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Life’s interesting sometimes, as it seems necessity and funding (or lack thereof) have forced me to finally learn about electronics (something my high school teacher could never do). The necessity has been light; and lots of it! Six weeks ago I had no clue about volts, amperes and of course the illusive watt… but I’ve entered the analogue world with vigour and have captured some of my LED findings in this post.

Having recently been thrust into the underwater videography world, I quickly came to the conclusion that we were woefully under equipped for the task at hand. Now, “average lights” may be good and well when you’re shooting a fish at 5m and close range, but a wreck or a cave at extended depth ranges are another matter altogether. To this end, we embarked on a series of DIY projects which have already yielded some seriously capable lights for under $100.

Now the crux of this is that LED technology has come a long way. It’s probably still got a long way to go, but none the less you can build some awesome lights for a song. Being lazy, phase one for us was simply upgrading existing lights (namely 10w HID setups), though we’ve moved on to bigger and better as we got more ambitious.

Step 1 – Housing and canister

Here you’ll typically need a light head and canister. An old HID head is great for this, especially the MR11 kind but the bigger the better in my mind so also consider the older MR16 halogen heads. The key point is that they need to be alloy and not plastic. The canister doesn’t really matter hear, but the size will probably come into play later when we talk about power and burn time.

Step 2 – Source the LEDs and Optics

Now you’ve got a few options here; so decide first what type of light you want. Think about beam angle, intensity and even colour temperature. We’ve been using the Cree XPG LEDs to date as they’ve the most lumen per watt at the moment and are readily available. An Australian company (Cutter Electronics) offer a great service and sell metal PCB boards with LEDs on them in various configurations (you can solder/glue these on yourself but that’s way beyond my skill). Decide what size board you can fit in your housing, number of LEDs, colour temperature and you’re on the way to illumination.

Naked LEDs on a board are great if you want a super wide beam angle; similar to a bare bulb effect. Most people however would consider them useless for a primary so this is where the optic comes in. The optic is a clear plastic widget thing that sits in front of the LEDs and channels the light. You can buy optics in different LEDs configurations and beam angles. Not all optics are created equal though, so do a little research on the one you want and don’t necessarily believe the beam angle figure stated as this may be per led and not the full board. Also remember that a light emitting 1000 lumens over 120 degrees is significantly more powerful than another emitting the same lumens over 10 degrees.

So you’ve now got a LED board and an optic – what next you ask? Well you need to mount these in your housing somehow. If you’re using a MR11 size housing this has a ~35mm internal diameter, but since it used to also house a HID ballast there is a ton of space in there when you put the LED and optic in. Our solution to this has been to machine up an alloy cylinder (~25mm long) that sits behind the LED board. It doesn’t take up all the space in the housing, but provides a good platform to mount the LED board on. Drill a hole for your wiring and you’re done. The space behind it can be taken up by some other spacer or even the cable (just pack it in behind, then tighten the tensioner). Alloy is important here as you need to give the LEDs a good thermal path to the outer housing, so that heat can escape to the water.

Step 4 – Powering the LEDs

Not surprisingly, LEDs require a certain amount of power in order to operate correctly. Too little and you’re holding a candle or they just don’t fire up, too much and they get too hot and burn out. A Cree XPG LEDfor example requires 3.3v @ 1000ma per LED (so 9.9v if you have 3 wired in series). Current is also important, as without this you won’t get enough volts and ultimately light so it is typically used as the indication of LED brightness. That is, most people assume that you can satisfy the voltage requirements of the LED, and will state milliamps are the controlling factor for lumen output.

So let’s assume for a second that you connect a 12v battery up to 3 XPG LEDs. You’ve got more than enough volts, but you also have unregulated current coming from the battery. The result is the LEDs will keep sucking power until the 12v limit is hit, at which point the LEDs would be drawing about 2200ma from the battery. Now you might think great, more current equals more lumens but you’d only be half right. The XPG LEDs can be run safely up to 1500ma (assuming good heat sinking), so anything past that and you’re producing a massive amount of heat and risk the LEDs burning out. Incidentally, this scenario is known as “direct drive” and should only be considered by the brave.

