, 250ma, 16~32V
• APC-8-350: 8W, 350ma, 11~23V
• APC-8-500: 8W, 500ma, 8~16V

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.

We’ve all been there, one or more of our plants isn’t doing well and you think, maybe it’s not getting enough light?

Is inadequate lighting really an issue? how do you tell if it’s affecting your plants? and if it is, what can you do about it? Let’s find out!

To Do: Add some pictures of well lit plants vs plants grow under inadequate lighting.

Symptoms

Stretching:

The easiest way to tell if your plant is not getting enough light is to look out for “stretching”; this is the term used to describe a plant that is unusually tall, with long gaps without leaves along the stem. This is also known as getting leggy, or having extended internodes. By contrast, a well lit plant will be short and stocky with short internodes and many leaves per inch/cm of plant stem.

In a garden or field, stretching is useful as getting tall helps a plant to compete with its neighbours and hopefully get above them and out of their shadows. In an indoor situation it’s less useful, gaining an extra few inches or centimeters is unlikely to result in more light and stretching is not a good strategy, but I’ve tried telling that to my plants and they just won’t listen!

Some plant varieties are more prone to leginess than other, for example I’ve always struggled to grow coriander/cilantro as I find my seedlings immediately get leggy as soon as they germinate. Perhaps I need to try germinating it in a propagator with lights.

Leaning:

Stretching causes the stem to be thin and weak which results in the next symptom: leaning. The plant will naturally bend towards what little light in available, that coupled with the thin stem will often cause it to fall over. A certain amount of bending towards the light is expected, but if the plant becomes so tall that it can’t support its own weight then it becomes problem. A well lit plant, especially one where the light is coming mostly from above, won’t have a significant lean and will be short and stocky enough to support the plant’s weight.

The stretching and leaning happens because a certain amount of light (especially blue light) is needed to inhibit cell growth and elongation. Without enough light, and particularly on the shaded side of the plant, the cells elongate and the stem stretches. If this happens more on one side than the other then the plant bends, hopefully pointing it in the direction of the light source and getting it more light.

Small Leaves:

If a plant is in the dark, there’s no point pouring energy into leave production. Instead the few leaves it does produce will be small and they will be spaced apart (see internode elongation above). Plants require light to create useful energy for growth, in low light conditions there’s not much energy to go around, so the energy is better off spent on getting close to the light than producing leaves that won’t get much light.

Yellow Leaves:

Plants get the green colour of their leaves from a chemical called chlorophyll. This chemical plays a vital role in turning light into useable energy in a process called photosynthesis. However, plants need to spend energy to produce chlorophyll. Under low light conditions, in a similar energy saving tactic to the small leaves above, the plant will conserve energy by not producing as much chlorophyll. This is visible to us as a yellowing of the leaves. By contrast, a plant grown under good lighting conditions will have a rich deep green colouration as it will be packed with chlorophyll.

If you’ve ever eaten forced rhubarb or asparagus then you might have noticed these foods are a different colour to their “normal” counterparts. Forcing is the process of growing a plant in the dark, and this challenging process is done to fruits and vegetables with special characteristics. Forced rhubarb is sweeter than normal rhubarb and is characterised by its pink (not green) stem. If you pay attention you might spot the tiny yellow leaves – a far cry from the usually huge green leaves of unforced Rhubarb. Asparagus spears are usually green, but when forced they are white rather than green.

To Do: Add a picture of forced Rhubarb.

Yellowing of leaves can be caused by other factors, such as mineral deficiencies, so shouldn’t be used as an indication for

Slow Growth:

As mentioned above, plants need energy to grow and they get that energy from light. Under low light conditions the plant will not be able to grow as quickly so will grow more slowly or may even appear to stop growing altogether.

If you’ve noticed some or all of these symptoms in your plant, then you should consider giving it access to more light to see if the plant’s health improves.

Fixes (How to give your plants more light)

Sunlight:

Often the easiest option is to give it more sunlight. If it’s warm and sunny consider putting your plants outside. If they are outside already consider moving them to a sunnier spot. If you can’t more them outside, consider moving them to a windowsill or to a windowsill that gets more direct sunlight.

These are great options if you have them, but it’s if it’s winter and you live far from the equator then there might not be many hours of sunlight to go round. If you live an a urban environment then you may not have access to outdoor space, even windows might be at a premium.

Existing Artificial Light:

Thankfully, artificial lighting can just as effective and is often easier. If you’re going to have lights on anyway then it can also be free. Take a look around your room and consider whether there’s an existing spot that gets more light. Perhaps you have a side light or a desk light that you could move your plant to be underneath. You could consider putting your lamps on a timer so they come on automatically in the evening. Even moving a plant from a dark corner to a brighter spot can make all the difference, especially for low light plants.

Switch to Low Light Plants:

Some plants need high light levels (basil, I’m looking at you!) and won’t grow well for me indoors over winter unless I give them some dedicated grow lights. One option you should consider is growing a different type of plant, I find Pothos has no problems growing next to a table lamp year round. I have plenty of rooted Pothos cuttings that I occasionally sell on eBay. Let me know in the comments if you’re interested.

To Do: Add an article on low light plants.

Grow in Summer:

The next is to consider growing at a different time of year – the growing season can be short, but it can be intense and fun to grow with the seasons as nature intended, enjoying the fruits of your label in autumn/fall with everyone else.

Dedicated Grow Lights:

If you’re determined to grow basil over winter then another option is dedicated grow lights. These can range from function to beautiful and can be expensive or made yourself cheaply and fairly easily. That said, there is an ongoing cost to running grow lights (see my grow running cost light calculator to get an idea). Personally, I light to integrate my growing lights with my home decor. By building beautiful lights that light both the plants and the room in a high quality glow, I get the benefit of healthy plants, lit beautifully while adding layered lighting that lifts the room. These lights can take a struggling houseplant in a dark corner and turnin it into a beautiful centrepiece for the room. Money well spent if you ask me.

To Do: Write up and link to the build of my chilli lamp/stand and add a picture here.

Not everyone will be hand crafting their own lights, so if you want to buy one then I’d say use whatever light you prefer. The amount of light is more important than it’s exact colour. I used to use the pink/purple (red/blue) glow lights as I felt they were more efficient, until I realised that for me, they took the joy out of growing as I couldn’t admire my plants. Instead I came to realise that white LEDs are as efficient (they have has far more research and development poured into them than grow lights) as more importantly for me, they can be beautiful. Cooler (bluer) shades of white light is probably better, but there’s not much in it, and I prefer to have warmer (redder) lights in my home so I personal built grow lights using warm white (2700k-3000k) LEDs.

Running costs:

A powerful 25W grow light (enough for about 6 plants) costs me around £3.60/year to run for four hours a day, six months a year. I keep this on a windowsill and those four hours gives extra light at the end of the day to extend the hours of daylight. This is with currently expensive 18p/kWh electricity tariff.

A smaller, 4W single plant grow light might cost me £1.8/year to run for 8 hours a day, six months a year. This assumes it’s not near a window so needs longer hours of light. The cost will depend on your electricity tarif, try plugging your numbers into the grow light calculator to see how much it would cost you.

To Do: add a write up for a single plant 4W grow light and link to it here.

Conclusion:

Hopefully you’ve learnt how to spot the stretching, leaning, small/yellowed leaves and stunted grow of a plant with inadequate lighting and you can easily tell if your plant is getting enough light. I hope you’re also got some ideas of how to fix the issue if it’s affecting you and your plants. Whatever fixes your choose, I hope they work well for you and let you keep on growing.

To Do: add a picture of some deep green stocky basil plants loving life under a grow light.