Basic primer on Light Emitting Diodes

[Hosebomber]

I posted this in Ronnie's grow journal and felt it would get a few more views and help some people make a more informed decision when it comes to LED lighting.

This is an attempt to clarify some of the information and misconceptions. Remember that this is in general terms and not all diode producers follow these rules exactly, and I will attempt to explain that a bit as well.

LEDs are rated by their max designed drive current. That current rating is as follows:
1 watt diodes: max drive current 350 mA (mil Amps)
2 watt diodes: max drive current 500 mA (mil Amps).... this is generally 2 single watt diode chips and are not made much any more
3 watt diodes: max drive current 750 mA (mil Amps)
5 watt diodes: max drive current 1000 mA (mil Amps)... or 1 Amp
10 watt diodes: max drive current 1500 mA (mil Amps)... or 1.5 Amps .... Note that once you get into this range there are a number of factors that change everything and the general rating of drive current to wattage may or may not be in effect.

The actual wattage of a diode will NEVER be the claimed wattage used (with a very small exception in some COB or chip on board designed diodes). The formula for wattage is as follows:

Forward Voltage x Amperage = Watts

This is displayed in data sheets as V for voltage, IF for Amperage, current forward, drive current.. and W for watts or power consumption.

Next is the Voltage. The voltage rises as current increases, but not all diodes use the same voltage. The following is taken from the datasheet for Cree XPE series of diodes.

Diode Color	Min Voltage	Max Voltage
Forward voltage (@ 350 mA) - white	3.05	3.9
Forward voltage (@ 350 mA) - royal blue, blue	3.1	3.9
Forward voltage (@ 350 mA) - green	3.3	3.9
Forward voltage (@ 350 mA) - amber, red-orange, red, photo red	2.1	2.5
Forward voltage (@ 500 mA) - amber	2.3
Forward voltage (@ 700 mA) - white	3.3
Forward voltage (@ 700 mA) - red-orange, red, photo red	2.3
Forward voltage (@ 1000 mA) - white, royal blue, blue	3.5
Forward voltage (@ 1000 mA) - green	3.8

As you can see, all of these 5 watt diodes have different Voltage forward depending on the current that they are driven at. Even though these are "5 watt" diodes the red, red-orange, and photo red all have a max current of 700 mA (Amber has a max current of 500 mA). This would technically make them a 3 watt diode (and 2 watt for Amber), but the manufacturer is not going to say "our 5 watt line that is sometimes a 3 or 2 watt depending on the diode you choose." Now lets see that the actual wattage of these diodes are. When Driven at 1 watt or 350mA the following is the actual power consumption of each:

Forward voltage (@ 350 mA) - white	1.07W
Forward voltage (@ 350 mA) - royal blue, blue	1.09
Forward voltage (@ 350 mA) - green	1.16
Forward voltage (@ 350 mA) - amber, red-orange, red, photo red	.74

The white diode (which is what they sell and market the most of) are pretty close to 1 watt. This is where/when the original naming of diodes by drive current occurred.

When Driven at their rated 5 watts (or max current) we get:

Forward voltage (@ 1000 mA) - white, royal blue, blue	3.5
Forward voltage (@ 1000 mA) - green	3.8
Forward voltage (@ 700 mA) - amber, red-orange, red, photo red	1.61

As you can see, the "5 watt" Red diodes, driven at 3 watt power, only pull 1.61 watts each. This is the diode that the majority of our panels are made from. This is also where a large portion of the misnaming and misrepresentation of LED panels come from. We have 100 three watt diodes in the panel so it's a 300 watt panel. But 80% of those diodes are red, this means our 300 watt panel only uses about 175 watts for powering the diodes.

The output of the diode will directly correlate to the drive current and the efficiency of the diode. In general terms (there are some new nano-tech COB diodes that do not follow this), the lower the drive current the more efficient the diode is, meaning that if you run the diode at a lower current it will have a higher radiant flux (light photons) output per watt (power input) than if you provide more current to it. However, you can drive the diode harder and get more radiant flux from the same diode at a lower efficiency. This gives us the ability to get more light in a small area at the cost of some efficiency. What this means is that you get a higher lumen per watt at a lower drive current, but more lumens per diode at a higher drive current.

Lenses and secondary lenses. In the above picture on the right hand side is the basic design of a diode. The chip is placed on top of a reflector cup, a membrane is added to protect the diode, a layer of phosphors are added to make it transmit the color wanted, then a resin dome is placed over the top to hold everything together and create the dispersion pattern of the light that comes out (in conjunction with the lower reflective lens). Nearly all high power diodes have a Lambertian pattern output. This designates how much of the light goes where. Again in general, high power diodes use a 120 degree Lambertian pattern (picture below). Secondary lenses change this pattern to a different angle. In doing so, more photons are directed in the area the lens determines, but at the cost of 7-15% of the radiant flux.