The solution is the LED driver. Drivers typically come in two flavours; buck and boost. A buck driver simply regulates current (and implicitly voltage) so that you don’t burn out LEDs. In the above scenario, a buck driver would limit the current to a specified level (e.g. 1000ma) and keep things in control. Since it limits the amount of power drawn from a battery, it’s also the key ingredient in making a battery last. Now buck drivers are fine when you have enough volts coming from your battery, but if you don’t you’ll need a boost driver. Let’s say we have 7 XPGs in series, which means 23.1v @ 1000ma (3.3v * 7) will be required. As most diving lights run a 12v Nimh battery, you won’t even get off the ground. In this scenario, a boost driver will suck more amps from the battery and convert this to volts. Boost drivers are typically more expensive, more complex and produce much more heat.

Step 5 – Battery

Batteries are rated in ampere hours (1amp = 1000ma), which is a measure of how much current can be drawn from them in one hour at a nominal voltage. For example, a 12v 3000ma battery will supply 12v @ 3000ma for 1 hour, or 1000ma for 3 hours. If you draw 1000ma at 9v, it will last longer (ask someone else how to calculate how long, and when you figure it out let me know).

Step 6 – Wire the sucker up

Basically the sequence is Battery –> Switch –> Driver –> LEDs, though depending on the driver type you can place the switch either side of the driver (this may be relevant if you have canister or head switches). Polarity is an issue with LEDs and drivers, so don’t get it wrong! I’ll admit to blowing up a few things in this way, but they’ve all gone to a good cause. A mate said it best when he told me that “Once the black smoke comes out [which happens when you blow up electronics], it’s like the jeannie coming out of the bottle – the magic is gone!”.

Step 7 – Thermal Path

Heat is the enemy so you’ll need to ensure that it can escape to the water at all times. Drivers typically shut down if they get too hot which means they’ll save your LEDs from burning out, but still very annoying if that happens mid dive. In addition to using alloy components (which will conduct heat), thermal grease is also used to ensure the contact between the alloy components will conduct heat.

LED Calculator

Scott Willan kindly provided me with a spread sheet that handles all the heavy lifting of LED calculations. Based on what you input, it’ll give you the theoretical lumen count, wattage and battery burn times.

Lux vs Lumens

Again I’m not expert here but lux appears to be the measure of brightness where lumen is actually light output. An interesting thing I discovered was that the warmer LEDs have less lux but the same lumens. I don’t really understand this fully yet, but the net result is that cooler LEDs appear brighter. The R5 bin of LEDs are amongst the coolest (8000K), so if you want a punching white light consider these as a primary. The R4 range are closer to the 6000K of daylight, so we’ve used them for video purposes.

10w HID/MR11 Header – Dive Rite, Sartek, Greenforce, et al

You basically have two options here; buck or boost. Assuming you’ve got a 12v battery, you can use a 3xXPG board with RCD-24 driver or a 4xXPG board with a Maxflex booster. It is all about lumens here, as maxed out these combos can output 1196 and 1691 respectively. The 3xXPG optics is tighter than than the GT4-XP optic listed by Cutter (or the 4xXRE optic which we use with XPGs and is tighter than GT4). Net result is the 3xXPG can appear to be a more intense light, is cheaper and less complex. I’m not convinced that either is superior though as it depends on what you can tolerate in terms or complexity, lumens and burn time. For video I use the Maxflex with 4xXPG and for primary I think the 3xXPG is more than adequate.

Latest News: Not into DIY? Find it all a bit confusing? I’ve started modifying lights commerically, get the Underwater Light Dude to upgrade your light for you at affordable prices. Checkout http://lightdude.com.au