DIY Metal Halide

Introduction

While everyone involved in reef keeping agrees good lighting is one of the keys to success, not everyone agrees on which type of lighting is the best. Clearly, no one type of lighting is best for all situations, and the ultimate decision about which type is best for your reef tank depends on a lot of factors.

If you decide to use metal halide lighting for your tank, then the first thing that becomes apparent is that the initial investment is quite substantial. Commercial metal halide lighting systems are not inexpensive. Most range from $500 to over $1,000. This represents about 1/3 to ½ of the total cost of setting up a reef tank.

Since commercial lighting systems are so expensive, the first question many people ask is “Can I build a MH lighting system?”. The answer is yes! Better yet, it is not difficult at all.

Obviously, one can go a little overboard and not really save any money. However, simply replicating what you can buy from any commercial vendor will end up saving you considerable money and give you some sense of pride when you’re done.

The Idea

The basic idea behind building a lighting system is twofold: Create a system that suits your needs and one which is safe and economical to use.

Since only you can address exactly what your needs are, it is assumed for the rest of this document that 2 MH lamps and 2 FL lamps will be sufficient to meet your needs. Additionally, it is further assumed the tank is 48″ long (the width is not really a critical factor). Notice wattages of those lamps were not mentioned. You can build the lighting system without addressing the details of wattage until the end (if you plan ahead).

Construction

There are two main components to any lighting system. The hood and the lamp assembly.

The Hood

The most critical thing to realize while designing your hood is that a 48″ FL lamp isn’t actually 48″ long. It is more near 47 1/2″ long. As such, the distance between your lamp holders should be around 47 1/2″. The exact dimension really depends on the specific lamp holder you use. For example, the popular BJB endcaps should be placed about 47 1/2″ apart. Additionally, these endcaps are thin enough to allow you to make the overall interior length of the hood be 48″ without impacting the insertion and removal of the FL lamps.

As a general rule, however, it is better to be safe than sorry. The interior dimension of the hood should be larger than the overall end to end distance of the lamp holders. An interior dimension of 49 to 50 inches should work fine.

Having an interior dimension of 49″ limits the type of wood we can use for our hood. Most planks come in 48″ and 96″ dimensions. For this reason, the easiest type of wood to work with is a plywood. A 4 foot by 8 foot sheet will allow you to build your entire hood with a lot of wood left over.

It is worth noting that you could build your hood out of planks if you wish to. You need to either make the dimensions ‘plank-friendly’, that is a multiple of 48″, or compensate for the odd lengths by purchasing longer planks. The latter, however, will leave you with a lot of extra wood. Just an excuse to build another hood ;).

Figures 1 and 2 show our hood with dimensions. The figure assumes the hood is constructed from 3/4″ plywood. Note, finished plywood produces a much nicer final product than construction grade, but any grade can be used.

Metal Halide Hood - Top — Figure 1 – Metal Halide Hood – Top

Metal Halide Hood - End — Figure 2 – Metal Halide Hood – End

This design is rather accommodating and should not need to be modified much to suit your particular application. However, the diagram does assume the hood will be suspended over the tank. This design does not make provisions for opening the hood on a regular basis for daily feeding or tank maintenance.

These figures give a general idea of the dimensions and construction of the hood. It is basically just a square box with a top, but no bottom. There is nothing complex about the hood assembly, however, there are a few notes:

For optimum air circulation, a 4″ fan should be placed at one end of the hood and an open screen on the opposing end. The fan should blow air into the hood, across the MH and FL lamps.

Secondly, to protect the lamps from water spray, an acrylic shield should be placed between the lamps and the water’s surface. An easy way to do this is to provide a channel near the bottom of the hood for an acrylic sheet to be inserted. However, the sheet needs to be removable so the lamps can be replaced. Therefore, one of the end boards should be about a 1/2″ shorter (in height) than the other. This will allow the shield to be slid in and out with ease.

Figure 3 shows a rather detailed picture of one end of the hood, with the lamp assembly in place. This picture gives a nice illustration of how to make the rail for the shield. An other option is to use hinges on the shield, or simply screw the shield to the bottom of the hood.

Lamp Assembly

The lamp assembly for the hood is somewhat more complex than the hood. However, its construction is not terribly difficult. It just requires some planning.

There are 2 main components of the lamp assembly, the sockets/wiring and the reflector. Figure 3 shows a diagram of the lamp assembly, which includes two MH lamps and 2 48″ FL lamps mounted on a reflector.

Due to the wide array of fluorescent endcaps available on the market and the all the possible variations regarding mounting the metal halide lamps, Figure 4 represents about as much detail as possible when it comes to constructing the reflector assembly. I realize this is not much information at all and I will try to expand on the information a little in the following sections.

The reflector needs to be rather rigid. If the reflector is highly flexible, it will sag in the center and possibly break the FL lamps. Any rigid, reflective material can be used for the reflector, but it must be resistant to corrosion. The combination of the high temperatures around the MH lamps and the water make a very harsh environment for most metals. The ideal metal is stainless steel, but this is usually hard to find and quite expensive. Therefore, a painted steel sheet or aluminum sheet, 1/16″ to 1/8″ thick will work just fine. If paint is used, it must be a high quality, high temperature paint. Normal acrylic paint will peel off when exposed to the high temperature of the MH lamps and that would be really, really bad.

The original construction of this hood used flashing for the reflector. This is an excellent material in terms of corrosion resistance and reflectiveness. However, it is incredibly flimsy, which made it very difficult to work with. The lamp assembly needed two reinforcing bars to stabilize it and it was still rather weak (still is).

