How to design a high power LED light

This page is going to show some things to consider when you start building a LED high powered light. As the technology is rapidly changing I will try to stay away from recommending brands and models as every 6 months this is likely to change. I'll try to provide info that I hope will stay helpful for a long time to come.

Which LED to choose

Before even looking at which brand or model to choose, there is a lot to consider. The following info should answer most questions, or start you looking in the right areas when comparing LEDs. Choosing the LED with the most raw lumen output is not going to make the best light if you don't consider a few fundamentals.

The biggest impact on how bright your light will turn out to be is the REFLECTOR or LENS design. Why choose a LED that can produce 10% more lumen when it either does not have the lens you're wanting, or only has a badly designed lens which will loose more than that 10% of that extra light.

Lens or Reflector

Which one to use depends on the brand of LED and what has been designed to work with each model of LED. A reflector works by capturing side light and focusing it forward, so to create a narrow beam of light a reflector must be deep taking up space from the length. A lens works by refocusing or bending the light which is already pointed forward. Lenses are more than just plastic, as you have probably heard of fibre optic cables which can carry light for very long distances and allow light to turn corners. Don't under estimate what a piece of plastic that is designed correctly can do, they can capture side light as well as forward light and focus it very well. Often they can fit into a smaller space.

Both a lens and a reflector need to be designed for the model of LED that you are purchasing, so you can't use one brand of LED with the optic for a competing brand LED. This is because the focal point (the most effective point at focusing the light) of the lens or reflector has to match the hot spot (where the most light is created) in the LED die. If you are wanting a wide beam of light, then this is less critical and sometimes the wrong optic can produce a good result. For a narrow beam of light then it must be carefully matched.

So before choosing the brand of LED you will use, take a look at the available lens and reflectors and find out how efficient they are at focusing the lumens into the beam they are rated at. For example my first LED light used 1100 lumens of LED light output and the lens is rated at focusing 85% of this light into an 8 degree beam. So I will end up with 935 lumen in the beam of light. 85% efficiency is very good with very few designs that can create a narrow beam having a higher rating. By choosing the wrong lens you can see how quickly you loose light if your design is <50% efficient because the two parts of the design simply were not designed to work together or because of a bad design.

One thing to be aware of is how manufacturers are bending the specs, some will quote +-9 degrees which is the same as an 18 degree beam. So don't be fooled when comparing a lens against another lens or reflector, if you see the + and - it means the beam is twice the size and hence the beam of light will not look as bright. A light which has the same amount of lumens focused into a smaller area will appear to be brighter. This becomes a problem when marketing departments run around advertising that their LED light is 9 degrees dropping the +-, when in fact the light is really a 18 degree light. It makes it very hard as a consumer to then work out which light is really what you are after.

I added a page with some beamshots here which demonstates the differences in optics. It also shows how a LED with less lumens is brighter when matched with a good narrow optic.

Common LEDs Compared

Is Series or Parallel better

From my experience with a few different brands and pushing LEDs to the limits to see when they will stop working, I have found that the LEDs will short out when they fail. I am yet to damage a LED which has gone open circuit. Whilst I am sure that it can happen it is far less likely to happen unless the LED bonding wires blow after the LED has already gone critical due to the driver not detecting and shutting down. This is called a cascading failure. So any LEDs which are Parallel like the P7 will completely blow up and stop producing light completely, if the driver does not shut down, then it will burn the bonding wires and go open circuit. With a cree wired in series you find that if one of the LEDs fails for what ever reason, the other LEDs will most likely still produce light as the blown LED is shorted and allows the same amount of current to safely run to the other functioning LEDs. This is a huge plus for using LEDs in series especially as a LED is less likely to fail in the first place as the series LED string will assist in controlling the power and help to prevent thermal runaway situations when compared to a Parallel string. In the instance of using the light in wrecks or caves where the light is critical in getting you safely back out of an overhead environment, having a light which is less likely to fail is very important as backup lights are never as bright as your primary light. Please note that if your LED Driver does not detect and auto shutdown on a short circuit (most do not) then an LED in parrallel which fails firstly as a short will quickly go open circuit due to the bonding wires turning into a fuse. It is actually easily explained why LED's in most cases turn into a short circuit, as all LED's are made from pure silicon which when pure does not conduct electricity until during manufacturing they deliberately put impurities into the silicon to turn it into an LED (this is called doping), when a part burns out the silicon is no longer pure and changes state and allows electricity to flow uncontrolled.