Now into some of the dirty parts. Epistar is one of the largest makers of chips. They are a Chinese company that started by attempting to copy the top producers and make more faster and cheaper. They did very well at the last two things, not so great at the first one. They produce mass quantities of chips at a super cheap price, however, they don't follow the general guidelines set forth above. They use the size of the die (the base the diode is placed on) to determine what they call their diode. Their 40x40mil diode is their 3 watt and 45x45mil diode is their 5 watt chip. These have max drive currents of 250mA and 500mA respectively. They do produce a wider range of colors than most companies, but in their quest to make super fast and cheap diodes, the resin they use for the diode lens (the dome over the chip itself) is very cheap as well. It degrades at a much faster rate than higher quality more costly chips and cannot withstand UV or IR light very well at all. This is also the reason you don't see the larger diode companies making low blue and UV diodes. You will be hard pressed to find a quality diode below 450nm and above 670nm. The resin and curing process for those resins to withstand the UV and IR light are much more expensive and time consuming to produce. Thus, they simply do not make diodes that require those methods. This saves them in material cost, production time, retooling between diode types, and a host of other money and time saving efforts.

If I left anything out or more clarification is needed please let me know.

 

[nofene]

Brilliant, coherent info.
Just a couple of practical things that I ran into:

Try not to go over 1A (if you want to use larger than 5W diodes it is usually better to go with COBs) with your drive current, everything heats much more which causes less efficiency. From this you can take cooling is important, it increases efficiency, also the better cooling the longer lifetime.
There are lots of discussion going on about secondary lenses. IMHO they can work well grow lights creating a much more homogenic light and can help with penetration. You have to look out for the design though also the material which polymer is it made out of, efficiency again how much light does it let through. In my experience it is more important on lights made of single chip emitters, COBs... I don't really know.

Try to se efficient LED drivers, and try to use most of the voltage that is provided by it! As a rule of thumb if you have a let's say 24V 700mA driver, put a chain of emitters on it on which at least 16-18V drops. In other terms, don't just hook two red emitters on it on which drops only about 4.4V oddly enough the more of the voltage available you use the smaller the heat the driver makes. This is true for drivers made with MOS-FETs. Count with a 0.6-0.7V overhead.

 

[Hosebomber]

Yes, you should use all but about 10% of the drivers available voltage to help keep the heat of the driver down.

You are mistaken on lenses. You will get less homogeneous blending of diodes with lenses on. This really doesn't matter in COBs because the diodes are so close together the blending happens prior to the lens. As for using 5 watt or higher diodes, it's a design choice. When you design your panel and layout you take into account the heat sink needed prior to ever buying the first component. COB's have a much higher heat output than a single high wattage diode.

 

[nofene]

Thanks for refining!
Of course the design choice is everyone's own, I just wanted to point out that if you are not familiar with DIY electronics or such it might be wise to stay under 1A.

Can I please ask your opinion on lenses? If you take a look at my small journal with an even smaller wattage light in there would you recommend using lenses on the single emitters? Also as a rule of thumb is it better to stay clear of them or it depends on the diodes?

Thanks ever so much!

 

[Hosebomber]

It's more of a grow style choice when it comes to lenses. If you are growing your plant taller without any type of training then lenses are usually a good option. If you are using a screen or LSTing your plants then there is no need for the deeper penetration and the use of lenses are just reducing your flux and your footprint.

 

[nofene]

Brilliant, makes absolute sense, thanks!

 

[Icemud]

Great writeup Hosebomber!! You definitely are the light guru!!!

What I find funny is that almost every LED manufacturer uses the "3 watt" and "5 watt" terms for advertising, however when you go to any of the LED chip manufacturers websites or data sheets, NONE of them list chips by wattage. (most are listed by Max current like you mentioned).

I think Cree and Nichia were the only ones that even mentioned chip wattage. Even Epistar listing their chips at 45x45 and 50x50 doesn't say anything about 3w or 5w.

I think the whole "3 watt" or "5 watt" is one of those terms used loosely by led grow light manufacturers like "penetration" to attract the buyer but the relevance really nothing. I just don't buy that the led grow light manufacturers are "unintentionally" using 3w or 5w as descriptions because the chip manufacturers list them as so (most of them dont). I think these LED manufacturers exactly know what they are doing by throwing around the terms to attract buyers.... knowing that the LED grow light world is so full of crap information, that most growers wont actually do the research to know what is really in their lights.

I mean, look at Mars II for example... a lot of buyers are attracted to the company because they use "5w" diodes...but do they? I think both of us know the answer is no... but if they advertised 3w diodes, then it would lose it uniqueness in the industry being one of few led grow lights labeled 5w... unlike some that actually do use 5w diodes like the new advanced xTE series about to be released and the new budmaster.