After selecting an appropriate reflector, the lamp sockets need to be mounted to the reflector. The method for mounting the FL lamps depends on the type of endcaps that are used. If BJB endcaps are used, then they can be mounted directly to the reflector, as shown in Figure 3. Other endcaps should mount in a similar fashion. If the endcaps can not be mounted directly to the reflector, then some method of attaching the endcaps to the reflector needs to be devised.

MH Lamp Holder

Lamp holders for FL lamps are readily available from many sources. Most mail order pet supply companies offer various different type of holders for FL lamps.

MH lamp holders, however, are not readily available. A good lighting store should stock sockets for a MH lamp, but the socket still needs to be mounted to the hood.

Constructing a mounting bracket for a MH lamp socket is fairly simple.It is basically a U-shaped bracket with wings, as shown below.

The sides of the bracket can be open, as shown above, or they can be closed. If the bracket is made from material that is not rigid, then the bracket should have closed sides to support the weight of the lamp. If the material is not rigid enough, the weight of the lamp will cause the lamp to droop and the lamp will not be parallel to the water’s surface. Certain lamps require a horizontal burn angle, so a sagging lamp may present a serious problem.

An incentive to make the bracket from a rigid material is that a bracket with open sides is much easier to assemble, and the wiring is also easier. A closed-side bracket makes it very difficult to mount the lamp socket. If the reflector is made from a rigid material (which it should be), then the bracket can be made from the same material.

Figure 5 shows a dimensioned drawing of an open-sided bracket. The dashed circle is where the MH lamp will go (don’t cut a hole there).

Before bending the lamp bracket to look like Figure 4, predrill holes for the MH lamp socket and the wires. Finding out exactly where to put the mounting holes can be tedious. To make things easier, we need to make a hole template.

You will need a piece of paper, a pen, a magic marker (or paint) and the MH socket. Remove the wiring screws from the bottom of the MH socket. Place the socket on the paper and trace the outline of the socket with the pen. Next, take the magic marker and color the wire terminals on the bottom of the socket. Quickly place the MH socket on the paper, being sure to align the socket to the circle made previously. The magic marker should make a perfect imprint of where the wire terminals are on the bottom of the socket. Finally, take the wire terminal screws and insert them through the paper. Ensure the holes are accurate by ‘screwing’ the paper to the socket. If everything works out, you should now have a perfect template for making the holes in the bracket. The final step is to make a hole in the center of the circle for the wires. A 1/4″ hole should be adequate, but use your best judgement.

Figure 5 shows the general location of the lamp socket on the bracket. Once the mounting bracket is formed, attach wires to the ceramic lamp socket and then attach the ceramic MH lamp socket to the bracket, as shown below.

Building the Lamp Assembly

The first step in building the lamp assembly is to attach the FL and MH lamp holders. Figure 4 shows some dimensions for attaching the end caps and mounting brackets. Use those dimensions to mark where the lamp holders will go. Then drill holes for the end caps and mounting brackets. Also drill holes for the wiring. Each end cap and mounting bracket should have a hole in the center of the mounting pattern for the wires to be fed through. Make sure the holes do not have any burrs. You don’t want the wiring insulation to come of and have the circuit short out.

Once all the holes are drilled, start by attaching the FL end caps. First attach wires to the end caps, and feed the wires through the center hole in the reflector. Then firmly attach the end cap to the reflector. Repeat this process for all the FL end caps and then the MH brackets.

It is worth noting at this point that the hood is designed for a remote ballast. So, you should run the wires to one end of the hood. This will be the junction point for the wires entering the hood from the ballasts.

The connection method at the junction is really a matter of preference. I ran the wires from the ballasts, about 6 feet to a connector. Then from the connector about 5 feet to the hood boundary. I connected the 5 foot section to the hood with wire nuts. I used a cinch connector for the MH wires and a standard DB-9 for the FL connector. In addition to the FL wires, I used the DB-9 for the feed to the fan as well.

After you are finished, the top of your lamp assembly will look like Figure 7.

An alternative is the following. Rather than running different wires from the lamps to one end of the hood and having a junction there, you could make the 5 foot section of wire 10 feet. Then, run those wires through the hood to the sockets. This means you need to run the cable through the hood boundary after you are finished wiring. (In hind site, I think this is a better way to go. Then you don’t have 12 wire nuts jammed into the hood where the cable leaves the hood.)

Wiring

Wire your ballasts by following the manufacturers directions. A general schematic of the lighting system is shown below.

Final Assembly

The assembly of the system is rather straight forward once the lamp assembly and the hood are constructed. Once again Figure 3 shows the overall construction.

To allow good air flow around the reflector and lamp assembly, there should be a 1/2″ to 3/4″ gap between the lamp assembly and the top of the hood. This allows air to flow between the lamp assembly and wood top, preventing overheating of the hood.

To create this gap, use a metal or wooden rail running the length of the hood. Then screw the reflector to the rail.

Once the hood and reflector are put together, the job is complete. The final step to to hand the hood over the tank. This can be accomplished by using eye bolts to each corner of the hood. Then attach chain or wire to the bolts. Each corner could be attached directly to the ceiling, or each side could be brought to a Y, and a single attachment could be made for each side.

Conclusion

I hope this helps. Use these plans at your own risk. I can not be held liable for any problems you may encounter. Feel free to email me with any questions – sasala at pop.dn.net.