So the point I wish to make is that series is by far a more robust and reliable way to wire up LED's when you have a choice of which one to use. You also increase your drivers efficiency by matching the forward voltage closer to the 12v battery voltage which is more common for dive lights, this will be seen as longer burn time.

What does the BIN label of a LED mean

LEDs are still being developed and improved at a rapid rate so it is hard for a factory to produce 100% cutting edge LEDs. Essentially the factory will mass produce LEDs and then test them to see how bright each LED is, the leds are graded and the brighter ones are sold for more as a better grade or Bin.

Each company will grade and label the LEDs differently, this is important to know as you will soon see.

SSC P7 are LEDs which are made from old XRE range Cree made dies which are then assembled in a Korean factory, because they are assembled in a cheaper factory they end up costing less however the Bin labels vary greatly so know what your buying.

• B Bin is 570 ~ 700 lm maximum output (at 4.2V, 2800mA)
• C Bin is 700 ~ 800 lm
• D Bin is 800 ~ 900 lm
• E Bin is 900 ~ 1100 lm

So if you buy a P7 from an online store you have no idea what Bin number it really is as it is not marked on the LED, your trusting the online shop. Other stores will advertise it is a B bin and try and claim it will give you 900 lumens which is not correct when in fact you could get a LED that produces 570 lumen. The LED could fall anywhere in the range above.

Cree on the other hand have a tighter range in the Bins so your more likely to get an LED closer to what you think your getting, however they still are not written on the LED so your trusting the store it is what they are selling it as. The XRE range will only vary by 6 lumens each step of the Bin compared to the 100 lumen step of the P7 and the 70 lumen step of the Cree MCE.

Cree MCE Bin labels at 700mA per die

• F Bin is 367 - 420 lm
• G Bin is 420 - 490 lm
• H Bin is 490 - 560 lm
• J Bin is 560 - 647 lm
• K Bin is 647 - 752 lm
• M Bin is 752 - 857 lm

How to work out burn times for different LEDs

As an example to compare 4 serially connected Cree XRE r2 bin LED to get 1064 lumen at 14.8 volts and 1000mA draw, to a 4 die Seoul Semiconductor P7 which will give 600 - 900 lumen at 3.6 volts and a 2800mA draw. Since the LED Genie uses PWM with very high efficiency you can run a 3.6v P7 from a 14.8 volt battery which the battery will see a 700mA draw due to the PWM. Here is how to work out the current draw from a higher voltage battery.

3.6 / 14.8 = 0.25 so the PWM will only be on for 25% of the time or 1/4 (a quarter). 0.25 * 2800 = 700 so this will mean the current will be essentially 700mA from a 14.8v voltage source whilst the LED will still be getting 2800mA of current.

You can use the same method above to work out the burn time of any LED however there is also another easier method. Once again using 4 Cree XRE r2 bin LED in the following example.

Voltage * Current(amps) = Watts

14.8v * 1a = 14.8 watts of power for the LED and my battery is 16.8v * 3.8ah = 63.8 watts so this will give 63.8 / 14.8 = 4.3 hours of light which will end up to be even longer as when the battery voltage of the 14 cell nimh pack drops below the 14.8v requirement of the LEDs the current will also drop down giving me extra time when the battery is going flat. This can be useful to design a light that will under power the LED when the battery is going flat which is how some lights are quoting 35+ hour burn times. I have found that I still have very usable light output after 6 hours of use when my buddy's HID just stopped working without any warning.