Metal Halide Lamp

What is a Metal Halide Lamp?
These are high light output lamps used for lighting freshwater, marine or reef aquariums.

Operation
Metal halide lamps are gas-discharge lamps having a low-pressure gas and metal halides inside. The light is generated by glowing gas heated by an arc striking between two electrodes. The lamp does not have a filament, like halogen lamp.

The lamps must be connected via an AC choke and a starting device. These must be chosen according to lamp parameters, to prevent a premature lamp wearing and possible explosion of the lamp. The starting device generates a 4000 volt pulse to start the lamp. This voltage can be deadly, and lamps must be constructed well, and they must be preferably equipped with a residual current device (RCD).

The lamps operates on elevated temperatures. If the lamp is allowed to overheat, it may explode. As the explosion risk is always present, the armature must always be equipped with a glass.

Lamp types for the aquarium

 * Type 'HQI-E40' is a lamp with E-40 base. Wattages 70, 150, 250, 400W
 * Type 'HQI-TS' is a lamp with RX7s base. Wattages 70, 150, 250W

The colour temperatures available for both types are 4200, 6000, 10000, 14000, 20000K. The larger the value, the 'colder' is the light. The warmer lamps are suitable for the freshwater, and the colder for the marine/reef aquariums.

DIY construction of a lamp armature
With all safety measures taken, the armature can be constructed on the diy basis out of a filament halogen armature. Please remember that making your own electric appliances might be dangerous and/or illegal. Remember, that a budget-hqi's beam quality will not compare with factory made twice-the-price units. The armatures should be mounted ca.0,15m from water surface for best results.

A filament halogen armature of 300/500watts can be mounted with 70W HQI-TS lamp. Do not exceed this power limit. The original unit is dismantled, cooling holes are drilled, and the original lamp base is replaced with suitable RX7s base. The original power-cord is replaced by the cable from the AC-choke/Ignitor circuit in a separate enclosure.

Manufacturers data states that a special type of cable is needed between the ignitor and the lamp. This is because of the 4kv start pulse. In the prototype, normal 1,5mm2 400vac rated cable was used, with no problems with insulation -so far.

Links

 * Wikipedia